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Chemical reaction dynamics II and Correlated systems, surfaces and catalysis: general discussion Henry Chapman, Majed Chergui, Jochen Ku ¨ pper, Martin Wolf, Katharine Reid, Wendy Flavell, Gwyn Williams, Isabella Gierz, Elaine Seddon, Hans Jakob Wo ¨ rner, Kai Rossnagel, Jonathan Underwood, Michael Woerner, Julia Weinstein, Christian Bressler, Grigory Smolentsev, Klaus von Haeften, Ben Spencer, Stefan Neppl and Nora Berrah
DOI: 10.1039/c4fd90015d
Gwyn Williams opened the discussion of the paper by Christian Bressler: How does the signal to noise at the rather uctuating output of the LCLS compare with that of a steady synchrotron?
Christian Bressler replied: The short answer is: horribly. SASE XFEL sources (without self-seeding), especially if monochromatized, deliver uctuations up to 100%, while synchrotrons deliver a photon number entirely governed by photon statistics (shot noise). For a physicist SR is the ideal probe with its extreme stability. However, XFEL sources can deliver superior (scientic) information (due to their incredible single-pulse intensities), if you can reliably measure the incident intensity on a shot-by-shot basis. I am not sure about the level of I0 accuracy these days (probably on the percent level, thus improvements can be expected), but overall the data quality resembles what has been achieved earlier in TR X-ray experiments at SR sources with kHz laser systems, but here with femtosecond time resolution. This is a developing eld, and improvements should be expected over time, similar as in the eld of MHz-SR X-ray experiments.
Henry Chapman addressed Christian Bressler and Stefan Neppl: How heroic are the MHz rate experiments at synchrotrons, and are technologies that are being developed for those experiments useful for the European XFEL, which also has MHz pulse rates? What combinations of technologies of sources and detectors are ideally required for the types of experiments you are doing? For example, would † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4fd90015d
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an energy recovery linac (ERL) be better than either a synchrotron or FEL for highrep rate experiments? What about diffraction-limited storage rings?
Stefan Neppl responded: High repetition rate X-ray sources are generally desirable for time-resolved XPS experiments. They compensate efficiently for the relatively low pulse uences that can be tolerated in these measurements (mainly due to space charge and sample damage limitations). There are, however, also limits to the pulse repetition rates that may be used in a particular experiment. These are usually dened by the recovery time of the system under investigation (it should have sufficient time to return to its unperturbed state between two consecutive pump pulses) and the speed by which the sample has to be renewed (i.e. scanned for large solid-state samples or ow for liquid/solvated samples). Dye-sensitized semiconductor systems such as the ones studied here usually have recovery times ranging from hundreds of nanoseconds up to milliseconds. Correspondingly, the most suitable pulse repetition rates oen range from kHz to MHz regimes. The use of time-resolving detectors and event tagging, as demonstrated in this paper for MHz synchrotron radiation (see e.g. Fig. 7A and B), is an interesting approach for experiments at future high-repetition rate XFEL light sources such as the European XFEL and LCLS II. The technique will resolve pump–probe signals corresponding to individual microbunches while recording events for all micro-bunches, simultaneously. Diffraction-limited storage rings provide extremely small source sizes that result in correspondingly small X-ray focal spot sizes at the sample. This characteristic will potentially have several advantages for the technique presented here. Small X-ray focal spots enable the use of correspondingly small (optical) pump laser focal spots and, in turn, lower pump pulse energies to achieve the same excitation density. This relaxes the requirements for the pump lasers, which can be operated at higher repetition rates. Smaller spot sizes may also enable lower sample scan speeds and, therefore, the use of smaller samples.
Christian Bressler responded: Time-resolved X-ray experiments at MHz repetition rates are not “heroic” at all, at least no longer, as these can make use of the full X-ray ux at the larger class of synchrotrons: ESRF, APS, Petra-III, can deliver electron bunches in the 0.1–10 MHz range, and—if a MHz laser system is synchronized accordingly—one can collect up to around 1 1012 photons/s. This has to be compared to the older TR experiments with only few kHz repetition rates: monochromatic beamlines typically deliver between 1 102 and 1 106 photons/pulse, thus one can do these experiments with only 1 105 – 1 109 photons/s, and this limits the class of systems, which can be studied, and certainly the information content due to the lower (by up to a few orders of magnitude!) signal-to-noise ratio.
Gwyn Williams asked: For absorption experiments one has to normalize to the incident ux. For a synchrotron this is steady, particularly if the beam current is
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constant, but for the LCLS the incident ux uctuates, so do you have to use an incident ux monitor? If so, how is this implemented?
Christian Bressler answered: At LCLS this is implemented via a quadrant backscattering foil into four diodes. Prior studies established its general functionality down to the percent level, but in principle accuracies down to 1 106 should be possible. The need for better I0 normalization is merely user-driven, thus these should assess their data quality and, if needed, demand better I0 monitors.
Jonathan Underwood said: In the discussion you mentioned that, for the X-ray spectroscopy and scattering techniques described in your paper, as long as high dynamic range detection is available, the low repetition rate and high photon numbers per pulse characteristics of FEL sources are as effective as high (MHz) repetition rate sources with low photon numbers per pulse with digital detection. Do you think there are nonetheless still advantages to high repetition rate sources with low photon numbers per pulse with regard to new opportunities incorporating coincidence detection of scattered photons (for example, such as proposed by Mukamel and colleagues in J. Phys. B: At. Mol. Opt. Phys., 47, 124037)?
Christian Bressler responded: NB: A large dynamic range is always convenient, but not necessarily paramount. E.g., for X-ray diffuse scattering a dynamic range of 100 (from single photons onwards) appears sufficient (in some cases only a dynamic range of ten is required). Mukamel and colleagues apply extreme intensities on the attosecond timescale, while chemical dynamics seeks to use the X-rays as a linear probe. In this spirit high-rep-rate laser and X-ray sources are preferred over sources with extreme peak brightness (and this includes XFELs with moderate focussing conditions), but Mukamel may prove me so wrong one day...
Katharine Reid addressed Christian Bressler and Stefan Neppl: There is a drive to keep reducing the pulse durations that can be achieved at FEL sources. Is there an optimum pulse duration to aim for, or do we need tools that provide a variety of pulse durations in order to probe dynamical processes on different timescales? Related to this, are there situations where the bandwidths associated with ultrashort pulse durations cause complications for measurements?
Stefan Neppl replied: The detection of transient chemical shis with timeresolved XPS requires an experimental energy resolution that is at least comparable to the magnitude of the expected shis. Since these shis may vary between different systems, one would ideally want to select the best compromise of photon bandwidth and pulse duration for a given experiment. For example, transient line-shape changes—as described in our paper—would be difficult to reveal with
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sub-femtosecond X-ray pulses, which are inherently associated with bandwidths larger than 1 eV.
Christian Bressler replied: I sincerely believe that any answer to this question will be revisited, as newer experiments are being performed. For now, and in view of ease of use of optical laser beams, the sweet spot may be set into the 15–50 fs range. This would already help settle questions on reaction intermediates, possibly even some transition states, in photochemical reaction pathways. However, I think there is no hard limit for elucidating the elementary steps in chemical reactivity, and therefore I foresee a need for even shorter pulse durations in the future.
Grigory Smolentsev asked: You have used a few time-resolved X-ray techniques. Which one will be the most efficient to monitor the formation of rst MLCT state in Fe(bpy)3?
Christian Bressler replied: I still think that XANES would be well suited, although the rst hard fact was delivered by K-beta XES.
Henry Chapman addressed Christian Bressler and Gwyn Williams: Gwyn Williams stated that there is a limit to the number of photons per unit time (i.e. peak X-ray power) you can generate from an electron bunch (e.g. in an FEL) which depends on the peak current that can be achieved, ultimately limited by space charge in the electron bunch. This is analogous to limitations in generating high pulse intensities in lasers, which is limited by damage to the gain medium by the high-power pulse. The problem is circumvented in that case by chirped pulse amplication, and the same should be true for FEL pulses. That is, use a long chirped electron bunch of limited peak current and then compress the generated chirped X-ray pulse with gratings. Such a scheme was originally proposed by Claudio Pellegrini, and efforts are underway in Hamburg to develop and test this method by Adrian Cavalieri and Sasa Bajt and their colleagues. For some experiments it might not actually be necessary to compress the X-ray pulse if your sample disperses the pulse and time-resolved information can be obtained, as proposed by Moffat in a previous Faraday Discussion (vol. 122, 65–77, 2003).
Gwyn Williams responded: In addition to this, it has been proposed that one can develop even shorter bunches by rotating the chirped bunch and passing it through a physical aperture. This is an area that would be of considerable interest in machine development. The calculations referred to above are here: 239. S. L. Hulbert and G. P. Williams, “Calculations of Synchrotron Radiation Emission in the Transverse Coherent Limit”, Rev. Sci. Instrum., 2009, 80, 106103.
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Christian Bressler responded: TR X-ray experiments on chemical and biologically relevant samples do not benet from a further increased peak power, au contraire. The goal in chemical reaction dynamics is to resolve the ensuing reaction, and not to perturb it by the probe beam.
Jochen K¨ upper communicated: Christian, the “Gedanken” experiment you suggested regarding the measurement of the change from molecule to atoms was already performed (B. Erk, R. Boll, S. Trippel, D. Anielski, L. Foucar, B. Rudek et al., ”Imaging charge transfer in iodomethane upon X-ray photoabsorption”, Science, 2014, 345(6194), 288–291, DOI: 10.1126/science.1253607).
Christian Bressler communicated in reply: Thanks. This refers to one observable, the charge states of iodine up to 14+ or so. I could imagine that effects concerning the valence orbitals have been completely missed, but this is a cool experiment in the right direction!
Wendy Flavell opened the discussion of the paper by Stefan Neppl: This is a rather philosophical question, but how do you interpret the surface photovoltage shi in the case of your nanoparticle samples? How do you understand the large difference between clean ZnO and ZnO + N3 dye?
Stefan Neppl answered: This is an interesting question. According to theory, we would expect the band bending of an extended semiconductor crystal to be larger as compared to a nanoparticle of the same material. In our case, the substrate is a sintered lm of ZnO nanoparticles and might therefore be considered as an intermediate between these two limiting cases. A quantitative description of the bend bending in such a nanoporous semiconductor network has—to the best of our knowledge—not been derived and clearly motivates comparative surface photovoltage studies on these systems. The large difference in the SPV response of the bare ZnO substrate and the N3sensitized ZnO sample can be explained by the fact that the photon energy of the 532 nm laser excitation is much lower than the ZnO band gap. Therefore, no electron-hole pairs are created and no SPV is observed for the bare ZnO substrate at this wavelength. For N3-ZnO, the 532 nm photons excite electrons in N3 from the HOMO to the LUMO, from which they are injected into the ZnO conduction band. These electrons partly compensate the positive space charges in the ZnO depletion layer which in turn gives rise to a transient SPV effect. The magnitude of this SPV shi (> 400 meV) is indeed surprisingly high. It might be related to an enhancement of the initial ZnO bend bending upon chemisorption of N3 molecules
Christian Bressler asked: How did you assess the excited state population? Is this assessment of general importance for any kind of TR experiment? If so, how important is this (prior or in situ) knowledge? This journal is © The Royal Society of Chemistry 2014
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Stefan Neppl responded: In our analysis, we t the Ru3d/C1s difference spectrum to a model that describes the entire difference spectrum as being due to a laser-induced shi of the Ru3d doublet. This model has only two t parameters, the intensity of the shied Ru3d component (¼ excited state population) and the shi in binding energy itself. Generally, these two parameters are correlated and may lead to ambiguous t results when the energy shi is much smaller than the linewidth of the spectral features. This, however, is not the case for the 2–3 eV shis observed in our TRXPS experiment and, therefore, both the population and the shi can be determined from a single t.
Majed Chergui asked: You show that the XPS transient can be reproduced by shiing the entire spectrum by 2 eV and subtract it from the unshied spectrum, but I wouldn't expect the Ru and C lines to be shied by the same amount?
Stefan Neppl replied: We observe two different shis in our time-resolved XPS experiment. The rst one is a rigid shi of the entire C1s/Ru3d spectrum (see Fig. 7C of the manuscript) induced by the surface photovoltage (SPV) effect in the semiconductor substrate. If we correct for this SPV shi, and compare the spectrum to the laser-off reference as shown in Fig. 8, we also detect a characteristic laser-induced change in the C1s/Ru3d spectral shape, which can partly be explained as the result of a ca. 2–3 eV transient shi of the Ru3d doublet in the excited molecules to higher binding energies. This trend might be expected, since the oxidation state of the Ru-metal center increases upon charge injection. When modeling the difference spectra (green line in Fig. 8B), we implicitly assumed that a similar transient shi of the C1s levels—due to the dynamic charge reconguration within the molecule—is much less pronounced. This is of course an approximation, which has to be tested in future experiments with a better signalto-noise ratio. Qualitatively, however, one might expect the effect on most of the carbon core levels to be smaller as compared to the Ru lines, since the laser excitation is associated with a HOMO–LUMO transition and subsequent charge transfer from the dye into the semiconductor substrate, leading to an electronic conguration that is well approximated by a hole in the HOMO of the dye and an additional electron in the substrate (ref. 41: Siefermann et al., J. Phys. Chem. Lett., 2014, 5(15), 2753–2759). The dye HOMO is highly localized on the Ru center and the thiocyanate groups. Thus, the net change in local electron density seen by an individual carbon atom in the pyridine groups is much smaller than for the ruthenium center and, therefore, the by far most intense C1s signal from the pyridine rings is expected to be much less affected by the photo-induced dynamics than the Ru3d lines. Detailed studies of core-level shis associated with other atoms in the system, in particular the sulfur atoms in the thiocyanate ligands and the nitrogen atoms bound to the Ru center, will be performed in future experiments.
Martin Wolf addressed Wendy Flavell and Stefan Neppl: The electronic structure of ZnO surfaces depends very much on surface preparation. On several single crystal surfaces hydrogen acts as a donor and adsorption may lead to a Faraday Discuss.
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metallic surface (charge accumulation layer). See, for example, Phys Rev. B, 2011, 83, 125406. This leads to a pinning of the conduction band and prevents a (signicant) surface voltage shi.
Wendy Flavell responded: Indeed that is the case. But what is observed at the surface of ZnO is, as you say, very dependent on surface preparation. In the work described in Phys. Rev. B, 2013, 88, 195301 (and paper 13) the combination of vacuum and oxygen annealing cycles always yields an SPV shi to high binding energy, consistent with a surface depletion layer when excited with band gap (or even slightly sub-band-gap radiation.) The size of the shi and its dynamics change with oxygen content/donor concentration in a consistent way that implicates oxygen vacancies in the persistent photoconductivity (as suggested by Lany and Zunger, Phys. Rev. B, 2005, 72, 035215). The involvement of hydrogen at these sites cannot be excluded.
Stefan Neppl replied: We would like to add that the ZnO nanoparticle lms used in our experiments are prepared ex situ, i.e. not under UHV conditions. This kind of preparation will likely result in the coexistence of different adsorbate- and/ or defect-induced electronic states, which all contribute to the bend bending. Future SPV experiments using both sub- and super-band gap illumination will provide more insight into the nature of the bend bending in this system.
Julia Weinstein asked: Dear Stefan, several questions: (i) Is it possible to distinguish between the two variants of the attachment of the dye to the surface of a semiconductor: two carboxylic groups from one bpyligand, or one carboxylic group per bpy ligand, e.g., from the electron density on the C atoms? (ii) Does the experiment capture the redistribution (delocalisation) of electron density between several bpy-type ligands on the metal centre following MLCT excitation? (iii) How do the timescales (and perhaps the yields?) of injection estimated by the method discussed compare with the results obtained by the optical methods?
Stefan Neppl replied: These are very interesting questions concerning the potential and limits of time-resolved XPS technique when applied to study dynamics in dye-semiconductor systems. (i) Unfortunately, this information is challenging to derive from XPS spectra. The C1s photolines of all carboxylic acid groups form essentially one single XPS peak (see, e.g., Mayor et al., J. Chem. Phys., 2008, 129, 114701 and J. Chem. Phys., 2009, 130, 164704). The O1s line is sensitive to protonation/deprotonation of the carboxylic acid groups but this does not provide clear-cut information on the adsorption geometry either since both deprotonated as well as protonated carboxylic acid groups may participate in the attachment of the dye to the substrate (see, e.g., Siefermann et al. J. Phys. Chem. Lett., 2014, 5, 2753).
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(ii) In a recent femtosecond time-resolved XPS study, we have determined that the electron leaves the dye within less than 500 fs aer the MLCT excitation (Siefermann et al., J. Phys. Chem. Lett., 2014, 5, 2753). Therefore, we expect that only experiments with a time resolution in the range of a few to, perhaps, a few tens of femtoseconds would be able to trace the electron delocalization step before electron injection into the substrate. Principally, carbon atoms in the individual bpy units which are located closer to the nitrogen or carboxylic acid groups, give rise to C1s emission lines which differ in binding energy by ca. 1 eV (see Mayor et al., J. Chem. Phys., 2009, 130, 164704). Given the above mentioned time resolution can be achieved, time-resolved XPS might therefore be used to trace intramolecular charge reconguration in N3 affecting these two parts in all the bpy-ligands differently, but cannot address the bpy-ligands individually. (iii) We have not (yet) been able to derive a timescale for electron injection from the ALS-based picosecond time-resolved XPS experiments. In our recent femtosecond time-resolved XPS study at the LCLS (Siefermann et al., J. Chem. Phys. Lett., 2014, 5, 2735 we found evidence that, while the electron leaves the dye within less than 500 fs, it remains in an interfacial charge-transfer state for more than 1 ps. This result is in agreement with studies in the (near) optical domain that measured time constants of tens of ps for the appearance of free carriers inside the semiconductor, e.g., via time-resolved THz spectroscopy (Nemec et al., Phys. Rev. Lett., 2010, 104, 197401, Tiwana et al., ACS Nano, 2011, 5, 5158). A more detailed and general comparison (which indeed would be very interesting) between time-resolved XPS (TRXPS) and ultrafast optical spectroscopy studies on the electron dynamics and injection yields at dye-semiconductor interfaces will be possible as soon as more TRXPS results become available.
Majed Chergui asked: You show the nsec recovery kinetics of the dye, which has at least 2 components. What are the origins of these two timescales? Injection is known to occur on several timescales, do you see a multicomponent growth of the cation signal in your measurements ?
Stefan Neppl answered: Multi-exponential recombination dynamics have been observed before for various dye-semiconductor systems, e.g., in all-optical transient absorption studies. For ruthenium-based dyes attached to ZnO and TiO2 substrates, Bauer et al. (J. Phys. Chem. B, 2001, 105, 5585) found bi-exponential electron back transfer kinetics with time constants in the range of hundreds of nanoseconds to a few microseconds. The kinetics were the same for both substrates, which implies that the general electronic structure of the substrate is not the main factor governing the recombination dynamics. In general, the different timescales observed in the electron injection and electron back transfer process are at least partly ascribed to sample inhomogeneities, e.g. the co-existence of different binding congurations with different electronic coupling to the semiconductor substrate or electronic trap states induced by defects in the semiconductor crystal structure (see, e.g., Tachibana et al., J. Phys. Chem. B, 2000, 104, 1198). Another possibility is that the dye-electron recombination dynamics may be affected by transient surface/interfacial potentials in a similar fashion as electron-hole recombination dynamics in band-gap excited semiconductors (see Faraday Discuss.
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the contribution of Wendy Flavell’s group to this Faraday Discussion). The feedback between interfacial electronic dynamics and the potentials governing them generally leads to transient signals that cannot be described by single exponentials (see Br¨ ocker et al., Chem. Phys., 2004, 299, 247). We will study these effects in greater detail in the near future. In our measurements, the transient shi of the Ru3d photoemission lines is the most direct probe for the cationic state of the dye molecule during charge injection. So far, we have measured this dynamical chemical shi only for two time delays (ref. 41 and Fig. 8 in the manuscript). The full time dependence of this shi will be studied in future experiments, which will also require access to fewfemtosecond X-ray light sources (e.g. FELs) to resolve the possibly fast (subpicosecond) dynamics of the involved electron injection pathways.
Wendy Flavell remarked: In the case of the decay transient for the ZnO SPV, why should the dynamics necessarily be bi-exponential with a fast and a slow part? The ‘self-decelerating’ model of Widdra and Broecker (e.g. W. Widdra et al., Surf. Sci., 2003, 543, 87) allows for the increase in lifetime as the SPV decays, i.e. a variation in the lifetime during the decay could be an intrinsic part of the physics, allowing one function to be tted to the decay transient.
Stefan Neppl replied: Indeed, the SPV decay in Fig. 9 can be tted equally well using the self-decelerating model with a (dark) carrier lifetime of 390 ns and an a-parameter of 3. An a-parameter of 3, which is an additional t parameter in this model, has so far not been observed in SPV experiments, and is signicantly larger than a parameters reported, e.g., for single crystalline ZnO (a < 1). Generally, the a parameter is related to the ideality factor used in the theory of Schottky diode performance, but its interpretation in the context of SPV measurements, and how this parameter can be related to different recombination mechanisms, is still an open question.
Majed Chergui enquired: You excited the dye and you probe it by XPS, but evidence that you have injected the electron into the substrate, is to probe the lines of Zn (or O). We did such an experiment on RuN719 adsorbed at the surface of TiO2 and observed the oxidation of Ru by Ru L edge spectroscopy and the reduction of Ti centres by Ti K edge absorption (M. Hannelore Rittmann-Frank, Chris J. Milne, Jochen Rittmann, Marco Reinhard, Thomas J. Penfold and Majed Chergui, Angew. Chem. Int. Ed., 2014, 53, 5858–5862). So in your case, what is the evidence that the electron is in the substrate.
Stefan Neppl answered: We think that the positive transient surface photovoltage (SPV) response (see Fig. 9 of the manuscript) is the spectroscopic signature for the electrons being transferred to the semiconductor: photo-excited electrons from the dye can effectively screen the positive excess charge inside the n-type ZnO substrate only when they diffuse into the tens of nanometer wide depletion layer. This additional screening mediated by the injected electrons This journal is © The Royal Society of Chemistry 2014
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causes reduction of the bending near the surface of the ZnO substrate, which is observed as a sudden increase in binding energy of all energy levels that are referenced to the semiconductor Fermi level. Since the dye molecules are chemisorbed on the ZnO substrate, their core-level photoemission can be expected to be as sensitive to the SPV effect as photoemission lines originating from the semiconductor material itself.
Kai Rossnagel said: In your talk, you also presented time-resolved XPS data that were acquired at the LCLS. Since free-electron lasers have signicantly lower repetition rates as compared to storage rings, the data collection efficiency of the presented photoelectron detection scheme is signicantly reduced. It is limited by the fact that the delay-line detector can only detect one electron per photon probe pulse. Will you continue to perform time-resolved XPS measurements at the LCLS and, if so, do you have plans to make these measurements more efficient, for example, by combining a time-of-ight spectrometer with a multi-segment delayline detector?
Stefan Neppl replied: The detector in the LCLS experiment was not a delay-line anode, but a phosphor screen combined with a fast CCD camera. Generally, the strongest constraints for time-resolved XPS experiments at FELs are imposed by space charge effects, which have been observed for X-ray pulse uences of less than 10 107 photons/pulse, i.e., several orders of magnitude lower than the maximum pulse uences available (see, e.g., Pietzsch et al., New J. Phys., 2008, 10, 033004). For the N3/ZnO experiment at the LCLS (ref. 41: Siefermann et al., J. Phys. Chem. Lett., in press), the photon ux had to be reduced to ca. 10 6 photons/ pulse to avoid space-charge induced peak distortions and shis. Under these conditions, less than one photoelectron was detected per X-ray pulse. Thus, detector saturation was not an issue and multi-segmented anodes would not be required for this particular experiment to improve data collection efficiency. However, they may still be advantageous, for example, in combination with timeof-ight spectrometers to increase the angular acceptance and sensitivity of an XPS setup. Cleary, time-resolved X-ray photoelectron spectroscopy will greatly benet from high-repetition rate XFELs, where femtosecond time-resolved X-ray measurements that require moderate pulse energies can still be efficiently performed.
Wendy Flavell addressed Stefan Neppl, Christian Bressler, Klaus von Haeen and Nora Berrah: As time resolution improves, I'd like to make the comment that we must also build in the ability to measure over quite long timescales, with this good resolution. For condensed matter systems of the type we are discussing, dynamics can be happening over very long timescales (in the case of paper 13, we needed to measure at delays up to ms to capture the persistent photoconductivity of ZnO). The ArToF analyser coupled with hybrid mode operation at synchrotron, as implemented for example at SPring8 BL07LSU seems a productive direction for
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the future. Synchronising to successive ‘rotations’ of the large bunch allows the dynamics to be measured over many orders of magnitude of time delay.
Klaus von Haeen replied: I agree. In our paper on the ‘Formation of coherent rotational wavepackets in molecule-helium clusters’ we record the propagation of a rotational wavepacket over 600 ps. In our example we have chosen laser conditions that allow for the excitation of rotational level close and beyond the dissociation limit of the He–C2H2 complex. It would be very interesting to observe the propagation of the rotational wavepacket for longer, for example up to 10 ns because dissociation times of related systems (CO2) have been reported to be 6 ns, M. J. Weida, J. M. Sperhac, D. J. Nesbitt and J. M. Hutson, J. Chem. Phys., 1994, 101, 8351. I would be particularly interested whether predissociation depends on the actual composition of all rotational states. A chemical reaction could then be triggered by shaping of the alignment laser eld.
Stefan Neppl replied: We strongly support this statement. The combination of active pump–probe synchronization and simultaneous time-tagging as implemented in our experiment at the Advanced Light Sources enables both efficient synchrotron-bunch length limited time-resolved experiments, and the probing of dynamics proceeding on micro- and even millisecond timescales. In addition, this setup benets from the adjustable repetition rate (single shot to MHz) of the used laser system, which allows to match the pump-pulse spacing to the timescale of the process to be investigated.
Christian Bressler answered: I would also agree, albeit the time resolution is no longer superimportant, when we enter the 100 ps range and much further above.
Nora Berrah responded: I agree and in fact the LCLS has extended its long timescales to 500 fs.
Jochen K¨ upper opened the discussion of the paper by Klaus von Haeen: You said that you had to use laser pulses for alignment and Coulomb explosion imaging that were parallel—why was that? Then, it seemed as if you said that you would not see alignment along the alignment laser axis if using a perpendicularly polarized ionization laser—did I understand that correctly? If so, is it valid to speak of “alignment”, in the sense of strong angular connement of the molecular rotational distribution, or would it be more appropriate to refer to the experiment as “rotational coherence spectroscopy” (RCS) as introduced by Felker (P. Felker, Rotational Coherence Spectroscopy—Studies of the Geometries of Large Gas-Phase Species by Picosecond Time-Domain Methods, J. Phys. Chem., 1992, 96(20), 7844–7857). My next question refers to Fig. 3 in your paper, or, actually, to the alignment trace you showed during the conference, which seems to signicantly deviate from the gure in the manuscript: This journal is © The Royal Society of Chemistry 2014
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Why do your ¼ (and /4 ) revivals show only alignment (or antialignment) but not the correspondingly pre- or post-anitalignment (alignment)? See, for instance, Nielsen, Phys. Chem. Chem. Phys., 13, 18971 where these are clearly visible. Samuel Leutwyler has recently performed a nice detailed analysis of this in his RCS experiments [(a) G. Br¨ ugger, H.-M. Frey, P. Steinegger, F. Balmer and S. Leutwyler, Accurate Determination of the Structure of Cyclohexane by Femtosecond Rotational Coherence Spectroscopy and Ab Initio Calculations, J. Phys. Chem. A, 2011, 115(34), 9567–9578, doi: 10.1021/jp2001546; (b) H. M. Frey, D. Kummli, S. Lobsiger and S. Leutwyler, High-resolution Rotational Raman Coherence Spectroscopy with Femtosecond Pulses, in Handbook of High-resolution Spectroscopy, Chichester, UK, John Wiley & Sons, Ltd., 2011, doi: 10.1002/ 9780470749593.hrs055]. He showed that accurate pump-probe delays are a very crucial effect and that is quite an effort to do this reasonably well. Therefore, it would be useful to learn what your statistical and systematic errors of the delay time were. 3. Since you suggest that you have no observable alignment with the laser polarizations perpendicular, is it correct to speak of “aligned molecules”, or should we really consider your experiment a rotational coherence spectroscopy (RCS) experiment as introduced by Kim and Felker in the 80s. 2. How did you calibrate your temporal axis, what statistical and systematic errors do you get on the time-axis of the RCS trace, or, what precision?
What is the error propagation into the rotational constants? What are the precision and accuracy of the temporal axis in your RCS experiment, i.e., what are the limits on its systematic errors? How do these uncertainties—statistical (precision) and systematic (accuracy) estimates—propagate into the rotational constants determined? In how far can you use these numbers to determine “structures”, as was done in other experiments (Ratzer, Chem. Phys., 283, 153 and references therein)?
Klaus von Haeen replied: 1. Indeed you must have misunderstood me. In all experiments in our paper the polarisation was parallel for consistency within the experimental run. Experiments with perpendicular polarisation were planned but the granted beamtime at the Rutherford Appleton Lab did not allow to begin such experiments. The orientation of polarisation had no consequences for our results. 2. I think it is fair to call our experiments RCS with the distinction to older work by Felker et al. that we use impulsive alignment to generate rotational wavepackets. This has specic, and very important advantages as outlined in the introduction of our paper 3. The temporal evolution of alignment depends on laser parameters and the target system. Our results shown are the rst of a target comprising of clusters of acetylene and a variable number of helium atoms. Hence, it makes no sense to compare these with other work on other target systems, with different laser parameters, etc. 4. Calibration can be achieved by investigating a known system, for example carbon monoxide. The accuracy that can be achieved is nicely illustrated in A. Przystawik, A. Kickermann, A. Al-Shemmary, S. Dusterer, A. M. Ellis, K. von Haeen, M. Harmand, S. Ramakrishna, H. Redlin, L. Schroedter, M. Schulz, T. Seideman, N. Stojanovic, J. Szekely, F. Tavella, S. Toleikis and T. Laarmann, Phys. Rev. A, 2012, 85, 052503. 5. Rotational constants only support structural models. I would therefore consider rotational spectroscopy not as a method to ‘determine’ structures. It is not uncommon, particularly for quantum mechanical systems that several observables add complementary information to the whole picture.
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Jochen K¨ upper communicated: 1. Typically the Coulomb explosion laser is polarized perpendicular to the alignment laser to avoid overestimating the degree of alignment—why did you not do that? What error, do you estimate, was introduced by that Sam Leutwyler has recently performed a nice detailed analysis of this in his RCS experiments and showed that this is a very crucial effect? 3. Since you suggest that you have no observable alignment with the laser polarizations perpendicular, is it correct to speak of “alignmed molecules”, or should we really consider your experiment a rotational coherence spectroscopy (RCS) experiment as introduced by Kim and Felker in the 80s. 2. How did you calibrate your temporal axis, what statistical and systematic errors do you get on the time-axis of the RCS trace, or, what precision?
What is the error propagation into the rotational constants?
Klaus von Haeen communicated in reply: 1. It is correct that in many conceptually similar experiments perpendicular polarisation between pump and probe laser is chosen. A requirement to observe impulsive alignment is that polarisation of the pump laser is parallel to the detector plane (for Coulomb explosion detection). Choosing a probe laser polarisation perpendicular to the detector plane will remove any possible anisotropy in the images that may be caused by the excitation process, which is good to remove background signal for the determination of pump-laser-induced alignment. An advantage of a parallel probe-laser polarisation is that anisotropies in the excitation can be used to detect alignment induced in the pump process. For example, we observed a modulation of the ion yield. 2. I think these questions overlap with a previous one.
Katharine Reid remarked: The authors of paper 3 were able to achieve quite substantial alignment of a polyatomic molecule ( ¼ 0.89), but the alignments shown in your paper are substantially lower. Is there a prospect of attaining much higher alignment of molecules embedded in helium clusters? Following this, is there a prospect of probing photoinduced dynamics of aligned molecules embedded in helium clusters?
Klaus von Haeen replied: The degree of alignment observed in our work was comparatively low, but the target was unusual. For a known molecule, e.g. with well-dened polarisability, in a laser eld of known intensity, pulse length, etc., the degree of alignment is dened and can be calculated. The degree of alignment is then reected by the respective values of and the characteristic time-dependence. For example, a sine-wave-type time dependence indicates involvement of only two rotational levels. Correspondingly, the level of alignment is low. In our case, the target was different. It consisted of molecular clusters comprising a single HCCH molecule and a variable number of helium atoms. Consequently, 1. (t) shows the superposition of the alignment of all the individual clusters 2. Alignment information is washed out during the Coulomb explosion process (see another question by H-J W¨ orner) Our work This journal is © The Royal Society of Chemistry 2014
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clearly shows that it is possible to generate coherent rotational wavepackets in very small molecule-helium clusters. Our results do not justify commenting on the prospects of achieving high alignment of molecules in helium clusters or droplets, which would certainly be of great advantage for investigating photoinduced dynamics of embedded molecules and molecular complexes.
Jonathan Underwood asked: In the paper presented by von Haeen, the observed Coulomb explosion images are a convolution of the molecular axis alignment produced by the pump pulse and the strong orientational dependence of the Coulomb explosion probe process. Because the probability for Coulomb explosion has a strong orientational dependence, it is non-trivial to quantitatively extract the molecular axis alignment, and it has become the norm in this eld to characterize the molecular axis alignment in terms of expectation values calculated for the resulting image. Recently, in collaboration with the Stapelfeldt group in Aarhus we have developed and demonstrated a technique for deconvoluting the orientational dependence of the Coulomb explosion process allowing for complete characterization of the molecular axis alignment through inversion of the images. This methodology requires an image to be recorded for the Coulomb explosion of isotropically oriented molecules ahead of doing the experiment with aligned molecules. This work will be published in the near future.
Hans Jakob W¨ orner commented: I have two questions. First, could you provide an estimate of the maximal cluster size or of the cluster-size distribution in your experiment? Second, have you obtained evidence for avalanche-like ionization of the helium clusters (Phys. Rev. Lett., 102, 128102) in other observables than the C+ kinetic energies (Fig. 2b)?
Klaus von Haeen replied: We are unable to provide gures for the maximal cluster size or the cluster size distribution. The clusters are produced using a pulsed expansion of a gas mixture in an Even-Lavie valve. While the cluster sizes in continuous expansion can be fairly reliably predicted for a large range of gases using the formalism developed by Hagena, this cannot be said for gas mixtures. Secondly, the shape of conical nozzles, in particular the opening angle controls the cluster size. Scaling laws have not been published for the Even-Lavie nozzle, although we can conrm by our own unpublished work on argon cluster formation in a continuous expansion through a conical E-L-nozzle that Hagena's formula can be applied and that an effectve half-opening angle of 20 can be conrmed. We have no rm evidence for avalanche-like ionization of the helium clusters, although we see indications that such an effect takes place. For expansion conditions favouring formation of larger clusters we observe higher kinetic energies of the Coulomb explosion carbon ion fragments. In our paper we suggest that an effect similar to that suggested by Mikaberidze, Saalmann and Rost, Phys. Rev. Lett., 2009, 102, 128102, may take place. We also explain in the paper that recombination between electrons and higher charged carbon ion fragment may
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contribute to the increased kinetic energies of C+, an idea originally proposed by J. M. Rost in a personal communication. The rather complicated fragmentation process following the Coulomb explosion has the consequence that angular information originally contained in the explosion process becomes washed out, leading to apparent lower levels of alignment.
Jochen K¨ upper remarked: You mentioned that you were using mixtures of gases. Which gases were you using, at what concentrations/partial pressures, and why was that necessary? How did you make sure that you were not observing signals from mixed clusters, or simply from clusters with other atoms?
Klaus von Haeen responded: In the presented paper HCCH–Hen clusters were produced by co-expansion of HCCH–He mixtures at stagnation pressures between 50 and 100 bar. The HCCH concentration was varied. Signatures of HCCH–He cluster formation were observed when the HCCH concentration was below 0.1%. Good results were achieved for even lower concentrations, for example, 0.03%. The concentrations used to produce the spectra shown in our paper are indicated. The signals observed were from pure HCCH, and indeed from mixed HCCH– He clusters. Signals from pure He clusters were not expected and not observed. The background pressure in the apparatus used was 5 109 mbar. Cluster formation with atoms other than helium can be safely excluded.
Michael Woerner opened the discussion of the paper by Kai Rossnagel: What is the energetic resolution of your experiments? What determines the energetic resolution of your experiments? Can you get energetic and spatial infomation simultaneously in the future ?
Kai Rossnagel answered: The effective energy resolution of our time-resolved ARPES experiments was determined from Fermi-edge spectra of a polycrystalline gold sample to be 260 meV. The two dominant contributions to the resolution are generally the spectral width of the XUV probe pulses and the resolution of the photoelectron spectrometer. For the presented experiments, an upper limit on the spectral width of the XUV pulses could be independently determined to be 170 meV, and the nominal spectrometer resolution was 200 meV. Note that we do not use a monochromator in our experimental setup, which would reduce the XUV pulse duration and photon ux. Monochromation of the XUV probe pulses is not necessary because by pumping with the second harmonic of a Ti:Sapphire laser at a fundamental wavelength of 790 nm we can generate an isolated, well-separated, and intense high harmonic pulse at about 22 eV. For experimental details, see DOI: 10.1016/j.elspec.2014.04.013. To get spectral and spatial information at the same time would be possible with a combined femtosecond time-resolved ARPES and X-ray photoelectron diffraction (XPD) experiment. Time-resolved ARPES would provide direct This journal is © The Royal Society of Chemistry 2014
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information on the dynamics of the band structure and Fermi surface, while timeresolved XPD would provide complementary information on the changes of structural parameters. Our plan is to realize such an experiment at the European XFEL.
Majed Chergui remarked: Since you mention determining lattice structure by photoelectron diffraction combined with ARPES, how would you distinguish the electronic structure from the lattice structure in this case?
Kai Rossnagel replied: ARPES and XPD, when applied in the mode with xed photon energy, measure the same thing: the angular distribution of photoelectrons at specic kinetic energies. The differences lie in the energies of the electronic states from which the photoemission occurs and in the information the photoemission intensity distribution contains. In the case of ARPES, valence electrons are knocked out and the recorded intensity maps directly reect the dispersion of the electronic states as a function of crystal momentum. In the case of XPD, on the other hand, core electrons are photoemitted and the measured intensity distributions are interpreted as interference patterns from which structural parameters can be extracted more or less directly. Electronic structure determination by ARPES works best at XUV photon energies, whereas the interpretation of XPD patterns is the simplest and most intuitive when so X-rays excite high-energy photoelectrons that are scattered by the atoms in the sample predominantly in the forward direction. So when I dream of a combined femtosecond time-resolved ARPES and XPD experiment, I think about two separate sets of measurements to be performed on the same sample, one XUV-ARPES measurement and one so-X-ray-XPD measurement. Presently, there is no photon source available where such experiments can be done. However, there is well-justied hope that, with up to 27000 pulses/s, the European XFEL, specically its so X-ray SASE3 beamline, will be such a source.
Martin Wolf enquired: You have pointed out that there are two timescales in the dynamics “melting” of the Mott and CDW gap in TaS2 und TaSe2. It is clear that the slow (200 fs) dynamics originates for ion motion by the amplitude mode, whereas you assign the ultrafast (
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Chemical reaction dynamics II and Correlated systems, surfaces and catalysis: general discussion Henry Chapman, Majed Chergui, Jochen Ku ¨ pper, Martin Wolf, Katharine Reid, Wendy Flavell, Gwyn Williams, Isabella Gierz, Elaine Seddon, Hans Jakob Wo ¨ rner, Kai Rossnagel, Jonathan Underwood, Michael Woerner, Julia Weinstein, Christian Bressler, Grigory Smolentsev, Klaus von Haeften, Ben Spencer, Stefan Neppl and Nora Berrah
DOI: 10.1039/c4fd90015d
Gwyn Williams opened the discussion of the paper by Christian Bressler: How does the signal to noise at the rather uctuating output of the LCLS compare with that of a steady synchrotron?
Christian Bressler replied: The short answer is: horribly. SASE XFEL sources (without self-seeding), especially if monochromatized, deliver uctuations up to 100%, while synchrotrons deliver a photon number entirely governed by photon statistics (shot noise). For a physicist SR is the ideal probe with its extreme stability. However, XFEL sources can deliver superior (scientic) information (due to their incredible single-pulse intensities), if you can reliably measure the incident intensity on a shot-by-shot basis. I am not sure about the level of I0 accuracy these days (probably on the percent level, thus improvements can be expected), but overall the data quality resembles what has been achieved earlier in TR X-ray experiments at SR sources with kHz laser systems, but here with femtosecond time resolution. This is a developing eld, and improvements should be expected over time, similar as in the eld of MHz-SR X-ray experiments.
Henry Chapman addressed Christian Bressler and Stefan Neppl: How heroic are the MHz rate experiments at synchrotrons, and are technologies that are being developed for those experiments useful for the European XFEL, which also has MHz pulse rates? What combinations of technologies of sources and detectors are ideally required for the types of experiments you are doing? For example, would † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4fd90015d
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an energy recovery linac (ERL) be better than either a synchrotron or FEL for highrep rate experiments? What about diffraction-limited storage rings?
Stefan Neppl responded: High repetition rate X-ray sources are generally desirable for time-resolved XPS experiments. They compensate efficiently for the relatively low pulse uences that can be tolerated in these measurements (mainly due to space charge and sample damage limitations). There are, however, also limits to the pulse repetition rates that may be used in a particular experiment. These are usually dened by the recovery time of the system under investigation (it should have sufficient time to return to its unperturbed state between two consecutive pump pulses) and the speed by which the sample has to be renewed (i.e. scanned for large solid-state samples or ow for liquid/solvated samples). Dye-sensitized semiconductor systems such as the ones studied here usually have recovery times ranging from hundreds of nanoseconds up to milliseconds. Correspondingly, the most suitable pulse repetition rates oen range from kHz to MHz regimes. The use of time-resolving detectors and event tagging, as demonstrated in this paper for MHz synchrotron radiation (see e.g. Fig. 7A and B), is an interesting approach for experiments at future high-repetition rate XFEL light sources such as the European XFEL and LCLS II. The technique will resolve pump–probe signals corresponding to individual microbunches while recording events for all micro-bunches, simultaneously. Diffraction-limited storage rings provide extremely small source sizes that result in correspondingly small X-ray focal spot sizes at the sample. This characteristic will potentially have several advantages for the technique presented here. Small X-ray focal spots enable the use of correspondingly small (optical) pump laser focal spots and, in turn, lower pump pulse energies to achieve the same excitation density. This relaxes the requirements for the pump lasers, which can be operated at higher repetition rates. Smaller spot sizes may also enable lower sample scan speeds and, therefore, the use of smaller samples.
Christian Bressler responded: Time-resolved X-ray experiments at MHz repetition rates are not “heroic” at all, at least no longer, as these can make use of the full X-ray ux at the larger class of synchrotrons: ESRF, APS, Petra-III, can deliver electron bunches in the 0.1–10 MHz range, and—if a MHz laser system is synchronized accordingly—one can collect up to around 1 1012 photons/s. This has to be compared to the older TR experiments with only few kHz repetition rates: monochromatic beamlines typically deliver between 1 102 and 1 106 photons/pulse, thus one can do these experiments with only 1 105 – 1 109 photons/s, and this limits the class of systems, which can be studied, and certainly the information content due to the lower (by up to a few orders of magnitude!) signal-to-noise ratio.
Gwyn Williams asked: For absorption experiments one has to normalize to the incident ux. For a synchrotron this is steady, particularly if the beam current is
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constant, but for the LCLS the incident ux uctuates, so do you have to use an incident ux monitor? If so, how is this implemented?
Christian Bressler answered: At LCLS this is implemented via a quadrant backscattering foil into four diodes. Prior studies established its general functionality down to the percent level, but in principle accuracies down to 1 106 should be possible. The need for better I0 normalization is merely user-driven, thus these should assess their data quality and, if needed, demand better I0 monitors.
Jonathan Underwood said: In the discussion you mentioned that, for the X-ray spectroscopy and scattering techniques described in your paper, as long as high dynamic range detection is available, the low repetition rate and high photon numbers per pulse characteristics of FEL sources are as effective as high (MHz) repetition rate sources with low photon numbers per pulse with digital detection. Do you think there are nonetheless still advantages to high repetition rate sources with low photon numbers per pulse with regard to new opportunities incorporating coincidence detection of scattered photons (for example, such as proposed by Mukamel and colleagues in J. Phys. B: At. Mol. Opt. Phys., 47, 124037)?
Christian Bressler responded: NB: A large dynamic range is always convenient, but not necessarily paramount. E.g., for X-ray diffuse scattering a dynamic range of 100 (from single photons onwards) appears sufficient (in some cases only a dynamic range of ten is required). Mukamel and colleagues apply extreme intensities on the attosecond timescale, while chemical dynamics seeks to use the X-rays as a linear probe. In this spirit high-rep-rate laser and X-ray sources are preferred over sources with extreme peak brightness (and this includes XFELs with moderate focussing conditions), but Mukamel may prove me so wrong one day...
Katharine Reid addressed Christian Bressler and Stefan Neppl: There is a drive to keep reducing the pulse durations that can be achieved at FEL sources. Is there an optimum pulse duration to aim for, or do we need tools that provide a variety of pulse durations in order to probe dynamical processes on different timescales? Related to this, are there situations where the bandwidths associated with ultrashort pulse durations cause complications for measurements?
Stefan Neppl replied: The detection of transient chemical shis with timeresolved XPS requires an experimental energy resolution that is at least comparable to the magnitude of the expected shis. Since these shis may vary between different systems, one would ideally want to select the best compromise of photon bandwidth and pulse duration for a given experiment. For example, transient line-shape changes—as described in our paper—would be difficult to reveal with
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sub-femtosecond X-ray pulses, which are inherently associated with bandwidths larger than 1 eV.
Christian Bressler replied: I sincerely believe that any answer to this question will be revisited, as newer experiments are being performed. For now, and in view of ease of use of optical laser beams, the sweet spot may be set into the 15–50 fs range. This would already help settle questions on reaction intermediates, possibly even some transition states, in photochemical reaction pathways. However, I think there is no hard limit for elucidating the elementary steps in chemical reactivity, and therefore I foresee a need for even shorter pulse durations in the future.
Grigory Smolentsev asked: You have used a few time-resolved X-ray techniques. Which one will be the most efficient to monitor the formation of rst MLCT state in Fe(bpy)3?
Christian Bressler replied: I still think that XANES would be well suited, although the rst hard fact was delivered by K-beta XES.
Henry Chapman addressed Christian Bressler and Gwyn Williams: Gwyn Williams stated that there is a limit to the number of photons per unit time (i.e. peak X-ray power) you can generate from an electron bunch (e.g. in an FEL) which depends on the peak current that can be achieved, ultimately limited by space charge in the electron bunch. This is analogous to limitations in generating high pulse intensities in lasers, which is limited by damage to the gain medium by the high-power pulse. The problem is circumvented in that case by chirped pulse amplication, and the same should be true for FEL pulses. That is, use a long chirped electron bunch of limited peak current and then compress the generated chirped X-ray pulse with gratings. Such a scheme was originally proposed by Claudio Pellegrini, and efforts are underway in Hamburg to develop and test this method by Adrian Cavalieri and Sasa Bajt and their colleagues. For some experiments it might not actually be necessary to compress the X-ray pulse if your sample disperses the pulse and time-resolved information can be obtained, as proposed by Moffat in a previous Faraday Discussion (vol. 122, 65–77, 2003).
Gwyn Williams responded: In addition to this, it has been proposed that one can develop even shorter bunches by rotating the chirped bunch and passing it through a physical aperture. This is an area that would be of considerable interest in machine development. The calculations referred to above are here: 239. S. L. Hulbert and G. P. Williams, “Calculations of Synchrotron Radiation Emission in the Transverse Coherent Limit”, Rev. Sci. Instrum., 2009, 80, 106103.
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Christian Bressler responded: TR X-ray experiments on chemical and biologically relevant samples do not benet from a further increased peak power, au contraire. The goal in chemical reaction dynamics is to resolve the ensuing reaction, and not to perturb it by the probe beam.
Jochen K¨ upper communicated: Christian, the “Gedanken” experiment you suggested regarding the measurement of the change from molecule to atoms was already performed (B. Erk, R. Boll, S. Trippel, D. Anielski, L. Foucar, B. Rudek et al., ”Imaging charge transfer in iodomethane upon X-ray photoabsorption”, Science, 2014, 345(6194), 288–291, DOI: 10.1126/science.1253607).
Christian Bressler communicated in reply: Thanks. This refers to one observable, the charge states of iodine up to 14+ or so. I could imagine that effects concerning the valence orbitals have been completely missed, but this is a cool experiment in the right direction!
Wendy Flavell opened the discussion of the paper by Stefan Neppl: This is a rather philosophical question, but how do you interpret the surface photovoltage shi in the case of your nanoparticle samples? How do you understand the large difference between clean ZnO and ZnO + N3 dye?
Stefan Neppl answered: This is an interesting question. According to theory, we would expect the band bending of an extended semiconductor crystal to be larger as compared to a nanoparticle of the same material. In our case, the substrate is a sintered lm of ZnO nanoparticles and might therefore be considered as an intermediate between these two limiting cases. A quantitative description of the bend bending in such a nanoporous semiconductor network has—to the best of our knowledge—not been derived and clearly motivates comparative surface photovoltage studies on these systems. The large difference in the SPV response of the bare ZnO substrate and the N3sensitized ZnO sample can be explained by the fact that the photon energy of the 532 nm laser excitation is much lower than the ZnO band gap. Therefore, no electron-hole pairs are created and no SPV is observed for the bare ZnO substrate at this wavelength. For N3-ZnO, the 532 nm photons excite electrons in N3 from the HOMO to the LUMO, from which they are injected into the ZnO conduction band. These electrons partly compensate the positive space charges in the ZnO depletion layer which in turn gives rise to a transient SPV effect. The magnitude of this SPV shi (> 400 meV) is indeed surprisingly high. It might be related to an enhancement of the initial ZnO bend bending upon chemisorption of N3 molecules
Christian Bressler asked: How did you assess the excited state population? Is this assessment of general importance for any kind of TR experiment? If so, how important is this (prior or in situ) knowledge? This journal is © The Royal Society of Chemistry 2014
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Stefan Neppl responded: In our analysis, we t the Ru3d/C1s difference spectrum to a model that describes the entire difference spectrum as being due to a laser-induced shi of the Ru3d doublet. This model has only two t parameters, the intensity of the shied Ru3d component (¼ excited state population) and the shi in binding energy itself. Generally, these two parameters are correlated and may lead to ambiguous t results when the energy shi is much smaller than the linewidth of the spectral features. This, however, is not the case for the 2–3 eV shis observed in our TRXPS experiment and, therefore, both the population and the shi can be determined from a single t.
Majed Chergui asked: You show that the XPS transient can be reproduced by shiing the entire spectrum by 2 eV and subtract it from the unshied spectrum, but I wouldn't expect the Ru and C lines to be shied by the same amount?
Stefan Neppl replied: We observe two different shis in our time-resolved XPS experiment. The rst one is a rigid shi of the entire C1s/Ru3d spectrum (see Fig. 7C of the manuscript) induced by the surface photovoltage (SPV) effect in the semiconductor substrate. If we correct for this SPV shi, and compare the spectrum to the laser-off reference as shown in Fig. 8, we also detect a characteristic laser-induced change in the C1s/Ru3d spectral shape, which can partly be explained as the result of a ca. 2–3 eV transient shi of the Ru3d doublet in the excited molecules to higher binding energies. This trend might be expected, since the oxidation state of the Ru-metal center increases upon charge injection. When modeling the difference spectra (green line in Fig. 8B), we implicitly assumed that a similar transient shi of the C1s levels—due to the dynamic charge reconguration within the molecule—is much less pronounced. This is of course an approximation, which has to be tested in future experiments with a better signalto-noise ratio. Qualitatively, however, one might expect the effect on most of the carbon core levels to be smaller as compared to the Ru lines, since the laser excitation is associated with a HOMO–LUMO transition and subsequent charge transfer from the dye into the semiconductor substrate, leading to an electronic conguration that is well approximated by a hole in the HOMO of the dye and an additional electron in the substrate (ref. 41: Siefermann et al., J. Phys. Chem. Lett., 2014, 5(15), 2753–2759). The dye HOMO is highly localized on the Ru center and the thiocyanate groups. Thus, the net change in local electron density seen by an individual carbon atom in the pyridine groups is much smaller than for the ruthenium center and, therefore, the by far most intense C1s signal from the pyridine rings is expected to be much less affected by the photo-induced dynamics than the Ru3d lines. Detailed studies of core-level shis associated with other atoms in the system, in particular the sulfur atoms in the thiocyanate ligands and the nitrogen atoms bound to the Ru center, will be performed in future experiments.
Martin Wolf addressed Wendy Flavell and Stefan Neppl: The electronic structure of ZnO surfaces depends very much on surface preparation. On several single crystal surfaces hydrogen acts as a donor and adsorption may lead to a Faraday Discuss.
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metallic surface (charge accumulation layer). See, for example, Phys Rev. B, 2011, 83, 125406. This leads to a pinning of the conduction band and prevents a (signicant) surface voltage shi.
Wendy Flavell responded: Indeed that is the case. But what is observed at the surface of ZnO is, as you say, very dependent on surface preparation. In the work described in Phys. Rev. B, 2013, 88, 195301 (and paper 13) the combination of vacuum and oxygen annealing cycles always yields an SPV shi to high binding energy, consistent with a surface depletion layer when excited with band gap (or even slightly sub-band-gap radiation.) The size of the shi and its dynamics change with oxygen content/donor concentration in a consistent way that implicates oxygen vacancies in the persistent photoconductivity (as suggested by Lany and Zunger, Phys. Rev. B, 2005, 72, 035215). The involvement of hydrogen at these sites cannot be excluded.
Stefan Neppl replied: We would like to add that the ZnO nanoparticle lms used in our experiments are prepared ex situ, i.e. not under UHV conditions. This kind of preparation will likely result in the coexistence of different adsorbate- and/ or defect-induced electronic states, which all contribute to the bend bending. Future SPV experiments using both sub- and super-band gap illumination will provide more insight into the nature of the bend bending in this system.
Julia Weinstein asked: Dear Stefan, several questions: (i) Is it possible to distinguish between the two variants of the attachment of the dye to the surface of a semiconductor: two carboxylic groups from one bpyligand, or one carboxylic group per bpy ligand, e.g., from the electron density on the C atoms? (ii) Does the experiment capture the redistribution (delocalisation) of electron density between several bpy-type ligands on the metal centre following MLCT excitation? (iii) How do the timescales (and perhaps the yields?) of injection estimated by the method discussed compare with the results obtained by the optical methods?
Stefan Neppl replied: These are very interesting questions concerning the potential and limits of time-resolved XPS technique when applied to study dynamics in dye-semiconductor systems. (i) Unfortunately, this information is challenging to derive from XPS spectra. The C1s photolines of all carboxylic acid groups form essentially one single XPS peak (see, e.g., Mayor et al., J. Chem. Phys., 2008, 129, 114701 and J. Chem. Phys., 2009, 130, 164704). The O1s line is sensitive to protonation/deprotonation of the carboxylic acid groups but this does not provide clear-cut information on the adsorption geometry either since both deprotonated as well as protonated carboxylic acid groups may participate in the attachment of the dye to the substrate (see, e.g., Siefermann et al. J. Phys. Chem. Lett., 2014, 5, 2753).
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(ii) In a recent femtosecond time-resolved XPS study, we have determined that the electron leaves the dye within less than 500 fs aer the MLCT excitation (Siefermann et al., J. Phys. Chem. Lett., 2014, 5, 2753). Therefore, we expect that only experiments with a time resolution in the range of a few to, perhaps, a few tens of femtoseconds would be able to trace the electron delocalization step before electron injection into the substrate. Principally, carbon atoms in the individual bpy units which are located closer to the nitrogen or carboxylic acid groups, give rise to C1s emission lines which differ in binding energy by ca. 1 eV (see Mayor et al., J. Chem. Phys., 2009, 130, 164704). Given the above mentioned time resolution can be achieved, time-resolved XPS might therefore be used to trace intramolecular charge reconguration in N3 affecting these two parts in all the bpy-ligands differently, but cannot address the bpy-ligands individually. (iii) We have not (yet) been able to derive a timescale for electron injection from the ALS-based picosecond time-resolved XPS experiments. In our recent femtosecond time-resolved XPS study at the LCLS (Siefermann et al., J. Chem. Phys. Lett., 2014, 5, 2735 we found evidence that, while the electron leaves the dye within less than 500 fs, it remains in an interfacial charge-transfer state for more than 1 ps. This result is in agreement with studies in the (near) optical domain that measured time constants of tens of ps for the appearance of free carriers inside the semiconductor, e.g., via time-resolved THz spectroscopy (Nemec et al., Phys. Rev. Lett., 2010, 104, 197401, Tiwana et al., ACS Nano, 2011, 5, 5158). A more detailed and general comparison (which indeed would be very interesting) between time-resolved XPS (TRXPS) and ultrafast optical spectroscopy studies on the electron dynamics and injection yields at dye-semiconductor interfaces will be possible as soon as more TRXPS results become available.
Majed Chergui asked: You show the nsec recovery kinetics of the dye, which has at least 2 components. What are the origins of these two timescales? Injection is known to occur on several timescales, do you see a multicomponent growth of the cation signal in your measurements ?
Stefan Neppl answered: Multi-exponential recombination dynamics have been observed before for various dye-semiconductor systems, e.g., in all-optical transient absorption studies. For ruthenium-based dyes attached to ZnO and TiO2 substrates, Bauer et al. (J. Phys. Chem. B, 2001, 105, 5585) found bi-exponential electron back transfer kinetics with time constants in the range of hundreds of nanoseconds to a few microseconds. The kinetics were the same for both substrates, which implies that the general electronic structure of the substrate is not the main factor governing the recombination dynamics. In general, the different timescales observed in the electron injection and electron back transfer process are at least partly ascribed to sample inhomogeneities, e.g. the co-existence of different binding congurations with different electronic coupling to the semiconductor substrate or electronic trap states induced by defects in the semiconductor crystal structure (see, e.g., Tachibana et al., J. Phys. Chem. B, 2000, 104, 1198). Another possibility is that the dye-electron recombination dynamics may be affected by transient surface/interfacial potentials in a similar fashion as electron-hole recombination dynamics in band-gap excited semiconductors (see Faraday Discuss.
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the contribution of Wendy Flavell’s group to this Faraday Discussion). The feedback between interfacial electronic dynamics and the potentials governing them generally leads to transient signals that cannot be described by single exponentials (see Br¨ ocker et al., Chem. Phys., 2004, 299, 247). We will study these effects in greater detail in the near future. In our measurements, the transient shi of the Ru3d photoemission lines is the most direct probe for the cationic state of the dye molecule during charge injection. So far, we have measured this dynamical chemical shi only for two time delays (ref. 41 and Fig. 8 in the manuscript). The full time dependence of this shi will be studied in future experiments, which will also require access to fewfemtosecond X-ray light sources (e.g. FELs) to resolve the possibly fast (subpicosecond) dynamics of the involved electron injection pathways.
Wendy Flavell remarked: In the case of the decay transient for the ZnO SPV, why should the dynamics necessarily be bi-exponential with a fast and a slow part? The ‘self-decelerating’ model of Widdra and Broecker (e.g. W. Widdra et al., Surf. Sci., 2003, 543, 87) allows for the increase in lifetime as the SPV decays, i.e. a variation in the lifetime during the decay could be an intrinsic part of the physics, allowing one function to be tted to the decay transient.
Stefan Neppl replied: Indeed, the SPV decay in Fig. 9 can be tted equally well using the self-decelerating model with a (dark) carrier lifetime of 390 ns and an a-parameter of 3. An a-parameter of 3, which is an additional t parameter in this model, has so far not been observed in SPV experiments, and is signicantly larger than a parameters reported, e.g., for single crystalline ZnO (a < 1). Generally, the a parameter is related to the ideality factor used in the theory of Schottky diode performance, but its interpretation in the context of SPV measurements, and how this parameter can be related to different recombination mechanisms, is still an open question.
Majed Chergui enquired: You excited the dye and you probe it by XPS, but evidence that you have injected the electron into the substrate, is to probe the lines of Zn (or O). We did such an experiment on RuN719 adsorbed at the surface of TiO2 and observed the oxidation of Ru by Ru L edge spectroscopy and the reduction of Ti centres by Ti K edge absorption (M. Hannelore Rittmann-Frank, Chris J. Milne, Jochen Rittmann, Marco Reinhard, Thomas J. Penfold and Majed Chergui, Angew. Chem. Int. Ed., 2014, 53, 5858–5862). So in your case, what is the evidence that the electron is in the substrate.
Stefan Neppl answered: We think that the positive transient surface photovoltage (SPV) response (see Fig. 9 of the manuscript) is the spectroscopic signature for the electrons being transferred to the semiconductor: photo-excited electrons from the dye can effectively screen the positive excess charge inside the n-type ZnO substrate only when they diffuse into the tens of nanometer wide depletion layer. This additional screening mediated by the injected electrons This journal is © The Royal Society of Chemistry 2014
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causes reduction of the bending near the surface of the ZnO substrate, which is observed as a sudden increase in binding energy of all energy levels that are referenced to the semiconductor Fermi level. Since the dye molecules are chemisorbed on the ZnO substrate, their core-level photoemission can be expected to be as sensitive to the SPV effect as photoemission lines originating from the semiconductor material itself.
Kai Rossnagel said: In your talk, you also presented time-resolved XPS data that were acquired at the LCLS. Since free-electron lasers have signicantly lower repetition rates as compared to storage rings, the data collection efficiency of the presented photoelectron detection scheme is signicantly reduced. It is limited by the fact that the delay-line detector can only detect one electron per photon probe pulse. Will you continue to perform time-resolved XPS measurements at the LCLS and, if so, do you have plans to make these measurements more efficient, for example, by combining a time-of-ight spectrometer with a multi-segment delayline detector?
Stefan Neppl replied: The detector in the LCLS experiment was not a delay-line anode, but a phosphor screen combined with a fast CCD camera. Generally, the strongest constraints for time-resolved XPS experiments at FELs are imposed by space charge effects, which have been observed for X-ray pulse uences of less than 10 107 photons/pulse, i.e., several orders of magnitude lower than the maximum pulse uences available (see, e.g., Pietzsch et al., New J. Phys., 2008, 10, 033004). For the N3/ZnO experiment at the LCLS (ref. 41: Siefermann et al., J. Phys. Chem. Lett., in press), the photon ux had to be reduced to ca. 10 6 photons/ pulse to avoid space-charge induced peak distortions and shis. Under these conditions, less than one photoelectron was detected per X-ray pulse. Thus, detector saturation was not an issue and multi-segmented anodes would not be required for this particular experiment to improve data collection efficiency. However, they may still be advantageous, for example, in combination with timeof-ight spectrometers to increase the angular acceptance and sensitivity of an XPS setup. Cleary, time-resolved X-ray photoelectron spectroscopy will greatly benet from high-repetition rate XFELs, where femtosecond time-resolved X-ray measurements that require moderate pulse energies can still be efficiently performed.
Wendy Flavell addressed Stefan Neppl, Christian Bressler, Klaus von Haeen and Nora Berrah: As time resolution improves, I'd like to make the comment that we must also build in the ability to measure over quite long timescales, with this good resolution. For condensed matter systems of the type we are discussing, dynamics can be happening over very long timescales (in the case of paper 13, we needed to measure at delays up to ms to capture the persistent photoconductivity of ZnO). The ArToF analyser coupled with hybrid mode operation at synchrotron, as implemented for example at SPring8 BL07LSU seems a productive direction for
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the future. Synchronising to successive ‘rotations’ of the large bunch allows the dynamics to be measured over many orders of magnitude of time delay.
Klaus von Haeen replied: I agree. In our paper on the ‘Formation of coherent rotational wavepackets in molecule-helium clusters’ we record the propagation of a rotational wavepacket over 600 ps. In our example we have chosen laser conditions that allow for the excitation of rotational level close and beyond the dissociation limit of the He–C2H2 complex. It would be very interesting to observe the propagation of the rotational wavepacket for longer, for example up to 10 ns because dissociation times of related systems (CO2) have been reported to be 6 ns, M. J. Weida, J. M. Sperhac, D. J. Nesbitt and J. M. Hutson, J. Chem. Phys., 1994, 101, 8351. I would be particularly interested whether predissociation depends on the actual composition of all rotational states. A chemical reaction could then be triggered by shaping of the alignment laser eld.
Stefan Neppl replied: We strongly support this statement. The combination of active pump–probe synchronization and simultaneous time-tagging as implemented in our experiment at the Advanced Light Sources enables both efficient synchrotron-bunch length limited time-resolved experiments, and the probing of dynamics proceeding on micro- and even millisecond timescales. In addition, this setup benets from the adjustable repetition rate (single shot to MHz) of the used laser system, which allows to match the pump-pulse spacing to the timescale of the process to be investigated.
Christian Bressler answered: I would also agree, albeit the time resolution is no longer superimportant, when we enter the 100 ps range and much further above.
Nora Berrah responded: I agree and in fact the LCLS has extended its long timescales to 500 fs.
Jochen K¨ upper opened the discussion of the paper by Klaus von Haeen: You said that you had to use laser pulses for alignment and Coulomb explosion imaging that were parallel—why was that? Then, it seemed as if you said that you would not see alignment along the alignment laser axis if using a perpendicularly polarized ionization laser—did I understand that correctly? If so, is it valid to speak of “alignment”, in the sense of strong angular connement of the molecular rotational distribution, or would it be more appropriate to refer to the experiment as “rotational coherence spectroscopy” (RCS) as introduced by Felker (P. Felker, Rotational Coherence Spectroscopy—Studies of the Geometries of Large Gas-Phase Species by Picosecond Time-Domain Methods, J. Phys. Chem., 1992, 96(20), 7844–7857). My next question refers to Fig. 3 in your paper, or, actually, to the alignment trace you showed during the conference, which seems to signicantly deviate from the gure in the manuscript: This journal is © The Royal Society of Chemistry 2014
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3
Why do your ¼ (and /4 ) revivals show only alignment (or antialignment) but not the correspondingly pre- or post-anitalignment (alignment)? See, for instance, Nielsen, Phys. Chem. Chem. Phys., 13, 18971 where these are clearly visible. Samuel Leutwyler has recently performed a nice detailed analysis of this in his RCS experiments [(a) G. Br¨ ugger, H.-M. Frey, P. Steinegger, F. Balmer and S. Leutwyler, Accurate Determination of the Structure of Cyclohexane by Femtosecond Rotational Coherence Spectroscopy and Ab Initio Calculations, J. Phys. Chem. A, 2011, 115(34), 9567–9578, doi: 10.1021/jp2001546; (b) H. M. Frey, D. Kummli, S. Lobsiger and S. Leutwyler, High-resolution Rotational Raman Coherence Spectroscopy with Femtosecond Pulses, in Handbook of High-resolution Spectroscopy, Chichester, UK, John Wiley & Sons, Ltd., 2011, doi: 10.1002/ 9780470749593.hrs055]. He showed that accurate pump-probe delays are a very crucial effect and that is quite an effort to do this reasonably well. Therefore, it would be useful to learn what your statistical and systematic errors of the delay time were. 3. Since you suggest that you have no observable alignment with the laser polarizations perpendicular, is it correct to speak of “aligned molecules”, or should we really consider your experiment a rotational coherence spectroscopy (RCS) experiment as introduced by Kim and Felker in the 80s. 2. How did you calibrate your temporal axis, what statistical and systematic errors do you get on the time-axis of the RCS trace, or, what precision?
What is the error propagation into the rotational constants? What are the precision and accuracy of the temporal axis in your RCS experiment, i.e., what are the limits on its systematic errors? How do these uncertainties—statistical (precision) and systematic (accuracy) estimates—propagate into the rotational constants determined? In how far can you use these numbers to determine “structures”, as was done in other experiments (Ratzer, Chem. Phys., 283, 153 and references therein)?
Klaus von Haeen replied: 1. Indeed you must have misunderstood me. In all experiments in our paper the polarisation was parallel for consistency within the experimental run. Experiments with perpendicular polarisation were planned but the granted beamtime at the Rutherford Appleton Lab did not allow to begin such experiments. The orientation of polarisation had no consequences for our results. 2. I think it is fair to call our experiments RCS with the distinction to older work by Felker et al. that we use impulsive alignment to generate rotational wavepackets. This has specic, and very important advantages as outlined in the introduction of our paper 3. The temporal evolution of alignment depends on laser parameters and the target system. Our results shown are the rst of a target comprising of clusters of acetylene and a variable number of helium atoms. Hence, it makes no sense to compare these with other work on other target systems, with different laser parameters, etc. 4. Calibration can be achieved by investigating a known system, for example carbon monoxide. The accuracy that can be achieved is nicely illustrated in A. Przystawik, A. Kickermann, A. Al-Shemmary, S. Dusterer, A. M. Ellis, K. von Haeen, M. Harmand, S. Ramakrishna, H. Redlin, L. Schroedter, M. Schulz, T. Seideman, N. Stojanovic, J. Szekely, F. Tavella, S. Toleikis and T. Laarmann, Phys. Rev. A, 2012, 85, 052503. 5. Rotational constants only support structural models. I would therefore consider rotational spectroscopy not as a method to ‘determine’ structures. It is not uncommon, particularly for quantum mechanical systems that several observables add complementary information to the whole picture.
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Jochen K¨ upper communicated: 1. Typically the Coulomb explosion laser is polarized perpendicular to the alignment laser to avoid overestimating the degree of alignment—why did you not do that? What error, do you estimate, was introduced by that Sam Leutwyler has recently performed a nice detailed analysis of this in his RCS experiments and showed that this is a very crucial effect? 3. Since you suggest that you have no observable alignment with the laser polarizations perpendicular, is it correct to speak of “alignmed molecules”, or should we really consider your experiment a rotational coherence spectroscopy (RCS) experiment as introduced by Kim and Felker in the 80s. 2. How did you calibrate your temporal axis, what statistical and systematic errors do you get on the time-axis of the RCS trace, or, what precision?
What is the error propagation into the rotational constants?
Klaus von Haeen communicated in reply: 1. It is correct that in many conceptually similar experiments perpendicular polarisation between pump and probe laser is chosen. A requirement to observe impulsive alignment is that polarisation of the pump laser is parallel to the detector plane (for Coulomb explosion detection). Choosing a probe laser polarisation perpendicular to the detector plane will remove any possible anisotropy in the images that may be caused by the excitation process, which is good to remove background signal for the determination of pump-laser-induced alignment. An advantage of a parallel probe-laser polarisation is that anisotropies in the excitation can be used to detect alignment induced in the pump process. For example, we observed a modulation of the ion yield. 2. I think these questions overlap with a previous one.
Katharine Reid remarked: The authors of paper 3 were able to achieve quite substantial alignment of a polyatomic molecule ( ¼ 0.89), but the alignments shown in your paper are substantially lower. Is there a prospect of attaining much higher alignment of molecules embedded in helium clusters? Following this, is there a prospect of probing photoinduced dynamics of aligned molecules embedded in helium clusters?
Klaus von Haeen replied: The degree of alignment observed in our work was comparatively low, but the target was unusual. For a known molecule, e.g. with well-dened polarisability, in a laser eld of known intensity, pulse length, etc., the degree of alignment is dened and can be calculated. The degree of alignment is then reected by the respective values of and the characteristic time-dependence. For example, a sine-wave-type time dependence indicates involvement of only two rotational levels. Correspondingly, the level of alignment is low. In our case, the target was different. It consisted of molecular clusters comprising a single HCCH molecule and a variable number of helium atoms. Consequently, 1. (t) shows the superposition of the alignment of all the individual clusters 2. Alignment information is washed out during the Coulomb explosion process (see another question by H-J W¨ orner) Our work This journal is © The Royal Society of Chemistry 2014
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clearly shows that it is possible to generate coherent rotational wavepackets in very small molecule-helium clusters. Our results do not justify commenting on the prospects of achieving high alignment of molecules in helium clusters or droplets, which would certainly be of great advantage for investigating photoinduced dynamics of embedded molecules and molecular complexes.
Jonathan Underwood asked: In the paper presented by von Haeen, the observed Coulomb explosion images are a convolution of the molecular axis alignment produced by the pump pulse and the strong orientational dependence of the Coulomb explosion probe process. Because the probability for Coulomb explosion has a strong orientational dependence, it is non-trivial to quantitatively extract the molecular axis alignment, and it has become the norm in this eld to characterize the molecular axis alignment in terms of expectation values calculated for the resulting image. Recently, in collaboration with the Stapelfeldt group in Aarhus we have developed and demonstrated a technique for deconvoluting the orientational dependence of the Coulomb explosion process allowing for complete characterization of the molecular axis alignment through inversion of the images. This methodology requires an image to be recorded for the Coulomb explosion of isotropically oriented molecules ahead of doing the experiment with aligned molecules. This work will be published in the near future.
Hans Jakob W¨ orner commented: I have two questions. First, could you provide an estimate of the maximal cluster size or of the cluster-size distribution in your experiment? Second, have you obtained evidence for avalanche-like ionization of the helium clusters (Phys. Rev. Lett., 102, 128102) in other observables than the C+ kinetic energies (Fig. 2b)?
Klaus von Haeen replied: We are unable to provide gures for the maximal cluster size or the cluster size distribution. The clusters are produced using a pulsed expansion of a gas mixture in an Even-Lavie valve. While the cluster sizes in continuous expansion can be fairly reliably predicted for a large range of gases using the formalism developed by Hagena, this cannot be said for gas mixtures. Secondly, the shape of conical nozzles, in particular the opening angle controls the cluster size. Scaling laws have not been published for the Even-Lavie nozzle, although we can conrm by our own unpublished work on argon cluster formation in a continuous expansion through a conical E-L-nozzle that Hagena's formula can be applied and that an effectve half-opening angle of 20 can be conrmed. We have no rm evidence for avalanche-like ionization of the helium clusters, although we see indications that such an effect takes place. For expansion conditions favouring formation of larger clusters we observe higher kinetic energies of the Coulomb explosion carbon ion fragments. In our paper we suggest that an effect similar to that suggested by Mikaberidze, Saalmann and Rost, Phys. Rev. Lett., 2009, 102, 128102, may take place. We also explain in the paper that recombination between electrons and higher charged carbon ion fragment may
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contribute to the increased kinetic energies of C+, an idea originally proposed by J. M. Rost in a personal communication. The rather complicated fragmentation process following the Coulomb explosion has the consequence that angular information originally contained in the explosion process becomes washed out, leading to apparent lower levels of alignment.
Jochen K¨ upper remarked: You mentioned that you were using mixtures of gases. Which gases were you using, at what concentrations/partial pressures, and why was that necessary? How did you make sure that you were not observing signals from mixed clusters, or simply from clusters with other atoms?
Klaus von Haeen responded: In the presented paper HCCH–Hen clusters were produced by co-expansion of HCCH–He mixtures at stagnation pressures between 50 and 100 bar. The HCCH concentration was varied. Signatures of HCCH–He cluster formation were observed when the HCCH concentration was below 0.1%. Good results were achieved for even lower concentrations, for example, 0.03%. The concentrations used to produce the spectra shown in our paper are indicated. The signals observed were from pure HCCH, and indeed from mixed HCCH– He clusters. Signals from pure He clusters were not expected and not observed. The background pressure in the apparatus used was 5 109 mbar. Cluster formation with atoms other than helium can be safely excluded.
Michael Woerner opened the discussion of the paper by Kai Rossnagel: What is the energetic resolution of your experiments? What determines the energetic resolution of your experiments? Can you get energetic and spatial infomation simultaneously in the future ?
Kai Rossnagel answered: The effective energy resolution of our time-resolved ARPES experiments was determined from Fermi-edge spectra of a polycrystalline gold sample to be 260 meV. The two dominant contributions to the resolution are generally the spectral width of the XUV probe pulses and the resolution of the photoelectron spectrometer. For the presented experiments, an upper limit on the spectral width of the XUV pulses could be independently determined to be 170 meV, and the nominal spectrometer resolution was 200 meV. Note that we do not use a monochromator in our experimental setup, which would reduce the XUV pulse duration and photon ux. Monochromation of the XUV probe pulses is not necessary because by pumping with the second harmonic of a Ti:Sapphire laser at a fundamental wavelength of 790 nm we can generate an isolated, well-separated, and intense high harmonic pulse at about 22 eV. For experimental details, see DOI: 10.1016/j.elspec.2014.04.013. To get spectral and spatial information at the same time would be possible with a combined femtosecond time-resolved ARPES and X-ray photoelectron diffraction (XPD) experiment. Time-resolved ARPES would provide direct This journal is © The Royal Society of Chemistry 2014
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information on the dynamics of the band structure and Fermi surface, while timeresolved XPD would provide complementary information on the changes of structural parameters. Our plan is to realize such an experiment at the European XFEL.
Majed Chergui remarked: Since you mention determining lattice structure by photoelectron diffraction combined with ARPES, how would you distinguish the electronic structure from the lattice structure in this case?
Kai Rossnagel replied: ARPES and XPD, when applied in the mode with xed photon energy, measure the same thing: the angular distribution of photoelectrons at specic kinetic energies. The differences lie in the energies of the electronic states from which the photoemission occurs and in the information the photoemission intensity distribution contains. In the case of ARPES, valence electrons are knocked out and the recorded intensity maps directly reect the dispersion of the electronic states as a function of crystal momentum. In the case of XPD, on the other hand, core electrons are photoemitted and the measured intensity distributions are interpreted as interference patterns from which structural parameters can be extracted more or less directly. Electronic structure determination by ARPES works best at XUV photon energies, whereas the interpretation of XPD patterns is the simplest and most intuitive when so X-rays excite high-energy photoelectrons that are scattered by the atoms in the sample predominantly in the forward direction. So when I dream of a combined femtosecond time-resolved ARPES and XPD experiment, I think about two separate sets of measurements to be performed on the same sample, one XUV-ARPES measurement and one so-X-ray-XPD measurement. Presently, there is no photon source available where such experiments can be done. However, there is well-justied hope that, with up to 27000 pulses/s, the European XFEL, specically its so X-ray SASE3 beamline, will be such a source.
Martin Wolf enquired: You have pointed out that there are two timescales in the dynamics “melting” of the Mott and CDW gap in TaS2 und TaSe2. It is clear that the slow (200 fs) dynamics originates for ion motion by the amplitude mode, whereas you assign the ultrafast (
