Relaxation channels of multi-photon excited xenon clusters P. Yu. Serdobintsev, L. P. Rakcheeva, S. V. Murashov, A. S. Melnikov, S. Lyubchik, N. A. Timofeev, A. A. Pastor, and M. A. Khodorkovskii
Citation: The Journal of Chemical Physics 143, 114302 (2015); doi: 10.1063/1.4930963 View online: http://dx.doi.org/10.1063/1.4930963 View Table of Contents: http://aip.scitation.org/toc/jcp/143/11 Published by the American Institute of Physics
THE JOURNAL OF CHEMICAL PHYSICS 143, 114302 (2015)
Relaxation channels of multi-photon excited xenon clusters P. Yu. Serdobintsev,1,2 L. P. Rakcheeva,1,a) S. V. Murashov,1 A. S. Melnikov,1,2 S. Lyubchik,3 N. A. Timofeev,2 A. A. Pastor,2 and M. A. Khodorkovskii1 1
Institute of Nanobiotechnologies, Peter the Great St.Petersburg Polytechnic University, Saint Petersburg 195251, Russia 2 Department of Physics, St. Petersburg State University, Saint Petersburg 198904, Russia 3 REQUIMTE, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica 2829-516, Portugal
(Received 7 April 2015; accepted 1 September 2015; published online 15 September 2015) The relaxation processes of the xenon clusters subjected to multi-photon excitation by laser radiation with quantum energies significantly lower than the thresholds of excitation of atoms and ionization of clusters were studied. Results obtained by means of the photoelectron spectroscopy method showed that desorption processes of excited atoms play a significant role in the decay of two-photon excited xenon clusters. A number of excited states of xenon atoms formed during this process were discovered and identified. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4930963]
I. INTRODUCTION
Studies of intermolecular energy conversion in polyatomic systems are of interest from both theoretical and experimental points of view. Atomic clusters are very useful for these purposes. In particular, van der Waals clusters of noble gases, formed by supersonic expansion through a nozzle, are an ideal target to observe these processes due to the lack of interaction with the environment. This fact has been widely used in a number of works1–16 to study energy relaxation channels for noble gases’ clusters excited in the conditions of crossed beams. In a number of works,1–6 it was shown that the result of relaxation of excited clusters can be with desorption of excited atoms and molecules. The linear and band spectra in the VUV range (100–200 nm) obtained by registration of radiation from argon, krypton, and xenon clusters excited by the electron impact were assigned1,2 to excited atoms and molecules desorbed from the corresponding clusters. The maximum yield of the excited atoms and molecules was registered at the average size of the clusters being about ⟨N⟩ = 50–100 atoms/cluster. It was shown that there are two competing relaxation channels depending on the cluster size. In addition to the usual relaxation channel of the exciton in solid state, for the clusters with the sizes below 160–190 atoms per cluster there is a new channel, named “nonradiative excimer dissociation” by the authors.1 Dependence of the relaxation processes on the cluster size was studied by the luminescent spectroscopy.3–6 Clusters of noble gases containing from a few to 105 atoms were excited by the tunable monochromatic synchrotron radiation. It was shown5,6 that the relaxation processes of the exited clusters lead to the desorption of the electronically excited dimers and, in some cases, excited atoms, as it was observed for the xenon clusters with the relatively small average size ⟨N⟩ = 10 atoms per cluster. a)[email protected]
0021-9606/2015/143(11)/114302/5/$30.00
It was shown5 that the important relaxation channel for excited clusters of helium is desorption of excited atoms and dimer molecules. While desorption of the excited helium atoms was observed for the clusters with the sizes of up to the several hundred atoms, the corresponding process in the argon clusters was observed only for the small clusters (N < 10). It is interesting to note that it comes into contradiction with the experimental results for the argon clusters.1,2 Photoelectron spectroscopy was used to study dependence of the ionization potential on the size of clusters at excitation by VUV radiation of helium discharge lamp or by synchrotron radiation.7–13 The analysis of the photoelectron spectra showed that the threshold of ionization of the small xenon clusters is about 10.5 eV and decreases with increase of the size of clusters. The threshold of ionization of the clusters is significantly lower than that of xenon dimers—11.176 eV14,15 and higher than the photoemission threshold of liquid and solid xenon (9.7–9.8 eV).16 It should be noted that the sources of excitation of clusters observed in the works mentioned above had the energies significantly higher than the ones needed for excitation of atoms and ionization of clusters. In this paper, the relaxation processes of the xenon clusters excited electronically by multi-photon absorption of laser radiation with the quantum energies significantly lower than the thresholds of excitation of atoms and ionization of clusters were studied using the method of photoelectron spectroscopy. This work continues a series of studies of the electron structures of dimers and clusters of the noble gases by the REMPI-TOF and REMPI-PES methods.17–24
II. EXPERIMENT
The photoelectron spectroscopy experimental setup was described in our previous paper.24 It consists of a pulsed supersonic 100 µm nozzle for production of xenon clusters at stagnation pressure of 4 bars of the Xe:Ar mixture ≈5%:95%,
143, 114302-1
© 2015 AIP Publishing LLC
114302-2
Serdobintsev et al.
a hand-made time-of-flight mass-spectrometer with limited mass range (about 1000 Da) for ion detection, a “magnetic bottle” type time-of-flight electron spectrometer for electron detection, and a tunable pulse laser light source (about 5 ns) for excitations of the xenon clusters. The wavelength was scanned in the range of 220.2–226.2 nm (45 413–44 208 cm−1, 5.48 eV–5.63 eV). The energy of two photons in this range is close to the energy of xenon atomic states 7p, 6p′, 6d. The intensity of the focused laser radiation was about 109 W/cm2. The experimental setup for measuring of jet composition consists of TOF mass spectrometer with the mass range about 200 000 Da (Bruker, TOF-1) equipped by the gas inlet system, pulse supersonic 100 µm nozzle, and electron gun with electron energy being about 70 eV. III. RESULTS AND DISCUSSION
The photoelectron spectra obtained from the area of intersection of laser radiation and supersonic molecular beam containing xenon clusters are presented in Figure 1. The energies of the exciting photons are plotted on the X-axis and the energy of the resulting photoelectrons is indicated on the Y-axis. This figure contains the data for the linear and band spectra formed by multi-photon ionization of free atoms and dimers, as well as for the continuous spectra originating from xenon clusters. In addition, some ordinary photoelectron spectra at selected photon energies are provided in Figures S1-S3 of the supplementary material.25 Electron signals due to direct three-photon ionization of xenon atoms (both resonant and non-resonant) form two bands marked “A” and “B”, corresponding to two states Xe+ 2P1/2 and Xe+ 2P3/2 of xenon ions. The distance between these two bands is equal to the difference between ionization potentials of these states −1.306 eV. These bands are parallel to each other and are tilted relative to the X-axis, since the energy of photoelectrons formed during the three-photon ionization of atomic xenon depends on the quantum energy of laser radiation: Ee = 3hν − I.P. Here, Ee is the photoelectron energy and I.P. is the ionization potential of atomic xenon. The signal intensity of these bands rapidly rises and reaches saturation when the three-photon ionization occurs through the intermediate
J. Chem. Phys. 143, 114302 (2015)
excited states allowed for the two-photon transition. The vertical lines observed at these wavelengths are caused by such signal saturation. In addition to the bands A and B (Figure 1), there are the weaker continuous bands tilted at the less angle to the X-axis in the electron energy range of 1.5–4.5 eV. These bands correspond to the photoelectrons formed during the single photon ionization of xenon atoms in various excited states: Ee = Ea + hν − I.P., where Ee—photoelectron energy, Ea—excitation potential of atomic xenon, and I.P.—ionization potential of atomic xenon. Because of continuous character of these bands, it can be concluded that these excited xenon atoms can only be produced by desorption from the two-photon excited xenon clusters, which are characterized by continuous absorption spectra. The possible second desorption channel of neutral Xe-atoms when most part of energy remains in excited Xe∗N−1 unfortunately cannot be observed experimentally since the concentration of the Xe atoms in stem is so high that contribution to the signal from relatively small amount of neutral atoms delivered by this way is negligible. The wide band marked “0” corresponds to the slow electrons with near-zero energy. The intensity of the band rises steadily with growing of the energy of excitation from 10.96 eV to 11.26 eV. The only plausible explanation of appearance of electrons with such energies is the two-photon ionization process of the clusters. As opposed to the interpretation presented in Ref. 26, such process should not be assigned to autoionization of two-photon excited xenon dimers, since it is not only observed at the corresponding resonant frequencies but also at the entire studied energy range. This process is specific for wide band absorption of clusters. The studied processes take place with the participation of the two or three photons, but the measured dependence of the signal intensity of photoelectrons on the laser intensity is quadratic (see Figures S4 and S5 in the supplementary material25). This dependence for electrons with zero energy (band “0”) is evident since they are presented by two-photon ionization of clusters resulting from the direct ionization process or autoionization mechanism. Ionizing three-photon transitions in xenon atoms and molecules have the same dependence because they occur through the real intermediate two-photon
FIG. 1. The photoelectron spectra of supersonic molecular beam containing xenon clusters 0—the two-photon ionization of xenon clusters and A and B—the bands formed by three-photon ionization of xenon atoms, corresponding to two states of xenon ions Xe+ 2P1/2 and Xe+ 2P3/2. The lines from 1 to 5 correspond to the three-photon ionization signals of xenon atoms from the ground state through the allowed two-photon transitions in both states of the Xe+ core. The notations of these transitions are 1—5p6 1S0-7p[5/2]2 88 351 cm−1 (10.955 eV); 2—5p6 1S0-7p[3/2]2 88 708 cm−1 (10.999 eV); 3—5p6 1S0-7p[1/2]0 88 842 cm−1 (11.016 eV); 4—5p6 1S0-6p[3/2]2 89 162 cm−1 (11.055 eV); 5—5p6 1S0-6p[1/2]0 89 860 cm−1 (11.142 eV). Vertical lines at the above mentioned energies of excitation are caused by saturation of signals by resonant enhancing of the three-photon ionization through the allowed two-photon resonance.
114302-3
Serdobintsev et al.
FIG. 2. The relaxation channels of two-photon excited XeN clusters. The right side of figure corresponds to the two-photon ionization of XeN clusters. The photoelectron spectrum labeled as “A” is taken from the Ref. 10 obtained by one-photon ionization of XeN clusters with average size ⟨N⟩ = 1000 using synchrotron radiation with the photon energy 61 eV. The left side of figure demonstrates ionization of excited xenon atoms desorbed from the two-photon excited XeN clusters. These excited atoms are then ionized by one-photon transition during the same laser pulse. Reproduced with permission from Feifel et al., Eur. Phys. J. D 30, 343 (2004). Copyright 2004 The European Physical Journal (EPJ).
excited state. The process of absorption of third photon is saturated since at the radiation intensity about 109 W/cm2 all excited atoms are ionized during the laser pulse. The onephoton ionization process of desorbed excited atoms is saturated too, but they appear from the two-photon excited clusters and intensity dependence is quadratic too. Thus, registration of fast and slow electrons reflects two distinct decay channels of excited clusters XeN∗. These processes are schematically shown in Figure 2. The first process is accompanied by desorption of the excited atoms Xe∗ in various states and their ionization by the next photon. As a result, the Xe+ 2P1/2 and Xe+ 2P3/2 ions and electrons with energies Ee = Ea + hν − I.P. are formed. The left part of the figure contains a diagram of the energy levels for atoms Xe* (Ea) and ions Xe+.
J. Chem. Phys. 143, 114302 (2015)
Based on this diagram, it is easy to explain the creation mechanism of electrons with the energy of 1.5–4.5 eV. The second process corresponds to two-photon ionization of clusters. The central part of the figure demonstrates two-photon excitation of the XeN∗ clusters. Figure 2(a) shows the photoelectron spectrum of outermost valence shells of xenon clusters XeN with average size ⟨N⟩ = 1000.10 Ionization energy of these clusters is characterized by the band shifted relative to the spectral line Xe+ 2P3/2 (I.P. = 12.13 eV) with the center of mass Ecgcl = 10.83 eV. In Ref. 7, the maximum of the corresponding band of clusters with the average size ⟨N⟩ < 100 is about 11.55 eV. In our experiment, maximum of the corresponding band is slightly higher than 11.26 eV. Comparing this value with the values of ionization energies of xenon atoms, molecules, and clusters of various sizes,7–9,11–13 it may be assumed that the average cluster size ⟨N⟩ observed during this experiment did not exceed 100 atoms. This estimation is in the size range of the result calculation of this value for the experimental conditions using the classical gas-dynamic approach.27–29 Data of direct measurements of average cluster size ⟨N⟩ collected by TOF mass-spectrometer setup described above are provided in Figure S6 of the supplementary material.25 As one can see, the average cluster size ⟨N⟩ at 4 bars stagnation pressure is about 70. Since the energy of two-photon excitation of clusters in our experiments did not exceed 11.26 eV, the direct ionization of clusters with ⟨N⟩ about 100 takes place on the low energy edge of the cluster ionization cross section. Appearance of low energy electrons (band “0”) and a monotonic growth of their intensity with increasing of the excitation energy could be explained in such manner. The photoelectron spectrum of xenon clusters excited by laser radiation at the fixed wavelength hν = 44 165 cm−1 (5.476 eV) is presented in Figure 3. The energy of two quanta (2hν = 88 330 cm−1 = 10.95 eV) does not coincide with any of the atomic or molecular transitions. The slow electrons with a nearly zero kinetic energy labeled by 0 are presented by a broad band in the range of 0–0.6 eV. The fast electrons
FIG. 3. The photoelectron spectrum of xenon clusters excited by laser radiation at the fixed wavelength hν = 44 165 cm−1 (exciting photon energy −5.476 eV). Red bars on the top represent the energy of excited xenon atom levels responsible for production Xe+ 2P1/2 ions (electron peaks 1, 2, and 5–15). The energy of ejected electron (Ee) is combined with the energy of the corresponding Xe∗ level (Eexc) according to the formula: Ee = Eexc + hv − Eion. The same formula is valid for the peaks 3, 4 but the ionization potential for these is the other ionic state Xe+ 2P3/2.
114302-4
Serdobintsev et al.
J. Chem. Phys. 143, 114302 (2015)
are presented by a number of bands labeled by 1–15. The labels correspond to those indicated in Figure 1. As it was mentioned above, the 0 band appears due to cluster ionization by two-photon process, and the other ones are formed as a result of ionization of the excited xenon atoms desorbed from the excited clusters. The short dashes on the upper Xaxis energetic scale correspond to the Xe* atomic states in the spectral range of 65 000–95 000 cm−1. The atomic energy levels and the photoelectron energy are related as follows: Ea(Xe∗) = Ee + I.P. (Xe+ 2P3/2) − hν,
(1)
where Ee—observed electron energy in cm−1, I.P. (Xe+ 2P3/2) = 97 833.8 cm−1 (12.13 eV)—ionization energy of Xe atom, and hν = 44 165 cm−1 (5.48 eV)—energy of laser radiation. Energies of the atomic states calculated by this formula are in good agreement with the values in Ref. 30. It can be seen in Figure 3 that the maxima of the bands 1–15 coincide with the position of the atomic levels. Table I demonstrates the kinetic energies and assignments of all observed bands 1–15. The bands 1, 2 correspond to formation of the Xe+ 2P3/2 ions resulting from absorption of third photon by the lowest excited states of xenon Xe* 6s[3/2]02,1. The 3, 4 and 13, 14 bands appear as a result of formation of the Xe+ 2P1/2 and Xe+ 2P3/2 ions correspondingly due to the same absorption process with participation of the Xe∗ 5d[3/2]01 and Xe∗ 7s[3/2]01,2 excited states. Changing of the atomic core state at ionization of excited xenon states was observed also in the works11,31,32 and is caused by configuration interaction. The bands 5–12 and 15 are assigned to the excited states responsible for formation of the Xe+ 2P3/2 ion only. It should be pointed out that the TABLE I. Kinetic energies and assignments of the recorded photoelectron peaks of xenon clusters. Peak label
E, eV
1 2 3 4
1.67 1.79 2.43 2.60
5 6
2.80 2.91
7
3.03
8
3.16
9
3.28
10 11 12 13 14
3.36 3.48 3.55 3.73 3.90
15
4.23
ν, cm−1 67 067.547 68 045.156 83 889.971 85 188.777 85 440.017 76 196.767 77 185.041 77 269.145 78 119.798 78 403.061 78 956.031 79 212.465 79 771.267 79 986.618 80 118.962 80 196.629 80 322.746 80 970.438 81 925.514 82 430.204 83 889.971 85 188.777 85 440.017 87 927.131
Assignment, Xe∗ → 2PJ Xe+ ∗
6s[3/2]0
2P
+
Xe 3/2 Xe 2→ Xe∗ 6s[3/2]01 → 2P3/2 Xe+ Xe∗ 5d[3/2]02 → 2P1/2 Xe+ Xe∗ 7s[3/2]02 → 2P1/2 Xe+ Xe∗ 7s[3/2]01 → 2P1/2 Xe+ Xe∗ 6s′[1/2]00 → 2P3/2 Xe+ Xe∗ 6s′[3/2]01 → 2P3/2 Xe+ Xe∗ 6p[3/2]1 → 2P3/2 Xe+ Xe∗ 6p[5/2]2 → 2P3/2 Xe+ Xe∗ 6p[5/2]3 → 2P3/2 Xe+ Xe∗ 6p[3/2]1 → 2P3/2 Xe+ Xe∗ 6p[3/2]2 → 2P3/2 Xe+ Xe∗ 5d[1/2]00 → 2P3/2 Xe+ Xe∗ 5d[1/2]01 → 2P3/2 Xe+ Xe∗ 6p[1/2]0 → 2P3/2 Xe+ Xe∗ 5d[7/2]04 → 2P3/2 Xe+ Xe∗ 5d[3/2]02 → 2P3/2 Xe+ Xe∗ 5d[7/2]03 → 2P3/2 Xe+ Xe∗ 5d[1/2]02 → 2P3/2 Xe+ Xe∗ 5d[5/2]03 → 2P3/2 Xe+ Xe∗ 5d[3/2]01 → 2P3/2 Xe+ Xe∗ 7s[3/2]02 → 2P3/2 Xe+ Xe∗ 7s[3/2]01 → 2P3/2 Xe+ Xe∗ 7p[1/2]1 → 2P3/2 Xe+ and other neighboring states
only excited Xe∗ 5d[3/2]02 and Xe∗ 7s[3/2]02,1 atoms can be ionized into both the Xe+ 2P1/2 and Xe+ 2P3/2 states, while all other excited states of the atomic xenon form the lowest Xe+ 2P3/2 state after ionization. It can be seen from the table that a number of bands have been assigned to several close atomic states, since difference of their energies is less or approximately equal to the resolution of the photoelectron spectrometer (240–320 cm−1). As follows from the analysis of the data, two processes are supposed to happen during relaxation of the excited xenon clusters: XeN + 2hν → XeN∗ → XeN−1 + Xe∗; Xe∗ + hν → Xe+ + e, (2) XeN + 2hν → XeN+ + e.
(3)
IV. CONCLUSIONS
As follows from the analysis of the photoelectron spectra of two-photon excited xenon clusters at excitation energies near the ionization threshold, two different processes were found. The first one is two-photon ionization process leading to the appearance of electrons with energies nearing the zero and the second one is the process of single-photon ionization of excited atoms desorbed from the two-photon excited xenon clusters. The desorption process leads not only to formation of two lowest excited states of atomic xenon1,2 but also to formation of the higher energy excited states of atomic xenon. ACKNOWLEDGMENTS
The work was carried out using the scientific equipment of the Center of Shared Usage “The analytical center of nano- and biotechnologies of SPbPU.” The authors gratefully acknowledge Antonina Alexandrovna Belyaeva for the significant invaluable contribution to the work. 1E. T. Verkhovtseva, E. A. Bondarenko, and Y. S. Doronin, Low Temp. Phys.
30, 34 (2004). 2E. T. Verkhovtseva, Y. S. Doronin, A. M. Ratner, and E. A. Bondarenko, Low
Temp. Phys. 32, 946 (2006). Moller, Z. Phys. D: At., Mol. Clusters 20, 1 (1991). 4R. von Pietrowski, M. Rutzen, K. von Haeften, S. Kakar, and T. Moller, Z. Phys. D: At., Mol. Clusters 40, 22 (1997). 5T. Moller, K. von Haeften, T. Laarmann, and R. von Pietrowski, Eur. Phys. J. D 9, 5 (1999). 6T. Laarmann, A. Kanaev, K. von Haeften, H. Wabnitz, R. von Pietrowski, and T. Möller, J. Chem. Phys. 116, 7558 (2002). 7F. Carnovale, J. B. Peel, and R. G. Rothwell, J. Chem. Phys. 95, 1473 (1991). 8F. Carnovale, J. B. Peel, R. G. Rothwell, J. Valldorf, and P. J. Kuntz, J. Chem. Phys. 90, 1452 (1989). 9A. Lindblad, T. Rander, I. Bradeanu, G. Öhrwall, O. Björneholm, M. Mucke, V. Ulrich, T. Lischke, and U. Hergenhahn, Phys. Rev. B 83, 125414 (2011). 10R. Feifel, M. Tchaplyguine, G. Öhrwall, M. Salonen, M. Lundwall, R. R. T. Marinho, M. Gisselbrecht, S. L. Sorensen, A. N. de Brito, L. Karlsson, N. Mårtensson, S. Svensson, and O. Björneholm, Eur. Phys. J. D 30, 343 (2004). 11P. M. Dehmer, S. T. Pratt, and J. L. Dehmer, J. Chem. Phys. 91, 2593 (1987). 3T.
114302-5 12P.
Serdobintsev et al.
M. Dehmer and J. L. Dehmer, J. Chem. Phys. 69, 125 (1978). M. Dehmer and J. L. Dehmer, J. Chem. Phys. 68, 3462 (1978). 14R. I. Hall, Y. Lu, Y. Morioka, T. Matsui, T. Tanaka, H. Yoshii, T. Hayaishi, and K. Ito, J. Phys. B: At., Mol. Opt. Phys. 28, 2435 (1995). 15C. Y. Ng, D. J. Trevor, B. H. Mahan, and Y. T. Lee, J. Chem. Phys. 65, 4327 (1976). 16N. Schwentner, M. Skibowski, and W. Steinmann, Phys. Rev. B 8, 2965 (1973). 17M. A. Khodorkovski˘i, A. A. Belyaeva, L. P. Rakcheeva, T. O. Artamonova, P. Yu. Serdobintsev, A. A. Pastor, A. S. Kozlov, S. V. Murashov, A. Z. Devdariani, R. Hallin, and K. Siegbahn, Opt. Spectrosc. 100, 497 (2006). 18M. A. Khodorkovski˘i, A. A. Belyaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, R. Hallin, and K. Siegbahn, Opt. Spectrosc. 102, 834 (2007). 19M. A. Khodorkovski˘i, A. A. Belyaeva, L. P. Rakcheeva, P. Yu. Serdobintsev, A. A. Pastor, A. S. Mel’nikov, N. A. Timofeev, R. Hallin, and K. Seigbahn, Opt. Spectrosc. 104, 674 (2008). 20M. A. Khodorkovskii, S. V. Murashov, T. O. Artamonova, A. A. Beliaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, I. A. Dement’ev, and J. Nordgren, J. Phys. B: At., Mol. Opt. Phys. 43, 155101 (2010). 21M. A. Khodorkovskii, S. V. Murashov, T. O. Artamonova, A. A. Beliaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, I. A. Dement’ev, and J. Nordgren, J. Phys. B: At., Mol. Opt. Phys. 43, 235101 (2010). 13P.
J. Chem. Phys. 143, 114302 (2015) 22M.
A. Khodorkovskii, S. V. Murashov, T. O. Artamonova, A. A. Belyaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, I. A. Dement’ev, R. Hallin, and J. Nordgren, Opt. Spectrosc. 108, 899 (2010). 23M. A. Khodorkovskii, A. A. Belyaeva, L. P. Rakcheeva, P. Yu. Serdobintsev, A. S. Melnikov, I. A. Shevkunov, N. A. Timofeev, and A. A. Pastor, Opt. Spectrosc. 112, 679 (2012). 24L. P. Rakcheeva, P. Yu. Serdobintsev, A. A. Belyaeva, I. A. Shevkunov, A. S. Melnikov, A. A. Nakozina, A. A. Pastor, and M. A. Khodorkovskii, J. Chem. Phys. 139, 174304 (2013). 25See supplementary material at http://dx.doi.org/10.1063/1.4930963 for the additional photoelectron and mass spectra and the dependence of the photoelectron signal intensity on the laser intensity. 26X. K. Hu, D. M. Mao, Y. J. Shi, S. S. Dimov, and R. H. Lipson, J. Chem. Phys. 109, 3944 (1998). 27O. F. Hagena, Surf. Sci. 106, 16 (1981). 28O. F. Hagena, Rev. Sci. Instrum. 63, 2374 (1992). 29J. Wörmer, V. Guzielski, J. Stapelfeldt, and T. Möller, Chem. Phys. Lett. 159, 321 (1989). 30Y. Ralchenko, A. E. Kramida, J. Reader, and NIST ASD Team, Atomic Spectra Database Version 3.1.5, National Institute of Standards and Technology, Gaithersburg, MD, 2008, http://physics.nist.gov/asd3. 31R. N. Compton, J. C. Miller, and A. E. Carter, Chem. Phys. Lett. 71, 87 (1980). 32K. Sato, Y. Achiba, and K. Kimura, J. Chem. Phys. 80, 57 (1984).
Citation: The Journal of Chemical Physics 143, 114302 (2015); doi: 10.1063/1.4930963 View online: http://dx.doi.org/10.1063/1.4930963 View Table of Contents: http://aip.scitation.org/toc/jcp/143/11 Published by the American Institute of Physics
THE JOURNAL OF CHEMICAL PHYSICS 143, 114302 (2015)
Relaxation channels of multi-photon excited xenon clusters P. Yu. Serdobintsev,1,2 L. P. Rakcheeva,1,a) S. V. Murashov,1 A. S. Melnikov,1,2 S. Lyubchik,3 N. A. Timofeev,2 A. A. Pastor,2 and M. A. Khodorkovskii1 1
Institute of Nanobiotechnologies, Peter the Great St.Petersburg Polytechnic University, Saint Petersburg 195251, Russia 2 Department of Physics, St. Petersburg State University, Saint Petersburg 198904, Russia 3 REQUIMTE, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica 2829-516, Portugal
(Received 7 April 2015; accepted 1 September 2015; published online 15 September 2015) The relaxation processes of the xenon clusters subjected to multi-photon excitation by laser radiation with quantum energies significantly lower than the thresholds of excitation of atoms and ionization of clusters were studied. Results obtained by means of the photoelectron spectroscopy method showed that desorption processes of excited atoms play a significant role in the decay of two-photon excited xenon clusters. A number of excited states of xenon atoms formed during this process were discovered and identified. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4930963]
I. INTRODUCTION
Studies of intermolecular energy conversion in polyatomic systems are of interest from both theoretical and experimental points of view. Atomic clusters are very useful for these purposes. In particular, van der Waals clusters of noble gases, formed by supersonic expansion through a nozzle, are an ideal target to observe these processes due to the lack of interaction with the environment. This fact has been widely used in a number of works1–16 to study energy relaxation channels for noble gases’ clusters excited in the conditions of crossed beams. In a number of works,1–6 it was shown that the result of relaxation of excited clusters can be with desorption of excited atoms and molecules. The linear and band spectra in the VUV range (100–200 nm) obtained by registration of radiation from argon, krypton, and xenon clusters excited by the electron impact were assigned1,2 to excited atoms and molecules desorbed from the corresponding clusters. The maximum yield of the excited atoms and molecules was registered at the average size of the clusters being about ⟨N⟩ = 50–100 atoms/cluster. It was shown that there are two competing relaxation channels depending on the cluster size. In addition to the usual relaxation channel of the exciton in solid state, for the clusters with the sizes below 160–190 atoms per cluster there is a new channel, named “nonradiative excimer dissociation” by the authors.1 Dependence of the relaxation processes on the cluster size was studied by the luminescent spectroscopy.3–6 Clusters of noble gases containing from a few to 105 atoms were excited by the tunable monochromatic synchrotron radiation. It was shown5,6 that the relaxation processes of the exited clusters lead to the desorption of the electronically excited dimers and, in some cases, excited atoms, as it was observed for the xenon clusters with the relatively small average size ⟨N⟩ = 10 atoms per cluster. a)[email protected]
0021-9606/2015/143(11)/114302/5/$30.00
It was shown5 that the important relaxation channel for excited clusters of helium is desorption of excited atoms and dimer molecules. While desorption of the excited helium atoms was observed for the clusters with the sizes of up to the several hundred atoms, the corresponding process in the argon clusters was observed only for the small clusters (N < 10). It is interesting to note that it comes into contradiction with the experimental results for the argon clusters.1,2 Photoelectron spectroscopy was used to study dependence of the ionization potential on the size of clusters at excitation by VUV radiation of helium discharge lamp or by synchrotron radiation.7–13 The analysis of the photoelectron spectra showed that the threshold of ionization of the small xenon clusters is about 10.5 eV and decreases with increase of the size of clusters. The threshold of ionization of the clusters is significantly lower than that of xenon dimers—11.176 eV14,15 and higher than the photoemission threshold of liquid and solid xenon (9.7–9.8 eV).16 It should be noted that the sources of excitation of clusters observed in the works mentioned above had the energies significantly higher than the ones needed for excitation of atoms and ionization of clusters. In this paper, the relaxation processes of the xenon clusters excited electronically by multi-photon absorption of laser radiation with the quantum energies significantly lower than the thresholds of excitation of atoms and ionization of clusters were studied using the method of photoelectron spectroscopy. This work continues a series of studies of the electron structures of dimers and clusters of the noble gases by the REMPI-TOF and REMPI-PES methods.17–24
II. EXPERIMENT
The photoelectron spectroscopy experimental setup was described in our previous paper.24 It consists of a pulsed supersonic 100 µm nozzle for production of xenon clusters at stagnation pressure of 4 bars of the Xe:Ar mixture ≈5%:95%,
143, 114302-1
© 2015 AIP Publishing LLC
114302-2
Serdobintsev et al.
a hand-made time-of-flight mass-spectrometer with limited mass range (about 1000 Da) for ion detection, a “magnetic bottle” type time-of-flight electron spectrometer for electron detection, and a tunable pulse laser light source (about 5 ns) for excitations of the xenon clusters. The wavelength was scanned in the range of 220.2–226.2 nm (45 413–44 208 cm−1, 5.48 eV–5.63 eV). The energy of two photons in this range is close to the energy of xenon atomic states 7p, 6p′, 6d. The intensity of the focused laser radiation was about 109 W/cm2. The experimental setup for measuring of jet composition consists of TOF mass spectrometer with the mass range about 200 000 Da (Bruker, TOF-1) equipped by the gas inlet system, pulse supersonic 100 µm nozzle, and electron gun with electron energy being about 70 eV. III. RESULTS AND DISCUSSION
The photoelectron spectra obtained from the area of intersection of laser radiation and supersonic molecular beam containing xenon clusters are presented in Figure 1. The energies of the exciting photons are plotted on the X-axis and the energy of the resulting photoelectrons is indicated on the Y-axis. This figure contains the data for the linear and band spectra formed by multi-photon ionization of free atoms and dimers, as well as for the continuous spectra originating from xenon clusters. In addition, some ordinary photoelectron spectra at selected photon energies are provided in Figures S1-S3 of the supplementary material.25 Electron signals due to direct three-photon ionization of xenon atoms (both resonant and non-resonant) form two bands marked “A” and “B”, corresponding to two states Xe+ 2P1/2 and Xe+ 2P3/2 of xenon ions. The distance between these two bands is equal to the difference between ionization potentials of these states −1.306 eV. These bands are parallel to each other and are tilted relative to the X-axis, since the energy of photoelectrons formed during the three-photon ionization of atomic xenon depends on the quantum energy of laser radiation: Ee = 3hν − I.P. Here, Ee is the photoelectron energy and I.P. is the ionization potential of atomic xenon. The signal intensity of these bands rapidly rises and reaches saturation when the three-photon ionization occurs through the intermediate
J. Chem. Phys. 143, 114302 (2015)
excited states allowed for the two-photon transition. The vertical lines observed at these wavelengths are caused by such signal saturation. In addition to the bands A and B (Figure 1), there are the weaker continuous bands tilted at the less angle to the X-axis in the electron energy range of 1.5–4.5 eV. These bands correspond to the photoelectrons formed during the single photon ionization of xenon atoms in various excited states: Ee = Ea + hν − I.P., where Ee—photoelectron energy, Ea—excitation potential of atomic xenon, and I.P.—ionization potential of atomic xenon. Because of continuous character of these bands, it can be concluded that these excited xenon atoms can only be produced by desorption from the two-photon excited xenon clusters, which are characterized by continuous absorption spectra. The possible second desorption channel of neutral Xe-atoms when most part of energy remains in excited Xe∗N−1 unfortunately cannot be observed experimentally since the concentration of the Xe atoms in stem is so high that contribution to the signal from relatively small amount of neutral atoms delivered by this way is negligible. The wide band marked “0” corresponds to the slow electrons with near-zero energy. The intensity of the band rises steadily with growing of the energy of excitation from 10.96 eV to 11.26 eV. The only plausible explanation of appearance of electrons with such energies is the two-photon ionization process of the clusters. As opposed to the interpretation presented in Ref. 26, such process should not be assigned to autoionization of two-photon excited xenon dimers, since it is not only observed at the corresponding resonant frequencies but also at the entire studied energy range. This process is specific for wide band absorption of clusters. The studied processes take place with the participation of the two or three photons, but the measured dependence of the signal intensity of photoelectrons on the laser intensity is quadratic (see Figures S4 and S5 in the supplementary material25). This dependence for electrons with zero energy (band “0”) is evident since they are presented by two-photon ionization of clusters resulting from the direct ionization process or autoionization mechanism. Ionizing three-photon transitions in xenon atoms and molecules have the same dependence because they occur through the real intermediate two-photon
FIG. 1. The photoelectron spectra of supersonic molecular beam containing xenon clusters 0—the two-photon ionization of xenon clusters and A and B—the bands formed by three-photon ionization of xenon atoms, corresponding to two states of xenon ions Xe+ 2P1/2 and Xe+ 2P3/2. The lines from 1 to 5 correspond to the three-photon ionization signals of xenon atoms from the ground state through the allowed two-photon transitions in both states of the Xe+ core. The notations of these transitions are 1—5p6 1S0-7p[5/2]2 88 351 cm−1 (10.955 eV); 2—5p6 1S0-7p[3/2]2 88 708 cm−1 (10.999 eV); 3—5p6 1S0-7p[1/2]0 88 842 cm−1 (11.016 eV); 4—5p6 1S0-6p[3/2]2 89 162 cm−1 (11.055 eV); 5—5p6 1S0-6p[1/2]0 89 860 cm−1 (11.142 eV). Vertical lines at the above mentioned energies of excitation are caused by saturation of signals by resonant enhancing of the three-photon ionization through the allowed two-photon resonance.
114302-3
Serdobintsev et al.
FIG. 2. The relaxation channels of two-photon excited XeN clusters. The right side of figure corresponds to the two-photon ionization of XeN clusters. The photoelectron spectrum labeled as “A” is taken from the Ref. 10 obtained by one-photon ionization of XeN clusters with average size ⟨N⟩ = 1000 using synchrotron radiation with the photon energy 61 eV. The left side of figure demonstrates ionization of excited xenon atoms desorbed from the two-photon excited XeN clusters. These excited atoms are then ionized by one-photon transition during the same laser pulse. Reproduced with permission from Feifel et al., Eur. Phys. J. D 30, 343 (2004). Copyright 2004 The European Physical Journal (EPJ).
excited state. The process of absorption of third photon is saturated since at the radiation intensity about 109 W/cm2 all excited atoms are ionized during the laser pulse. The onephoton ionization process of desorbed excited atoms is saturated too, but they appear from the two-photon excited clusters and intensity dependence is quadratic too. Thus, registration of fast and slow electrons reflects two distinct decay channels of excited clusters XeN∗. These processes are schematically shown in Figure 2. The first process is accompanied by desorption of the excited atoms Xe∗ in various states and their ionization by the next photon. As a result, the Xe+ 2P1/2 and Xe+ 2P3/2 ions and electrons with energies Ee = Ea + hν − I.P. are formed. The left part of the figure contains a diagram of the energy levels for atoms Xe* (Ea) and ions Xe+.
J. Chem. Phys. 143, 114302 (2015)
Based on this diagram, it is easy to explain the creation mechanism of electrons with the energy of 1.5–4.5 eV. The second process corresponds to two-photon ionization of clusters. The central part of the figure demonstrates two-photon excitation of the XeN∗ clusters. Figure 2(a) shows the photoelectron spectrum of outermost valence shells of xenon clusters XeN with average size ⟨N⟩ = 1000.10 Ionization energy of these clusters is characterized by the band shifted relative to the spectral line Xe+ 2P3/2 (I.P. = 12.13 eV) with the center of mass Ecgcl = 10.83 eV. In Ref. 7, the maximum of the corresponding band of clusters with the average size ⟨N⟩ < 100 is about 11.55 eV. In our experiment, maximum of the corresponding band is slightly higher than 11.26 eV. Comparing this value with the values of ionization energies of xenon atoms, molecules, and clusters of various sizes,7–9,11–13 it may be assumed that the average cluster size ⟨N⟩ observed during this experiment did not exceed 100 atoms. This estimation is in the size range of the result calculation of this value for the experimental conditions using the classical gas-dynamic approach.27–29 Data of direct measurements of average cluster size ⟨N⟩ collected by TOF mass-spectrometer setup described above are provided in Figure S6 of the supplementary material.25 As one can see, the average cluster size ⟨N⟩ at 4 bars stagnation pressure is about 70. Since the energy of two-photon excitation of clusters in our experiments did not exceed 11.26 eV, the direct ionization of clusters with ⟨N⟩ about 100 takes place on the low energy edge of the cluster ionization cross section. Appearance of low energy electrons (band “0”) and a monotonic growth of their intensity with increasing of the excitation energy could be explained in such manner. The photoelectron spectrum of xenon clusters excited by laser radiation at the fixed wavelength hν = 44 165 cm−1 (5.476 eV) is presented in Figure 3. The energy of two quanta (2hν = 88 330 cm−1 = 10.95 eV) does not coincide with any of the atomic or molecular transitions. The slow electrons with a nearly zero kinetic energy labeled by 0 are presented by a broad band in the range of 0–0.6 eV. The fast electrons
FIG. 3. The photoelectron spectrum of xenon clusters excited by laser radiation at the fixed wavelength hν = 44 165 cm−1 (exciting photon energy −5.476 eV). Red bars on the top represent the energy of excited xenon atom levels responsible for production Xe+ 2P1/2 ions (electron peaks 1, 2, and 5–15). The energy of ejected electron (Ee) is combined with the energy of the corresponding Xe∗ level (Eexc) according to the formula: Ee = Eexc + hv − Eion. The same formula is valid for the peaks 3, 4 but the ionization potential for these is the other ionic state Xe+ 2P3/2.
114302-4
Serdobintsev et al.
J. Chem. Phys. 143, 114302 (2015)
are presented by a number of bands labeled by 1–15. The labels correspond to those indicated in Figure 1. As it was mentioned above, the 0 band appears due to cluster ionization by two-photon process, and the other ones are formed as a result of ionization of the excited xenon atoms desorbed from the excited clusters. The short dashes on the upper Xaxis energetic scale correspond to the Xe* atomic states in the spectral range of 65 000–95 000 cm−1. The atomic energy levels and the photoelectron energy are related as follows: Ea(Xe∗) = Ee + I.P. (Xe+ 2P3/2) − hν,
(1)
where Ee—observed electron energy in cm−1, I.P. (Xe+ 2P3/2) = 97 833.8 cm−1 (12.13 eV)—ionization energy of Xe atom, and hν = 44 165 cm−1 (5.48 eV)—energy of laser radiation. Energies of the atomic states calculated by this formula are in good agreement with the values in Ref. 30. It can be seen in Figure 3 that the maxima of the bands 1–15 coincide with the position of the atomic levels. Table I demonstrates the kinetic energies and assignments of all observed bands 1–15. The bands 1, 2 correspond to formation of the Xe+ 2P3/2 ions resulting from absorption of third photon by the lowest excited states of xenon Xe* 6s[3/2]02,1. The 3, 4 and 13, 14 bands appear as a result of formation of the Xe+ 2P1/2 and Xe+ 2P3/2 ions correspondingly due to the same absorption process with participation of the Xe∗ 5d[3/2]01 and Xe∗ 7s[3/2]01,2 excited states. Changing of the atomic core state at ionization of excited xenon states was observed also in the works11,31,32 and is caused by configuration interaction. The bands 5–12 and 15 are assigned to the excited states responsible for formation of the Xe+ 2P3/2 ion only. It should be pointed out that the TABLE I. Kinetic energies and assignments of the recorded photoelectron peaks of xenon clusters. Peak label
E, eV
1 2 3 4
1.67 1.79 2.43 2.60
5 6
2.80 2.91
7
3.03
8
3.16
9
3.28
10 11 12 13 14
3.36 3.48 3.55 3.73 3.90
15
4.23
ν, cm−1 67 067.547 68 045.156 83 889.971 85 188.777 85 440.017 76 196.767 77 185.041 77 269.145 78 119.798 78 403.061 78 956.031 79 212.465 79 771.267 79 986.618 80 118.962 80 196.629 80 322.746 80 970.438 81 925.514 82 430.204 83 889.971 85 188.777 85 440.017 87 927.131
Assignment, Xe∗ → 2PJ Xe+ ∗
6s[3/2]0
2P
+
Xe 3/2 Xe 2→ Xe∗ 6s[3/2]01 → 2P3/2 Xe+ Xe∗ 5d[3/2]02 → 2P1/2 Xe+ Xe∗ 7s[3/2]02 → 2P1/2 Xe+ Xe∗ 7s[3/2]01 → 2P1/2 Xe+ Xe∗ 6s′[1/2]00 → 2P3/2 Xe+ Xe∗ 6s′[3/2]01 → 2P3/2 Xe+ Xe∗ 6p[3/2]1 → 2P3/2 Xe+ Xe∗ 6p[5/2]2 → 2P3/2 Xe+ Xe∗ 6p[5/2]3 → 2P3/2 Xe+ Xe∗ 6p[3/2]1 → 2P3/2 Xe+ Xe∗ 6p[3/2]2 → 2P3/2 Xe+ Xe∗ 5d[1/2]00 → 2P3/2 Xe+ Xe∗ 5d[1/2]01 → 2P3/2 Xe+ Xe∗ 6p[1/2]0 → 2P3/2 Xe+ Xe∗ 5d[7/2]04 → 2P3/2 Xe+ Xe∗ 5d[3/2]02 → 2P3/2 Xe+ Xe∗ 5d[7/2]03 → 2P3/2 Xe+ Xe∗ 5d[1/2]02 → 2P3/2 Xe+ Xe∗ 5d[5/2]03 → 2P3/2 Xe+ Xe∗ 5d[3/2]01 → 2P3/2 Xe+ Xe∗ 7s[3/2]02 → 2P3/2 Xe+ Xe∗ 7s[3/2]01 → 2P3/2 Xe+ Xe∗ 7p[1/2]1 → 2P3/2 Xe+ and other neighboring states
only excited Xe∗ 5d[3/2]02 and Xe∗ 7s[3/2]02,1 atoms can be ionized into both the Xe+ 2P1/2 and Xe+ 2P3/2 states, while all other excited states of the atomic xenon form the lowest Xe+ 2P3/2 state after ionization. It can be seen from the table that a number of bands have been assigned to several close atomic states, since difference of their energies is less or approximately equal to the resolution of the photoelectron spectrometer (240–320 cm−1). As follows from the analysis of the data, two processes are supposed to happen during relaxation of the excited xenon clusters: XeN + 2hν → XeN∗ → XeN−1 + Xe∗; Xe∗ + hν → Xe+ + e, (2) XeN + 2hν → XeN+ + e.
(3)
IV. CONCLUSIONS
As follows from the analysis of the photoelectron spectra of two-photon excited xenon clusters at excitation energies near the ionization threshold, two different processes were found. The first one is two-photon ionization process leading to the appearance of electrons with energies nearing the zero and the second one is the process of single-photon ionization of excited atoms desorbed from the two-photon excited xenon clusters. The desorption process leads not only to formation of two lowest excited states of atomic xenon1,2 but also to formation of the higher energy excited states of atomic xenon. ACKNOWLEDGMENTS
The work was carried out using the scientific equipment of the Center of Shared Usage “The analytical center of nano- and biotechnologies of SPbPU.” The authors gratefully acknowledge Antonina Alexandrovna Belyaeva for the significant invaluable contribution to the work. 1E. T. Verkhovtseva, E. A. Bondarenko, and Y. S. Doronin, Low Temp. Phys.
30, 34 (2004). 2E. T. Verkhovtseva, Y. S. Doronin, A. M. Ratner, and E. A. Bondarenko, Low
Temp. Phys. 32, 946 (2006). Moller, Z. Phys. D: At., Mol. Clusters 20, 1 (1991). 4R. von Pietrowski, M. Rutzen, K. von Haeften, S. Kakar, and T. Moller, Z. Phys. D: At., Mol. Clusters 40, 22 (1997). 5T. Moller, K. von Haeften, T. Laarmann, and R. von Pietrowski, Eur. Phys. J. D 9, 5 (1999). 6T. Laarmann, A. Kanaev, K. von Haeften, H. Wabnitz, R. von Pietrowski, and T. Möller, J. Chem. Phys. 116, 7558 (2002). 7F. Carnovale, J. B. Peel, and R. G. Rothwell, J. Chem. Phys. 95, 1473 (1991). 8F. Carnovale, J. B. Peel, R. G. Rothwell, J. Valldorf, and P. J. Kuntz, J. Chem. Phys. 90, 1452 (1989). 9A. Lindblad, T. Rander, I. Bradeanu, G. Öhrwall, O. Björneholm, M. Mucke, V. Ulrich, T. Lischke, and U. Hergenhahn, Phys. Rev. B 83, 125414 (2011). 10R. Feifel, M. Tchaplyguine, G. Öhrwall, M. Salonen, M. Lundwall, R. R. T. Marinho, M. Gisselbrecht, S. L. Sorensen, A. N. de Brito, L. Karlsson, N. Mårtensson, S. Svensson, and O. Björneholm, Eur. Phys. J. D 30, 343 (2004). 11P. M. Dehmer, S. T. Pratt, and J. L. Dehmer, J. Chem. Phys. 91, 2593 (1987). 3T.
114302-5 12P.
Serdobintsev et al.
M. Dehmer and J. L. Dehmer, J. Chem. Phys. 69, 125 (1978). M. Dehmer and J. L. Dehmer, J. Chem. Phys. 68, 3462 (1978). 14R. I. Hall, Y. Lu, Y. Morioka, T. Matsui, T. Tanaka, H. Yoshii, T. Hayaishi, and K. Ito, J. Phys. B: At., Mol. Opt. Phys. 28, 2435 (1995). 15C. Y. Ng, D. J. Trevor, B. H. Mahan, and Y. T. Lee, J. Chem. Phys. 65, 4327 (1976). 16N. Schwentner, M. Skibowski, and W. Steinmann, Phys. Rev. B 8, 2965 (1973). 17M. A. Khodorkovski˘i, A. A. Belyaeva, L. P. Rakcheeva, T. O. Artamonova, P. Yu. Serdobintsev, A. A. Pastor, A. S. Kozlov, S. V. Murashov, A. Z. Devdariani, R. Hallin, and K. Siegbahn, Opt. Spectrosc. 100, 497 (2006). 18M. A. Khodorkovski˘i, A. A. Belyaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, R. Hallin, and K. Siegbahn, Opt. Spectrosc. 102, 834 (2007). 19M. A. Khodorkovski˘i, A. A. Belyaeva, L. P. Rakcheeva, P. Yu. Serdobintsev, A. A. Pastor, A. S. Mel’nikov, N. A. Timofeev, R. Hallin, and K. Seigbahn, Opt. Spectrosc. 104, 674 (2008). 20M. A. Khodorkovskii, S. V. Murashov, T. O. Artamonova, A. A. Beliaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, I. A. Dement’ev, and J. Nordgren, J. Phys. B: At., Mol. Opt. Phys. 43, 155101 (2010). 21M. A. Khodorkovskii, S. V. Murashov, T. O. Artamonova, A. A. Beliaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, I. A. Dement’ev, and J. Nordgren, J. Phys. B: At., Mol. Opt. Phys. 43, 235101 (2010). 13P.
J. Chem. Phys. 143, 114302 (2015) 22M.
A. Khodorkovskii, S. V. Murashov, T. O. Artamonova, A. A. Belyaeva, L. P. Rakcheeva, A. A. Pastor, P. Yu. Serdobintsev, N. A. Timofeev, I. A. Shevkunov, I. A. Dement’ev, R. Hallin, and J. Nordgren, Opt. Spectrosc. 108, 899 (2010). 23M. A. Khodorkovskii, A. A. Belyaeva, L. P. Rakcheeva, P. Yu. Serdobintsev, A. S. Melnikov, I. A. Shevkunov, N. A. Timofeev, and A. A. Pastor, Opt. Spectrosc. 112, 679 (2012). 24L. P. Rakcheeva, P. Yu. Serdobintsev, A. A. Belyaeva, I. A. Shevkunov, A. S. Melnikov, A. A. Nakozina, A. A. Pastor, and M. A. Khodorkovskii, J. Chem. Phys. 139, 174304 (2013). 25See supplementary material at http://dx.doi.org/10.1063/1.4930963 for the additional photoelectron and mass spectra and the dependence of the photoelectron signal intensity on the laser intensity. 26X. K. Hu, D. M. Mao, Y. J. Shi, S. S. Dimov, and R. H. Lipson, J. Chem. Phys. 109, 3944 (1998). 27O. F. Hagena, Surf. Sci. 106, 16 (1981). 28O. F. Hagena, Rev. Sci. Instrum. 63, 2374 (1992). 29J. Wörmer, V. Guzielski, J. Stapelfeldt, and T. Möller, Chem. Phys. Lett. 159, 321 (1989). 30Y. Ralchenko, A. E. Kramida, J. Reader, and NIST ASD Team, Atomic Spectra Database Version 3.1.5, National Institute of Standards and Technology, Gaithersburg, MD, 2008, http://physics.nist.gov/asd3. 31R. N. Compton, J. C. Miller, and A. E. Carter, Chem. Phys. Lett. 71, 87 (1980). 32K. Sato, Y. Achiba, and K. Kimura, J. Chem. Phys. 80, 57 (1984).
