Supersonic molecular beam experiments on surface chemical reactions.

Personal Account

THE CHEMICAL RECORD

Supersonic Molecular Beam Experiments on Surface Chemical Reactions Michio Okada Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043 (Japan) E-mail: [email protected]

Received: February 17, 2014 Publised online: ■■

ABSTRACT: The interaction of a molecule and a surface is important in various fields, and in particular in complex systems like biomaterials and their related chemistry. However, the detailed understanding of the elementary steps in the surface chemistry, for example, stereodynamics, is still insufficient even for simple model systems. In this Personal Account, I review our recent studies of chemical reactions on single-crystalline Cu and Si surfaces induced by hyperthermal oxygen molecular beams and by oriented molecular beams, respectively. Studies of oxide formation on Cu induced by hyperthermal molecular beams demonstrate a significant role of the translational energy of the incident molecules. The use of hyperthermal molecular beams enables us to open up new chemical reaction paths specific for the hyperthermal energy region, and to develop new methods for the fabrication of thin films. On the other hand, oriented molecular beams also demonstrate the possibility of understanding surface chemical reactions in detail by varying the orientation of the incident molecules. The steric effects found on Si surfaces hint at new ways of material fabrication on Si surfaces. Controlling the initial conditions of incoming molecules is a powerful tool for finely monitoring the elementary step of the surface chemical reactions and creating new materials on surfaces. DOI 10.1002/tcr.201402003 Keywords: molecular beams, oxidation, steric effects, surface chemistry

1. Introduction The initial conditions of a gas-phase molecule approaching a surface, i.e., the translational energy, internal states and so on, significantly affect the dynamical processes after the first encounter. It is expected that the resulting surface chemical reactions, if they occur, depend strongly on such initial conditions. Precise control of such parameters of the incoming molecule enables us to understand the surface chemical reactions in detail and manipulate them in space and time. Such information becomes important in complex systems like biomaterials. Molecular beam techniques allow a detailed investigation of several aspects of molecular interactions and molecular

Chem. Rec. 2014, ••, ••–••

dynamics,[1,2] as shown in Figure 1. The main advantages of a supersonic molecular beam are: (a) narrow velocity distribution, (b) variable kinetic energy, and (c) a large degree of control over the internal states. A typical example of the molecule–surface interaction is shown in the schematic potential curve in Figure 1. It is expected that the interaction and the resulting surface reaction depend sensitively on the abovementioned parameters of a molecule (a–c). The energy exchange processes induced by the incident molecule will have a substantial effect on the resulting processes such as adsorption, excitation of internal molecular modes, and

© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim

www.tcr.wiley-vch.de

THE CHEMICAL RECORD

Fig. 1. Schematic representation of the topic of this report: supersonic molecular beams controlling the conditions of incident molecules are used to provide detailed understanding of elementary processes in surface chemical reactions. A typical example of the molecule–surface interaction is shown in the schematic potential curve. It is expected that the interaction and the resulting surface chemical reaction depend sensitively on the conditions of the incoming molecule.

surface chemical reactions (see Figure 1).[3–5] The dynamics of atom–surface scattering can be described with simple models,[6] such as the hard-cube model.[7] In some cases, scattering of molecules is considered as well.[8] Such simple models are insightful enough to understand the underlying physics as a problem of energy exchange. One of the important energy transfer processes induced by incoming atoms and molecules is phonon excitation. In the case of NO or N2 scattering from typical metal surfaces, a beam with incident energy on the order of 1 eV can lose 25–75% of its energy into substrate phonons.[9] This will cause the local heating of the surface,

Michio Okada received his Doctor of Science in Chemistry from the University of Tokyo in 1993. Following a position as a Postdoctoral Research Associate at the University of Tokyo and the University of Tennessee, he joined Osaka University (OU) as an Assistant Professor in the Department of Chemistry, Graduate School of Science, in 1995. He became a Full Professor in the Renovation Center of Instruments for Science Education and Technology, OU, in 2008. Since 2013, he has been a Full Professor in the Department of Chemistry, Graduate School of Science, OU. His research interests lie in the field of surface chemistry explored by molecular beams and ion beams, as well as in the area of surface physics of materials with low dimensionality.


possibly resulting in a chemical reaction specific to the hyperthermal energy region. In molecule–surface scattering, another aspect of the energy transfer processes also appears, namely the exchange of energy between molecular degrees of freedom induced in the interaction with the surface. Such energy exchange processes[1–3,10–21] in gas–surface scattering have been studied extensively by both experimental and theoretical techniques. The interesting aspects of nonadiabaticity also appear in gas–surface scattering;[22–25] for example, energy dissipation by scattering of highly vibrationally excited molecules, vibrational energy relaxation, dissociative adsorption, and associative desorption. The energy transfer processes are vital in the dynamical processes induced by incoming molecules and also have the possibility of inducing new pathways of surface chemical reactions. The dependence of the involved molecules of elastic, inelastic and reactive molecular collisions on spatial orientation distribution is of particular interest in the above-mentioned context.[26–30] Such a geometrical dependence is known as a steric effect. The first trials to control the molecular orientation experimentally were carried out on polar molecules exploiting an electrostatic quadrupolar field.[31] This technique was applied successfully to obtain the anisotropy in some two-body gas-phase van der Waals interactions.[32,33] Later, hexapole focusing techniques were applied to various gas-phase reactions.[26–28] However, the investigation of the stereodynamics of elementary processes[34–37] still represents one of the main themes of advanced research in several areas of molecular and surface sciences. Since it is well known that

Chem. Rec. 2014, ••, ••–••


Supersonic Molecular Beams for Surface Chemical Reaction Analysis

molecules often adsorb with a well-defined orientation and that the surface itself can serve to orient the reactants,[38,39] molecular beam–surface experiments can provide unique information on the anisotropic nature of stereodynamics in chemical and physical processes. The sticking probability, S, of reactants coming from the gas phase, resulting in the evolution of the uptake of the molecule onto the surface is one of the key parameters determining the reaction rate in catalytic processes in the heterogeneous phase. Therefore, it was characterized as precisely as possible for a large variety of incoming molecular conditions, and its dependence on the chemical properties of the gas-phase species as well as of the surface was investigated in detail by performing state-resolved experiments and theoretical simulations. From a quantum mechanical point of view, the state of a gas-phase species is described by its momentum vector, containing the condition on translational energy and polar and azimuthal angles of impingement onto the surface, and also by the quantum numbers related to electronic, vibrational and rotational degrees of freedom. On the other hand, the surface condition is also characterized by a variety of parameters, which include surface temperature, surface coverage of the reactant and of other co-adsorbed species, surface crystallographic condition, surface defectivity and, more generally, surface morphology. Moreover, the surface chemical properties can be tuned by alloying. These parameters are quite important in the model catalysis system.[5] Here, in the present Personal Account, I will focus on our recent works on the hyperthermal translational energy effects and the molecular orientation effects on the surface chemical reactions.

2. Translational Energy Effects The translational energy of an incoming molecule plays an important role in the surface reaction dynamics. Translationalenergy-induced effects in gas–surface reactions have been studied extensively in terms of gas–surface chemical reactions,[4,40–53] thin-film growth,[42] and so on. When adsorption is translationally activated, hyperthermal molecular beams allow us to simulate the fate of molecules populating the highenergy tail of the Boltzmann distribution. The tails represent indeed only a small fraction of the total statistical population but are the only ones that matter, if all the others are prevented from adsorption and bounced back into the gas phase upon collision with the surface. The hyperthermal O2 molecular beams (HOMB) (≥0.5 eV) may improve the quality of thin oxide film growth, for example as demonstrated for organic films,[42] and allows the production of oxide layers at lower crystal temperatures, avoiding contamination problems and reducing film defectivity. The dynamics of O2 molecules on the surface in the hyperthermal energy region were also studied in detail on metal and semiconductor surfaces.[40,48–53] In the hyperthermal energy region, we also expect the efficient collision-induced processes proposed by Ceyer.[47] Collisioninduced absorption (CIA)[43,54] and local heating of the substrate[55] were indeed shown to be effective for inducing oxide nucleation, opening up new possibilities for the production of nanostructured metal oxides. The energy transfer from hyperthermal beams can also be applied to the measurement of the friction coefficients of the adsorbed molecular motions.[56] A cross-sectional top view of the apparatus in the BL23SU beamline at SPring-8 in Japan is shown in Figure 2.[57] Soft

Fig. 2. Cross-sectional top view of the main parts of the surface chemistry experimental station SUREAC2000 installed at BL23SU at the SPring-8 facilities.[57,91]

Chem. Rec. 2014, ••, ••–••



THE CHEMICAL RECORD

X-rays from about 300 eV to 2000 eV are available with a high spectral resolution in a grazing incidence type monochromator system.[58] A HOMB generator, consisting of differentially pumped expansion (nozzle) and chopper stages, is coupled to the surface reaction analysis chamber. A two-step differential pumping system separates the HOMB generator and the surface reaction analysis chamber. The HOMB is generated by adiabatic expansion of O2 seeded in He and/or Ar. The combination of HOMB and the synchrotron radiation (SR) X-ray photoemission spectroscopy (XPS) is unique and very useful in the research of surface chemical reactions. The kinetic energy of incident O2 is varied by changing the seeding ratio and the nozzle temperature. The value is determined from the formula

Ei =

Mi M

γ

1− γ

kBTn

(1)

where Ei and Mi are the translational energy and mass of the i-component of the gas mixture, 〈M〉 is the average mass, γ is the ratio of constant pressure and constant volume specific heat, Tn is the nozzle temperature and kB is Boltzmann’s constant. We note that Equation (1) is valid only for an almost perfect adiabatic expansion, i.e., for complete transformation of enthalpy into translational energy. For a seeded beam (0.5% O2 in He) with Tn = 300 and 1400 K the experimentally estimated upper limits at SPring-8 are Ei = 0.5 and 2.3 eV, respectively, indicating a very good adiabatic expansion. Herein we offer an overview on recent detailed studies of oxygen adsorption and of the initial stages of Cu2O and CuO formation on low Miller index and vicinal Cu surfaces and also Cu3Au alloy surfaces. 2.1. HOMB Oxidation of Cu Understanding the chemistry of the copper–oxygen interaction is one of the outstanding open issues of solid-state physics, due to the implications for the understanding of high-Tc superconducting oxides, whose basic units are Cu–O chains or layers.[59] Cuprous oxide (Cu2O) and cupric oxide (CuO) are regarded as meaningful benchmark materials for theories and experiments. The O-uptake curve for a 2.3 eV HOMB[54] at normal incidence on Cu(100), determined from the integration of the O-1s spectra, is reported in Figure 3. In region A, O2 dissociatively adsorbs on the surface and Θ increases steeply with increasing HOMB dose (Figure 3 – note the logarithmic scale for exposure). From comparison of the uptake curve for the 2.3 eV HOMB exposure (full circles) with that for thermal O2 (blue dashed line),[54] the increased efficiency of HOMB for dissociative adsorption is evident. This result suggests that HOMB helps in overcoming the activation barrier of dissociative adsorption.


Fig. 3. Oxygen-uptake curves on Cu(100) for 2.3 eV HOMB incidence (full circles) and ambient O2 exposure (dashed line). Regions A and B are indicated by arrows. A schematic representation of the CIA mechanism is shown in the inset.[54].

For Θ ≥ 0.5 ML (region B) the oxidation of Cu proceeds gradually, as seen in Figure 3. In this region, the penetration of O atoms into the subsurface and/or bulk occurs, which initiates the Cu2O growth. Further experiments to verify Cu2O formation have been performed on Cu(410), finding that thicker Cu2O films can be grown efficiently. Cu2O formation was confirmed by the features present in the valence-band spectra, as shown in Figure 4 (upper panel).[60] After a 2.2 eV HOMB dose at 300 K, the main peak gets narrower and becomes very similar to the one of bulk Cu2O.[61] The prominent decrease of the density of state at the Fermi level indicates the opening of a band gap and the formation of a semiconductor Cu2O thin film. Cu2O formation is also confirmed by L3M4,5 M4,5 Auger electron spectra (Figure 4, lower panel). From the O-uptake curve in Figure 3 we can calculate the O-coverage dependence of the sticking probability for dissociative adsorption of 2.3 eV O2 beams. It decreases from 0.03 to ∼10−5 in region A.[54] In region B, it is as small as ∼10−5. This rather slow process corresponds to the initial stage of copper oxide formation (vide ante). In the exposure range corresponding to region B no gradual increase of oxygen coverage was observed for thermal O2 exposure at ∼300 K. The oxidation efficiency is thus significant for HOMB doses and almost negligible for thermal O2 exposure, suggesting an important role of translational energy of the incoming molecules. We proposed[54] that a collision-induced absorption mechanism[62,63] is responsible for such a slow oxidation process of Cu (see the inset of Figure 3). In this mechanism, the direct energy transfer from incident hyperthermal O2 molecules to the pre-adsorbed O atoms and their surroundings induces their incorporation into the subsurface. We directly confirmed the occurrence of CIA by the increase of subsurface and/or bulk O

Chem. Rec. 2014, ••, ••–••



Fig. 4. Upper panel: valence-band SR-XPS spectra of Cu(410) clean (full blue) and covered by 2.07 ML O (dashed red) dosed by 2.2 eV HOMB at room temperature.[60,65] The spectra are measured at 70° from the surface normal. The thin dotted line corresponds to the XPS spectrum of bulk Cu2O.[61] The calculated valence-band peak positions of Cu2O are marked by vertical bars at 7.95, 6.70, 2.78, and 1.29 eV. Bottom panel: Cu L3M4,5 M4,5 Auger electron spectra measured at 70° from the surface normal after 2.2 eV HOMB incidence along the surface normal.[60,65] From bottom: clean surface, 1.74 ML prepared at room temperature. Spectra corresponding to bulk CuO (dashed line) and Cu2O (thin line) are also shown.[61]

atoms for the 3 eV Ar incidence on the 0.5 ML O atom precovered surface.[54] The O-1s component corresponding to subsurface and/or bulk O atoms increases with increasing Ar dose, suggesting the energy transfer in the inert gas collision induces the CIA process. In order to unravel the role of the local surface structures, such as defects, in the HOMB oxidation process, we performed O2 uptakes on the stepped Cu(410) and Cu(511) at 300 K using a 2 eV HOMB impinging along the surface normal.[60,64] In Figure 5 (left panel) we compare the O-uptake curves during 2 eV HOMB oxidation on Cu(410) and Cu(511) with that recorded for Cu(100)[54] (see the discussion of Figure 3). It is evident that a dependence of the uptake curves on crystal face symmetry is clearly present for Θ ≥ 0.5 ML and that the behavior of Cu(511) is intermediate between (410) and (100). The stepped surface readily accommodates O atoms into the subsurface in the CIA process due to the surface relaxation, compared to the flat surface. The open step facet of (110) for the (410) stepped surface can be relaxed more easily than the

Chem. Rec. 2014, ••, ••–••

close-packed step facet of (111) for the (511) stepped surface. These effects cause the difference in the uptake of O atoms in the CIA process. To further elucidate the mechanism underlying Cu2O formation on stepped Cu surfaces we also performed other experiments by dosing O2 at oblique incidence (see Figure 5 for details on the angles of incidence).[60,64] Indeed, the experiment shows that this scattering condition of the on-terrace incidence is more efficient for oxidation than the on-step-rise incidence. In CIA the important factor is the energy transferred to oxygen adatoms in the direction pointing towards the subsurface region. A closer inspection of the Cu(410) and Cu(511) geometry in Figure 5 shows that at an incidence angle θ = −30° (off normal towards the (100) nanoterrace), O2 molecules hit the O moiety pre-adsorbed at terrace sites at nearly the same angle as for θ = 0°. The larger cross section for oxide formation for θ = −30° indicates therefore that subsurface incorporation is favored when O adatoms move towards the fourfold hollow below the step. No traces of oxide formation are detected in O-1s and valence-band SR-XPS spectra, when dosing at Ei = 0.5 eV, indicating that this translational energy is not high enough even for the stepped surfaces to induce oxygen incorporation. The combination of the HOMB and the surface temperature control leads to the formation of various phases of Cu oxides. At the low temperature of ∼100 K, we obtained mixed CuO and Cu2O phases on Cu(410).[65] The antiferromagnetic CuO phase, which turns to Cu2O during annealing, is metastable. At high temperatures, the pyramid-like faceting structure is formed on Cu(511), which can be used as a nanoscale template in the fabrication of nanostructures.[64,66] 2.2. Protective Layer Formation on Cu3Au Figure 6 shows the O-uptake curves produced by the integration of O-1s spectra recorded after successive exposures of Cu3Au(100) at a surface temperature (Ts) of 300 K to HOMB at kinetic energies of 2.3, 0.6 and 0.3 eV and to thermal O2 backfilling.[55,67] For the latter dosing condition only 0.15 ML O atoms are dissociatively adsorbed on the surface even after massive exposure (∼9000 L). On the other hand, at 2.3 eV, a coverage of about 0.6 ML was obtained after a comparable exposure. Interestingly, decreasing the HOMB kinetic energy from 2.3 to 0.3 eV results in an O-uptake curve in between the thermal and 0.6 eV O2 doses. Thus, dissociative adsorption of O2 on the Cu3Au(100) surface is an activated process and an incident energy above 0.6 eV is enough to surmount the barrier. HOMB exposure induces structural changes to the Cu3Au(100) surface, where Cu atoms segregate and produce the O-adsorbed Cu layer. Schemes of the clean and the O-covered surfaces are shown in Figure 6. This scheme is supported by the low-energy electron diffraction observations and



THE CHEMICAL RECORD

Fig. 5. Left panel: O-uptake curves for HOMB at normal incidence on Cu(410) (full red circles), Cu(511) (full blue circles), and Cu(100) (open squares) at Ts = 300 K.[60,64] Incident energies are 2.3 eV for (100), and 2.2 eV for (511) and (410). Middle and right panels: in the off-normal incidence, O-uptake curves measured for Cu(511) and Cu(410) for on-terrace (full red circles) and on-step (open blue circles) rise incidence (see the top schematic representations) at 2.2 eV and for Ts = 300 K are compared. A top view of each stepped surface is shown in the inset.

Fig. 6. Oxygen-uptake curves on Cu3Au(100) for the 2.3 (full red circles), 0.6 (open black triangles) and 0.3 eV (full black triangles) HOMB incidence and for thermal O2 exposure (open red circles).[55] The uptake curve on Cu(100) for 2.3 eV HOMB incidence (full blue circles) is shown for comparison. The incidence direction is along the surface normal and the surface temperature is 300 K. The oxidation scheme is also shown.

the XPS measurements of the core levels of O, Cu and Au.[55,67] Moreover, this O-induced structure of Cu3Au(100) was also produced with an ion beam.[68] With respect to Cu(100) (see also the uptake curve reported in Figure 6 for comparison), dissociative adsorption on Cu3Au(100) has a higher activation barrier. Moreover, it did not occur on Au(111) even when using 2.3 eV HOMB,[69] suggesting that the activation barrier on Cu3Au(100) falls in


between those of the pure Cu and Au surfaces. The reduced reactivity (high activation barrier) may be explained by the shift of the d-band center due to Au alloying:[70] the dissociation barrier is higher for the isolated Cu atoms (possible O2 dissociation sites) surrounded by Au atoms than for the ones in a pure Cu surface, in analogy with that reported for the Au/Ni system.[71] When comparing the different behavior of Cu3Au and Cu at high O2 doses (≥1018 molecules·cm−2 in Figure 6) it is concluded that the first Cu–O layer and the Au-rich interface work as protection layers against bulk-like Cu2O formation. The former prevents the dissociative adsorption of gas-phase O2 and the latter reduces the diffusion of O atoms into the bulk by making the CIA process ineffective. A similar protective nature of an oxidized surface was also observed for a Cu3Au(111) surface, as shown in Table 1.[72] From the angular distribution of the Au 4f photoelectrons, we determined the layer profile of the oxidized surface. The first layer consists of the Cu–O, while the second and third layers contain ∼50% Au atoms. The Au layer profile demonstrates the strong protective nature against further bulk oxidation (the uptake curve is not shown here but supports the protective nature).

3. Molecular Orientation Effects The steric parameters of the incoming molecules in space are one of the most important controllable parameters in the

Chem. Rec. 2014, ••, ••–••



Table 1. The layer profile of Au atomic fraction (%) for clean and O-covered Cu3Au(111).[72] O coverage

1st layer

2nd layer

3rd layer

4th layer

0 0 0.48 0.5

51 50 0 0

32 25 47 50

bulk (25) bulk (25) 45 50

bulk (25) bulk (25) bulk (25) bulk (25)

Clean Cu3Au(111) O-covered Cu3Au(111)

Experimental measurements Theoretical calculations Experimental measurements Theoretical calculations

Fig. 7. Schematic representation of orientation and alignment.

molecular beam technique. It is expected that the so-called molecular orientation and alignment (see Figure 7) affect the surface chemical reactions via the differing overlap of the molecular orbitals of the incoming molecule and the surface. We can distinguish two types of steric effect, one depending on the orientation and the other depending on the alignment of the spatial distribution W(cos γs) of the molecular axis, with 1

∫ W (cos γ

s

) d (cos γ s ) = 1

(2)

Fig. 8. Evolution over time of the sticking probability measured for a vicinal Si(100) surface at a translational energy of 0.25 eV while alternating two of the three geometries shown in the inset.[76] In Si(100), two surface Si atoms form dimers and are shown as short bars.

−1

where γs is the orientation angle of the molecular axis with respect to the surface normal. The first moment of W(cosγs), the so-called orientation 〈cos γs〉, is defined as 1

∫ cos γ W (cos γ s

s

) d (cos γ s )

(3)

−1

This informs us about the importance of the difference between the head and tail orientations of the molecule for the collision and reaction dynamics. Alignment is defined as the second moment and accounts for the importance of the difference between the long-side (either the head or tail) and broadside approaches of the molecule. The alignment effects in surface chemical reactions were extensively studied by Rocca’s group[73] and by Kurahashi and Yamauchi.[74–77]

Chem. Rec. 2014, ••, ••–••

Kurahashi et al., combining a supersonic O2 beam with a magnetic hexapolar field, have developed a single spinrotational state selected O2 beam, for which both the molecular alignment and the spin direction relative to the magnetic field (H) are well defined.[74–77] Since the O2 axis is mainly perpendicular to the H direction, the three different geometries depicted in Figure 8 can be realized by controlling the H direction at the sample. They measured the O2 sticking probability on a Si(100)-(2 × 1) surface while modulating the O2 geometries, and observed clear differences among them.[76] The helicopter geometry results in the highest sticking probability, showing the preference for the side-on collision. The difference in the two cartwheel geometries indicates that O2 is more reactive when its axis is perpendicular to the Si dimer.[76] They also applied the alignment-resolved sticking experiment to the analysis of O2 adsorption on Al(111).[77] The dynamics of O2



THE CHEMICAL RECORD

adsorption on Al(111) remained unclear despite many years of research.[40] They clarified that O2 reacts with its axis nearly parallel to the surface at kinetic energies of less than 0.2 eV. Their results elucidated that the abstraction process, which occurs when the O2 axis is perpendicular to the surface, is a minor event at low-energy conditions. The first direct observation of molecular orientation effects was reported by Kleyn’s group[78] for NO scattering from an Ag surface. The angular distribution of the scattered molecules demonstrated that approach in the least attractive orientation, with the O end towards the surface, results in a higher adsorption probability than for N-end incidence.[78] The NO/Ag(111) system is weakly bound. These results can be understood by a strong anisotropic or orientation-dependent repulsion and the resulting preferential rotationally mediated adsorption for the O-end incidence.[79] The steric effect depends strongly on the substrates, i.e., the interaction potential. The N-end approach is more attractive than the O-end approach in the strongly bound NO/Pt(111) system.[80,81] The trapping probability is higher for the N-end approach. The highly reactive nature of the N-end approach has also been demonstrated for NO/Al(111), and is understood well with DFT calculations.[82] Molecular orientation effects in reactive systems were also investigated extensively by Heinzmann’s group, with measurements of the sticking probability and detection of gas-phase products on metal surfaces.[83] They found that the N-end approach is more reactive in the strongly bound NO/Ni(100)[83] and Pt(100)[84] systems, while in the reaction of oriented N2O with alkali-metal-adsorbed Pt(100) and Rh(100),[85] harpooning reactions occur preferentially with the O-end approach, resulting in the effective production of N2 molecules. For more complex polyatomic systems, Bernstein’s group systematically studied scattering of alkyl halide molecules from

highly oriented pyrolytic graphite (HOPG), and found steric effects with magnitudes depending on the alkyl chains and the halogens of the alkyl halide.[86–88] They demonstrated steric effects in the weak interaction of polyatomic molecules with more degrees of freedom than NO. The stereodynamics on surfaces were discussed by Auerbach and co-workers[37] and were reviewed in more detail by Vattuone et al.[73] 3.1. Steric Effects in CH3Cl/Si(100) This topic is illustrated by reference to steric effects appearing in polyatomic surface chemical reactions of CH3Cl/Si(100). The oriented CH3Cl beam can be obtained by hexapolar techniques,[89] which filter the rotational states in high purity. Although the development of laser selection of rotational states has recently been reported, hexapolar techniques are still the best way to investigate the reactive surface system. One of our apparatuses is shown in Figure 9. A CH3Cl molecule is a typical symmetric top molecule, one of the best candidates for testing the basic concepts of stereodynamics in surface chemical reactions. Figure 10 shows the orientation distribution function for the Cl-end in our apparatus.[89,90] A CH3Cl molecule adsorbs dissociatively on Si(100) via precursor states that have been considered to scramble the incoming molecular orientation effect. Steric effects of reactants have been reported in the collision dynamics and reactions for several systems.[73,81,91] In particular, the role of the incoming molecular orientation in the precursor-mediated adsorption on Si has been understood, suggesting the general importance of steric effects in the chemical reactions of organic molecules with Si.[90–94] Figure 11a shows the surfacetemperature dependence of initial sticking probability SRandom for the random-orientation CH3Cl of |JKM〉= |111〉 (full red circles) and |212〉 (open blue circles) states at Ei = 120 meV.[90]

Fig. 9. Schematic top view of the oriented molecular beamline and surface-reaction analysis chamber at Osaka University.[89] Shown are the pulsed valve (PV), the skimmer (SK), the beam collimator (C), the hexapole device (HP), the beam stop (BS), the guiding electrode (GE), the orientation electrode (OE), the King–Wells flag (HOPG), the sample (SA), and the quadrupole mass spectrometer (QM).


Chem. Rec. 2014, ••, ••–••



Fig. 10. Polar plots of orientation distribution for CH3Cl |111〉 (red) and |212〉 (blue), estimated from the experimental focusing curve.[90] The Cl-end distribution of CH3Cl is shown in the case of the indicated electric field. The black circle around the center corresponds to the random-orientation distribution.

Fig. 11. (a) Ts dependence of SRandom for CH3Cl |111〉 (full circles) and |212〉 (open circles) incident on Si(100) at Ei = 120 meV. (b) Ts dependence of SCl/SRandom and SCH3/SRandom for CH3Cl of |111〉 (full and open red circles, respectively) and |212〉 states (full and open blue squares, respectively), incident at Ei = 120 meV.[90].

Chem. Rec. 2014, ••, ••–••

Here, J, K and M are the typical rotational quantum numbers for the symmetric top molecule in an electric field.[89,90] Strong Ts dependence of SRandom suggests that the dissociative adsorption of CH3Cl occurs via precursors. The precursor-mediated adsorption is also supported by the result of no obvious incidence-angle dependence of SRandom. Figure 11b shows the Ts dependence of SCl/SRandom and SCH3/SRandom for the |111〉 (full and open red circles, respectively) and |212〉 (full and open blue squares, respectively) incidences at Ei = 120 meV, where SCl and SCH3 are the initial sticking probabilities for the Cl-end and CH3-end approaches, respectively.[90] A value of 1 corresponds to no molecular orientation effects. If there is a contribution of the alignment effects, there should be a systematic offset from 1. Above 280 K, an obvious orientation effect appears and its onset agrees well with the decrease of SRandom. The molecular orientation effects appearing in the precursor-mediated adsorption can be understood by the molecular-orientation dependence of trapping into the precursor state. The correlation between the onset of the steric effect and the decrease of SRandom in Figure 11 suggests that the observed steric effect strongly couples with the desorption from the potential well of the precursor state. If the desorption occurs from the stabilized (thermalized) precursors, the memory of the initial incoming molecular orientation will be lost. Thus, the observed desorption occurs before the complete thermalization is established. This process is attributed to the transient trapping where the molecule is at the surface without equilibration of the degrees of freedom.[49] It is considered that desorption occurs in a transient trapping state before the molecule is eventually stabilized into a precursor state via energy dissipation processes of phonon and/or electron-hole-pair excitations.[95] It is expected that a transient-trapping molecule feels the molecular-orientation-dependent potential well. Such desorption may occur, possibly coupling with phonons, within a time scale of thermalization of ∼100 ps[95] just after being transiently trapped into the precursor potential well. An upperlevel (more weakly bound) molecule in the potential well desorbs more easily than a deeper (more strongly bound) one. According to theoretical calculations,[96,97] the stable precursor is weakly bound to the surface Si atom with its Cl end. Possible precursor geometries are shown in Figure 12. In both geometries, the Cl-end molecule is more attractive than the CH3-end one. Therefore, the Cl-end collision is expected to suffer more effective energy dissipation than the CH3-end collision. The Si dimer can buckle for adjusting the charge state according to the approach of the highly negative charge end of CH3Cl. This causes the further attractive interaction. During such physical processes of transient trapping, the successive chemical processes also contribute to the large steric effect. The molecular-orientation-dependent direct access to the dissociation channel during the transient trapping process enhances the observed steric effect. Chemical reaction path



THE CHEMICAL RECORD

Fig. 12. Two possible precursors for dissociative adsorption of CH3Cl on Si(100).[96,97].

selection may be possible in such a process, depending on the molecular orientation.[92] It is quite interesting that when the |212〉 state at Ei = 120 meV was dosed on the surface, no large orientation effects were observed, as shown in Figure 11. According to theoretical calculations,[96,97] one of the possible precursors is almost parallel to the surface, with CH3 between the two dimer rows and Cl close to the lower Si of a dimer, as shown in Figure 12. The average orientation is closer to the surface parallel for the |212〉 state (〈cos γs〉 = 0.33) than the |111〉 state (〈cos γs〉 = 0.47). Moreover, from the significant contribution of the average alignment relative to the average orientation, the rotational motion of the |212〉 state is also closer to the stable precursor geometry and thus the orientation effect is smeared out effectively by the steering effect[93] in the attractive well of the precursor state. 3.2. Steric Effects in CH3Cl Scattering from HOPG and Si(111) In Section 3.1, we proposed that the transient trapping into the weakly bound precursor states and also possibly the chemical interactions play key roles in the observed steric effects. In order to elucidate the role of the weak interaction potential, we performed scattering experiments for the CH3Cl/HOPG and Si(111) system. The CH3Cl/HOPG system is a weakly bound physisorption system,[98] while CH3Cl/Si(111) is a weakly bound physisorption/chemisorption system.[99] As we mentioned above, Bernstein et al. also studied extensively the steric effects in the interaction between alkyl halide molecules and HOPG.[86–88] Below 100 K, physisorption occurred on HOPG and its growth kinetics were discussed in detail.[100] On the other hand, above 100 K, no stable adsorption layer was formed on HOPG. The steric effect appears in the direct inelastic scattering for the 320 meV oriented CH3Cl incidence on HOPG and the CH3-end approach is scattered more strongly than the Cl-end approach. From the time-of-flight (TOF) measurements,[101] we plot the stereo-asymmetric factor (R) defined as


Fig. 13. Ts dependence of steric asymmetry for CH3Cl scattered from HOPG.[100,101].

R=

I CH3 − I Cl I random

(4)

in Figure 13, where I CH3 , ICl and Irandom correspond to the intensities upon integrating the TOF spectra for the direct inelastic scattering of the CH3-end, Cl-end, and random orientation, respectively. In these measurements, we measured the TOF spectra at a fixed angle of θi + θs = 90°, where θi (θs) is the angle enclosed by the surface normal and the incoming (scattered) beam. Positive values of R were mostly observed for the present experimental conditions of surface temperatures and scattering geometries. That is, the trapping probability of the Cl-end molecule is higher than the CH3-end molecule. Similar molecular-orientation dependence was also reported by Bernstein et al. Furthermore, the Ts dependence of R is negligibly small within our experimental accuracy. The interactions between a permanent dipole and its image dipole contribute to the scattering of CH3Cl on the HOPG surface in addition to the van der Waals dispersion interaction. It is expected that the dipole versus image-dipole attractive interaction depends strongly on the molecular orientation of an incoming molecule.[88] The Cl-end approach induces more attractive interaction than the CH3-end approach because of the higher charge density of Cl. Thus, the Cl-end collision suffers larger energy dissipation due to the acceleration near the surface than the CH3-end collision. As a result, the surface-trapping probability will be higher for the Cl-end collision. In a series of works by Bernstein and co-workers, the trapping probability was always higher for the side of the highest charge density.[86–88] The interaction between a permanent dipole and its image dipole may be described with a simple point-charge model. In the CF3H scattering from the

Chem. Rec. 2014, ••, ••–••



HOPG surface, such a simple approximation succeeded in explaining the observed steric effects.[88] A similar trend was also observed in the scattering of 320 meV CH3Cl |111〉 from the Si(111) surface.[94] The scattering intensity for the CH3-end collision is higher than that for the Cl-end collision. The steric effect appears on the direct inelastic scattering. The sign of R is positive for the specular scattering. These results confirm that the surface-trapping probability is higher for the Cl-end collision than for the CH3-end collision. The incoming CH3Cl molecule in the oblique incidence interacts with the outermost dangling bonds of the adatoms preferably on Si(111). Therefore, it is expected that the interaction may depend on the molecular orientation. The negatively charged Cl end induces more attractive interaction in the approach to the electrophilic Si adatoms than the positively charged CH3 end. Thus, it is expected that the Cl-end collision will suffer larger energy dissipation due to the acceleration in the more attractive potential well than the CH3-end collision. The permanent-dipole approach will induce further charge redistribution both on the Si surface and CH3Cl, leading to the most stable interaction. The charge redistribution among the adatoms and the rest atoms will occur in the CH3Cl approach, depending on the molecular orientation of an incoming molecule. Although the higher trapping in the Cl-end collision on Si(111) is consistent with molecular-orientation dependence of the initial sticking probability reported for CH3Cl on Si(100),[90] the observed steric effect is much larger on Si(100) than on Si(111). The difference may be characterized by the chemical interactions. On Si(100), it may be possible that the molecular-orientation-dependent direct access to the dissociation channel of the C–Cl bond during the transient trapping process enhances the observed steric effect.

3.3. Steric Effects in NO/Si(111) The stereodynamics of NO on a surface have been a particular target of several studies using various experimental approaches (vide ante).[78–85] In these experiments, little attention was given to the resulting surface products. However, it should be noted that elucidation of the molecular-orientation effects in the reaction products on the surface is necessary in order to completely understand the surface reactions. The total flux of incident NO molecules is divided into the fluxes of scattered and reacted NO. The flux of reacted NO can be further divided into the fluxes of the gas-phase products and the surface products. Therefore, we can expect that the molecular-orientation dependence found in the scattered NO need not be reversed in the surface products. Thus, we perform the direct observation of the surface products created by the oriented NO beam.[91,102,103] Figure 14 shows the orientation dependence of the O-1s and N-1s XPS spectra, measured at ΘTotal ≈ 0.28 ML for the 58 meV oriented NO |0.5 0.5 0.5〉 beam at Ts = 400 K. It is clear that an N-end collision is more reactive than an O-end collision. Figure 15 shows the total coverage ΘTotal dependence of the stereo-asymmetric factor R. Here, we define R as follows

R=

I N − IO 0.5 ( I N + IO )

(5)

where IN and IO correspond to the integrated O-1s XPS intensities at the same exposures of the N-end and O-end beams, respectively. Here t0

I i (t 0 ) = F ∫ Si (t ) dt 0

Fig. 14. Orientation dependence of the O-1s and N-1s XPS spectra for 3 min exposures of 58 meV oriented NO beams at Ts = 400 K. The N-end and O-end collisions are indicated by solid orange squares and open blue triangles, respectively. The spectra correspond to θTotal ≈ 0.28 ML.[103].

Chem. Rec. 2014, ••, ••–••



(6)

THE CHEMICAL RECORD

Fig. 15. Steric asymmetry parameter R as a function of ΘTotal at Ts = 330 K (triangles), 400 K (circles) and 600 K (squares).[103] The ΘTotal for exposure of randomly oriented NO is used for the plot of R. The possible interaction potential of NO/Si(111) is also shown.

where Si(t), with i = N or O, is the reactive sticking probability for the N- or O-end collision, respectively. In Figure 15, we plot R as a function of ΘTotal obtained at the same exposure of random-orientation NO. We obtained the following insights: (i) R is positive and, thus, the N-end collisions are more reactive than the O-end collisions. (ii) R depends on Ts. R becomes smaller at Ts = 600 K, while ΘN/ΘO, revealing no steric effects, increases from 1 to 1.2 upon increasing Ts from 400 to 600 K. The anisotropy in ΘN/ΘO may come from the direct reactions between stabilized NO precursors formed on the adatom and the nearest rest atom. (iii) R at Ts = 400 K is larger than at the other Ts compared at lower ΘTotal. Results (i–iii) are also supported by the N-1s measurements. The strong Ts dependence of the uptake curves[103] suggests that the precursor-mediated reaction is a dominant process in NO dissociative adsorption on Si(111). The smaller R at 600 K suggests little contribution of the direct dissociative adsorption to the steric effect. In the precursor-meditated reaction,[104] two kinds of precursor potentials to the dissociative adsorption can be considered. One is a physisorption well with a typical depth of ∼130 meV,[105] and the other is a molecular chemisorption well. The “atop” state (Si–N bonding), with a binding energy of ca. 0.6–0.8 eV, is the most possible molecularly chemisorbed state leading to dissociation.[106] If only trapping into the physisorption well contributes to the observed steric effect, we would expect a negative R due to the rotational excitation of NO.[107] In the present study, the positive R suggests an additional contribution from the chemisorption well, as expected from the potential diagram shown in Figure 15. The chemisorption well for the N-end approach is deeper than that for the O-end approach, resulting in the higher reactivity.


The moderate orientation-dependent depth of the molecular chemisorption potential contributes mainly to the steric effect, depending on how far the first contact position is from the final chemisorption site on Si(111). The interaction is more attractive for the N-end approach.[108] This orientation dependence causes the observed steric asymmetry of S at Ts = 330 and 400 K. Elevating the Ts to 600 K enhances the probability of desorption from transient weakly bound states, while direct and strongly bound state mediated dissociative adsorption remains due to the strong attractive interaction. Both of these effects lead to a smearing out of steric effects at low energies due to the steering effect.[109,110] The rotational steering into the preferred orientation may be dominant, as seen in the CH3Cl/ Si(100) system.[93] Thus, orientation effects become smaller at Ts = 600 K. Steric effects depending strongly on Ts and Ei appear on trapping into a transient precursor state in the CH3Cl/Si(100) system, as shown in Section 3.1 and reference [90]. The largest R is observed at Ts = 400 K, as shown in Figure 15, despite the monotonic decrease of S with increasing Ts, which suggests a similar strong dependence of the steric effect on Ts.

4. Summary and Outlook I have reviewed our recent studies on the chemical reactions of single-crystalline Cu and Si surfaces by hyperthermal oxygen molecular beams and oriented molecular beams, respectively, and present the following conclusions. (1) Oxidation of various faces of Cu by HOMB was investigated with high-resolution XPS with SR. The advantage

Chem. Rec. 2014, ••, ••–••



in oxide formation by HOMB dosing is clearly demonstrated and attributed to the high translational energy available in the gas–surface interaction. Copper oxide formation on lowindexed Cu surfaces is induced mainly by CIA operating with HOMB irradiation. The efficiency of oxide formation is quite sensitive to the crystal face. The stepped surface is more effective in oxide formation via CIA. We have demonstrated that nearly perfect Cu2O films can be grown on Cu(410) and Cu(511) by HOMB, even at ∼300 K. These HOMB-induced oxide-formation processes are important for fabricating copper oxide at and below RT, which is quite important for device technology. Moreover, HOMB also has great potential for fabricating various metastable phases for the synthesis of new nanostructured materials. We have demonstrated this by producing the interesting CuO phase in the Cu2O phase at low temperature using HOMB.[65] (2) I have also reported our recent studies on chemical reactions of single-crystalline Si surfaces induced by oriented molecular beams in comparison with the steric effects appearing on the HOPG surface. Steric effects were found in the dissociative adsorption of NO on Si(111) and CH3Cl on Si(100) surfaces. Trapping into a shallower precursor well plays an important role in the appearance of the observed steric effects. This conclusion is supported by the observation of steric effects in scattering processes in weakly bound systems of CH3Cl on Si(111) and HOPG. Oriented molecular beams demonstrate the possibility of understanding and controlling surface chemical reactions in detail and hint at new ways of material fabrication on Si surfaces. Many kinetic studies of these chemical reactions have been carried out at a well-controlled temperature, viz., under conditions of thermal equilibrium. The measured thermal reaction rate constants refer to an average over all accessible reactant

states weighted by the populations of those states at the corresponding temperature. Such averages smear detailed information about which factors really cause the reaction to proceed. Thus, what we need for detailed understanding of the chemical reactions is the ability to examine the individual processes with the energetic (and orientation) states of the reactants.[111] Our studies introduced in this Personal Account are one of the present and also future directions in this context. In spite of the growing number of recent investigations, the influence of stereodynamics on gas–surface interactions is still a young topic that is likely to reach significant new achievements in the near future. New frontiers can be explored using newly developed experimental techniques[112–114] and increasing computing power, enabling us to perform fully multidimensional ab initio calculations of the potential energy surface (PES) and the wavepacket motion. When talking about the perspectives of investigations aimed at understanding the stereodynamics at surfaces, the system under investigation should be able to be extended to more complex ones in the future, i.e., model catalytic systems, biomolecular systems, and so on. Figure 16 shows our new apparatus combining the oriented molecular beam and infrared reflection–absorption spectroscopy, aimed at allowing us to interpret the stereodynamics in more complex surface chemical reactions in the near future. On the other hand, when talking about the perspectives for investigations aiming at using these tools for the growth of ordered layers, the one direction may be what I demonstrated with respect to the copper oxide formation. In conclusion, I have reported an overview of our recent works in the field of gas–surface interactions, focusing on our experiments of stereodynamics exploiting the orientation of polar molecules achieved by hexapole devices and oxide layer growth using HOMB. I hope to have offered with this review

Fig. 16. Schematic top view of the setup. The sample, optics for FTIR, QMS and oriented molecular beam are located at the same level. The LEED and ion gun are fixed 135 mm below the FTIR level. A polarizer is inserted in the incident beam. The solid red line from the interferometer to the MCT is the IR light path. Its incident angle is 80° and it is focused with gold-coated parabolic mirrors (not shown). The dashed blue line indicates the path of the oriented molecular beam. The sample is fixed at the center of the UHV chamber and the inlet and outlet for IR light are sealed with BaF2 windows. The light path outside of the UHV chamber is purged with dried air.[91].

Chem. Rec. 2014, ••, ••–••



THE CHEMICAL RECORD

a flavor of the many exciting possibilities of this field for the fundamental understanding of surface chemical reactions in complex systems.

Acknowledgements I thank Y. Teraoka, K. Moritani, M. Rocca, L. Vattuone, S. Goto, T. Fukuyama, T. Kasai and H. Ito for their contributions in the beam experiments and fruitful discussions. I also thank M. Kurahashi for his valuable suggestions. I gratefully acknowledge MEXT for a Grant-in-Aid for Scientific Research (Nos. 25620013, 2035005 and 22655005). This work was also financially supported by PRESTO of JST, the Mitsubishi Foundation, the Sumitomo Foundation and the Murata Science Foundation. The synchrotron radiation experiments were performed at the BL23SU in the SPring-8 facilities with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) and the Japan Atomic Energy Agency (JAEA).

REFERENCES [1] Atomic and Molecular Beam Methods, Vol. I (Eds.: G. Scoles, D. Bassi, U. Buck, D. Lainé), Oxford University Press, USA, 1988. [2] Atomic and Molecular Beam Methods, Vol. II (Eds.: G. Scoles, D. Lainé, U. Valbusa), Oxford University Press, USA, 1992. [3] J. A. Barker, D. J. Auerbach, Surf. Sci. Rep. 1984, 4, 1–99. [4] M. P. D’Evelyn, R. J. Madix, Surf. Sci. Rep. 1983, 3, 413–495. [5] J. Libuda, H. J. Freund, Surf. Sci. Rep. 2005, 57, 157–298. [6] A. W. Kleyn, in Dynamics (Eds.: E. Hasselbrink, B. I. Lundqvist), Elsevier, Amsterdam, 2008, pp. 29–52. [7] S. Yamamoto, R. E. Stickney, J. Chem. Phys. 1970, 53, 1594– 1604. [8] T. Kondo, H. S. Kato, T. Yamada, S. Yamamoto, M. Kawai, J. Chem. Phys. 2005, 122, 244713. [9] J. R. Manson, in Dynamics (Eds.: E. Hasselbrink, B. I. Lundqvist), Elsevier, Amsterdam, 2008, pp. 53–93. [10] Atomic and Molecular Beams: The State of the Art 2000 (Ed.: R. Campargue), Springer, Berlin/Heidelberg, 2001. [11] Helium Atom Scattering from Surfaces (Ed.: E. Hulpke), Springer, Berlin, 1992. [12] V. Bortolani, A. C. Levi, Riv. Nuovo Cimento Soc. Ital. Fis. 1986, 9, 1–77. [13] J. P. Toennies, in Surface Phonons (Eds.: W. Kress, F. W. de Wette), Springer, Heidelberg, 1991. [14] C. T. Rettner, J. Kimman, D. J. Auerbach, J. Phys. Chem. 1991, 94, 734–750. [15] V. Celli, in Many-Body Phenomena at Surfaces (Eds.: D. Langreth, H. Suhl), Academic Press, New York, 1984. [16] J. P. Toennies, Phys. Scr. 1987, T19, 39–46. [17] G. Benedek, J. P. Toennies, Surf. Sci. 1994, 299, 587–611. [18] B. Gumhalter, Phys. Rep. 2001, 351, 1–159.


[19] G. R. Darling, in Dynamics (Eds.: E. Hasselbrink, B. I. Lundqvist), Elsevier, Amsterdam, 2008, pp. 141–195. [20] G. O. Sitz, in Dynamics (Eds.: E. Hasselbrink, B. I. Lundqvist), Elsevier, Amsterdam, 2008, pp. 197–229. [21] W. A. Diño, H. Kasai, A. Okiji, Prog. Surf. Sci. 2000, 63, 63–134. [22] J. W. Gadzuk, in Dynamics (Eds.: E. Hasselbrink, B. I. Lundqvist), Elsevier, Amsterdam, 2008, pp. 1–28. [23] A. M. Wodtke, D. Matsiev, D. J. Auerbach, Prog. Surf. Sci. 2008, 83, 167–216. [24] N. Shenvi, S. Roy, J. C. Tully, Science 2009, 326, 829–832. [25] B. Gergen, H. Nienhaus, W. H. Weinberg, E. W. MacFarland, Science 2001, 294, 2521–2523. [26] K. H. Kramer, R. B. Bernstein, J. Chem. Phys. 1965, 42, 767–770. [27] P. R. Brooks, Science 1976, 193, 11–16. [28] S. Stolte, J. Reuss, H. L. Schwartz, Physica 1973, 66, 211–216. [29] R. N. Zare, Ber. Bunsen-Ges. 1982, 86, 422–425. [30] B. Friedrick, D. R. Herschbach, Nature 1991, 353, 412– 414. [31] H. G. Bennewitz, W. Paul, Ch. Schlier, Z. Phys. 1955, 141, 6–15. [32] H. G. Bennewitz, K. H. Kramer, W. Paul, J. P. Toennies, Z. Phys. 1964, 177, 84–110. [33] J. P. Toennies, Z. Phys. 1965, 182, 257–277. [34] R. B. Bernstein, D. R. Herschbach, R. D. Levine, J. Phys. Chem. 1987, 91, 5363–5377. [35] H. J. Loesch, J. Bulthuis, S. Stolte, A. Durand, J. C. Loison, J. Vigué, Europhys. News 1996, 27, 12–15. [36] P. R. Brooks, Int. Rev. Phys. Chem. 1995, 14, 327–354. [37] H. Hou, S. J. Goulding, C. T. Rettner, A. M. Wodtke, D. J. Auerbach, Science 1997, 277, 80–82. [38] E. B. D. Bourdon, P. Das, I. Harrison, J. C. Polanyi, J. Segner, C. D. Stanners, R. J. Williams, P. A. Young, Faraday Discuss. Chem. Soc. 1986, 82, 343–358. [39] St. J. Dixon-Warren, E. T. Jensen, J. C. Polanyi, G.-Q. Xu, S. H. Yang, H. C. Zeng, Faraday Discuss. Chem. Soc. 1991, 91, 451–463. [40] A. J. Komrowski, J. Z. Sexton, A. C. Kummel, M. Binetti, O. Weisse, E. Hasselbrink, Phys. Rev. Lett. 2001, 87, 246103. [41] J. A. Jensen, C. Yan, A. C. Kummel, Science 1995, 267, 493– 496. [42] L. Casalis, M. F. Danisman, B. Nickel, G. Bracco, T. Toccoli, S. Iannotta, G. Scoles, Phys. Rev. Lett. 2003, 90, 206101. [43] L. Vattuone, P. Gambardella, U. Burghaus, F. Cemic, A. Cupollilo, U. Valbusa, M. Rocca, J. Chem. Phys. 1998, 109, 2490–2502. [44] C. T. Rettner, H. A. Michelsen, D. J. Auerbach, Phys. Rev. Lett. 1992, 68, 1164–1167. [45] A. Al-Halabi, A. W. Kleyn, G. J. Kroes, Chem. Phys. Lett. 1999, 307, 505–510. [46] Y. Teraoka, I. Nishiyama, Appl. Phys. Lett. 1993, 63, 3355– 3357. [47] S. T. Ceyer, Science 1990, 249, 133–139. [48] T. Miyake, S. Soeki, H. Kato, T. Nakamura, A. Namiki, H. Kamba, T. Suzaki, Phys. Rev. B 1990, 42, 11801–11807.

Chem. Rec. 2014, ••, ••–••



[49] A. Raukema, A. W. Kleyn, Phys. Rev. Lett. 1995, 74, 4333– 4336. [50] A. Raukema, D. A. Butler, F. M. A. Box, A. W. Kleyn, Surf. Sci. 1996, 347, 151–168. [51] A. Raukema, D. A. Butler, A. W. Kleyn, J. Phys.: Condens. Matter 1996, 347, 2247–2263. [52] A. Raukema, D. A. Butler, A. W. Kleyn, Surf. Sci. 1996, 363, 29–41. [53] A. Raukema, D. A. Butler, A. W. Kleyn, J. Chem. Phys. 1997, 106, 2477–2491. [54] M. Okada, K. Moritani, S. Goto, T. Kasai, A. Yoshigoe, Y. Teraoka, J. Chem. Phys. 2003, 119, 6994–6997. [55] M. Okada, M. Hashinokuchi, M. Fukuoka, T. Kasai, K. Moritani, Y. Teraoka, Appl. Phys. Lett. 2006, 89, 201912. [56] T. Takaoka, T. Komeda, Phys. Rev. Lett. 2008, 100, 046104. [57] Y. Teraoka, A. Yoshigoe, Jpn. J. Appl. Phys. 1999, 38 (Suppl. 1), 642–645; Y. Teraoka, A. Yoshigoe, Appl. Surf. Sci. 2001, 169– 170, 738–741. [58] Y. Saitoh, K. Nakatani, T. Matsushita, A. Agui, A. Yoshigoe, Y. Teraoka, A. Yokoya, Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 474, 253–258. [59] J. R. Waldram, Superconductivity of Metals and Cuprates, IOP Publishing Ltd., London, 1996. [60] M. Okada, L. Vattuone, A. Gerbi, L. Savio, M. Rocca, K. Moritani, Y. Teraoka, T. Kasai, J. Phys. Chem. C 2007, 111, 17340–17345. [61] J. Ghijsen, L. H. Tjeng, J. van Elp, H. Eskes, M. T. Czyzyk, Phys. Rev. B 1988, 38, 11322–11330. [62] L. Vattuone, U. Burghaus, L. Savio, M. Rocca, G. Costantini, F. Buatier de Mongeot, C. Boragno, S. Rusponi, U. Valbusa, J. Chem. Phys. 2001, 115, 3346–3355. [63] K. J. Maynard, A. D. Johnson, S. P. Daley, S. T. Ceyer, Faraday Discuss. Chem. Soc. 1991, 91, 437–449. [64] M. Okada, L. Vattuone, M. Rocca, Y. Teraoka, J. Chem. Phys. 2012, 136, 094704. [65] M. Okada, L. Vattuone, K. Moritani, L. Savio, Y. Teraoka, T. Kasai, M. Rocca, Phys. Rev. B 2007, 75, 233413. [66] A. N. Chaika, S. S. Nazin, S. I. Bozhko, Surf. Sci. 2008, 602, 2078–2088. [67] M. Okada, K. Moritani, T. Fukuyama, H. Mizutani, A. Yoshigoe, Y. Teraoka, T. Kasai, Surf. Sci. 2006, 600, 4228– 4232. [68] H. Niehus, C. Achete, Surf. Sci. 1993, 289, 19–29. [69] No O-1s intensity in SR-XPS spectra was observed on Au(111) even after doses of ca. 1019–1020 O2 molecules/cm2 with a 2.3 eV HOMB. [70] B. Hammer, J. K. Nørskov, in Impact of Surface Science on Catalysis (Eds.: B. C. Gates, H. Knözinger), Academic Press, San Diego, 2000, pp. 71–129. [71] F. Besenbacher, I. Chorkendorff, B. S. Clausen, B. Hammer, A. M. Molenbroek, J. K. Nørskov, I. Stensgaard, Science 1998, 279, 1913–1915. [72] Y. Tsuda, K. Oka, M. Okada, W. A. Diño, M. Hashinokuchi, A. Yoshigoe, Y. Teraoka, H. Kasai, Phys. Chem. Chem. Phys. 2014, 16, 3815–3822.

Chem. Rec. 2014, ••, ••–••

[73] L. Vattuone, L. Savio, F. Pirani, D. Cappeletti, M. Okada, M. Rocca, Prog. Surf. Sci. 2010, 85, 92–160. [74] M. Kurahashi, Y. Yamauchi, Rev. Sci. Instrum. 2009, 80, 083103. [75] M. Kurahashi, Y. Yamauchi, Phys. Rev. B 2012, 85, 161302. [76] M. Kurahashi, Y. Yamauchi, J. Chem. Phys. 2014, 140, 031102. [77] M. Kurahashi, Y. Yamauchi, Phys. Rev. Lett. 2013, 110, 246102. [78] E. W. Kuipers, M. G. Tenner, A. W. Kleyn, S. Stolte, Nature 1988, 334, 420–422. [79] F. H. Geuzebroek, A. E. Wiskerke, M. G. Tenner, A. W. Kleyn, S. Stolte, A. Namiki, J. Phys. Chem. 1991, 95, 8409–8421. [80] E. W. Kuipers, M. G. Tenner, A. W. Kleyn, S. Stolte, Phys. Rev. Lett. 1989, 62, 2152–2155. [81] A. W. Kleyn, Prog. Surf. Sci. 1997, 54, 407–420. [82] A. J. Komorowski, H. Ternow, R. Razaznejad, B. Berenbak, J. Z. Sexton, I. Zoric, B. Kasemo, B. I. Lundqvist, S. Stolte, A. W. Kleyn, A. C. Kummel, J. Chem. Phys. 2002, 117, 8185–8189. [83] G. H. Fecher, N. Böwering, M. Volkmer, B. Pawlitzky, U. Heinzmann, Surf. Sci. 1990, 230, L169–L172. [84] M. Brandt, H. Müller, G. Zagatta, O. Wehmeyer, N. Böwering, U. Heinzmann, Surf. Sci. 1995, 331–333, 30–34. [85] M. Brandt, T. Gerber, N. Böwering, U. Heinzmann, Phys. Rev. Lett. 1998, 81, 2376–2379. [86] T. J. Curtiss, R. B. Bernstein, Chem. Phys. Lett. 1989, 161, 212–218. [87] T. J. Curtiss, R. S. Mackay, R. B. Bernstein, J. Chem. Phys. 1990, 93, 7387–7405. [88] I. V. Ionova, S. I. Ionov, R. B. Bernstein, J. Phys. Chem. 1991, 95, 8371–8376. [89] M. Okada, K. Moritani, S. Goto, T. Kasai, Jpn. J. Appl. Phys. 2005, 44, 8580–8589. [90] M. Okada, S. Goto, T. Kasai, Phys. Rev. Lett. 2005, 95, 176103. [91] M. Okada, J. Phys.: Condens. Matter 2010, 22, 263003. [92] M. Okada, S. Goto, T. Kasai, J. Am. Chem. Soc. 2007, 129, 10052–10053. [93] M. Okada, S. Goto, T. Kasai, J. Phys. Chem. C 2008, 112, 19612–19615. [94] H. Ito, M. Okada, T. Kasai, J. Phys. Chem. A 2010, 114, 3080–3086. [95] W. Mönch, Semiconductor Surfaces and Interfaces, Springer, Berlin, 1993. [96] A. H. Romero, C. Sbraccia, P. L. Silvestrelli, F. Ancilotto, J. Chem. Phys. 2003, 119, 1085–1092. [97] M. Preuss, W. G. Schmidt, F. Bechstedt, J. Phys. Chem. B 2004, 108, 7809–7813. [98] J. Wilkes, C. L. A. Lamont, L. Siller, J. M. Coquel, R. E. Palmer, Surf. Sci. 1997, 390, 237–242. [99] R. Küster, K. Chritmann, Ber. Bunsen-Ges. 1997, 101, 1799– 1810. [100] M. Hashinokuchi, T. Fukuyama, M. Okada, T. Kasai, Phys. Chem. Chem. Phys. 2011, 13, 6584–6589. [101] T. Fukuyama, M. Okada, T. Kasai, J. Phys. Chem. A 2009, 113, 14749–14754.



THE CHEMICAL RECORD

[102] M. Okada, M. Hashinokuchi, K. Moritani, T. Kasai, Y. Teraoka, Jpn. J. Appl. Phys. 2008, 47, 3686–3691. [103] M. Hashinokuchi, M. Okada, H. Ito, T. Kasai, K. Moritani, Y. Teraoka, Phys. Rev. Lett. 2008, 100, 256104. [104] W. H. Weinberg, in Kinetics of Interface Reactions (Eds.: M. Grunze, H. J. Kreuzer), Springer, New York, 1987, pp. 94–124. [105] C. E. Brown, P. G. Hall, J. Colloid Interface Sci. 1973, 42, 334–341. [106] Z. C. Ying, W. Ho, J. Chem. Phys. 1989, 91, 2689–2705. [107] M. G. Tenner, E. W. Kuipers, A. W. Kleyn, S. Stolte, Surf. Sci. 1991, 242, 376–385. [108] Z. Su, X. Lu, Q. N. Zhang, Chem. Phys. Lett. 2003, 375, 106–112.


[109] C. W. Muhlhausen, L. R. Williams, J. C. Tully, J. Chem. Phys. 1985, 83, 2594–2606. [110] A. Groß, Theoretical Surface Science, Springer, Berlin, 2002. [111] R. D. Levine, Molecular Reaction Dynamics, Cambridge University Press, Cambridge, 2005. [112] S. De, I. Znakovskaya, D. Ray, F. Anis, N. G. Johnson, I. A. Bocharova, M. Magrakvelidze, B. D. Esry, C. L. Cocke, I. V. Litvinyuk, M. F. Kling, Phys. Rev. Lett. 2009, 103, 153002. [113] S. Y. T. van de Meerakker, H. L. Bethlem, N. Vanhaecke, G. Meijer, Chem. Rev. 2012, 112, 4828–4878. [114] T. Schäfer, N. Bartels, N. Hocke, X. Yang, A. M. Wodtke, Chem. Phys. Lett. 2012, 535, 1–11.

Chem. Rec. 2014, ••, ••–••


Growth dynamics in supersonic molecular beam deposition of pentacene sub-monolayers on SiO2.

Cluster chemical ionization and deuterium exchange mass spectrometry in supersonic molecular Beams.

Surface Nano-Structuring by Adsorption and Chemical Reactions.

A microvascular system for chemical reactions using surface waste heat.

[Quantum chemical experiments on drug receptor complexes].

Fingerprints of energy dissipation for exothermic surface chemical reactions: O2 on Pd(100).

Tracking chemical reactions on the surface of filamentous phage using mass spectrometry.

Surface excitations in the modelling of electron transport for electron-beam-induced deposition experiments.

Reaction efficiency effects on binary chemical reactions.

Electron impact mass spectrometry of alkanes in supersonic molecular beams.

Experiments on the formation of particles by chemical reactions in the dark and under influence of light.

Chemical reactions of conformationally selected 3-aminophenol molecules in a beam with Coulomb-crystallized Ca+ ions.

Intermolecular interaction in the H2S-H2 complex: molecular beam scattering experiments and ab-inito calculations.

Surface reactions of adhesives on dentin.

Two dimensional imaging of the virtual source of a supersonic beam: helium at 125 K.

My Research History on the Chemical Standpoint-From Molecular Structure to Surface Science.

Effect of Alkali-Acid-Heat Chemical Surface Treatment on Electron Beam Melted Porous Titanium and Its Apatite Forming Ability.

Comment on "the ocular surface chemical burns".

Integrating molecular dynamics simulations with chemical probing experiments using SHAPE-FIT.

Knudsen cell mass spectrometry using restricted molecular beam collimation. I. Optimization of the beam from the vaporizing surface.

Standardization of RNA chemical mapping experiments.

Acceptor-compensated charge transport and surface chemical reactions in Au-implanted SnO₂ nanowires.

Thermodynamic coupling in chemical reactions.

Influence of exothermic chemical reactions on laser-induced shock waves.