J. Phys. Chem. C 2007, 111, 9961-9967
9961
Effects of Vibrational and Rotational Excitations on the Dissociative Adsorption of O2 on Cu Surfaces Kousuke Moritani,*,† Muneyuki Tsuda,‡ Yuden Teraoka,† Michio Okada,§,| Akitaka Yoshigoe,† Tetsuya Fukuyama,§ Toshio Kasai,§ and Hideaki Kasai‡ Synchrotron Radiation Research Center, Japan Atomic Energy Agency, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan, Department of Applied Physics, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, Department of Chemistry, Graduate School of Science, Osaka UniVersity, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan, and PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan ReceiVed: March 7, 2007; In Final Form: April 25, 2007
We report an X-ray photoemission study of the dissociative adsorption of O2 on Cu(111), (001), and (110) surfaces with an O2 molecular beam generated with a variable temperature nozzle. The O-uptake curves, which are produced from precisely measured O-1s peaks, indicate that the dissociative absorption is enhanced as the nozzle temperature is increased up to ∼1000 K for the normal incidence of O2 at a kinetic energy of 0.5 eV. However, further increasing the nozzle temperature Tn to 1400 K reduces the probability of dissociative adsorption. These results suggest that vibrational excitations of incident O2 assist dissociative adsorption while rotational excitations hinder it.
Introduction The oxidation process of transition metals plays a crucial role in various applications such as oxide layer formation,1 homogeneous catalysis,2 and corrosion.3 The dissociative adsorption of O2 on the surface is the first step of oxidation and, thus, must be elucidated. Among transition metals, copper is one of the most useful materials in practical applications because of its electrical and thermal conductivities. Thus, numerous experimental4-7 and theoretical8-10 studies have been devoted to elucidating oxygen adsorption on copper surfaces. However, little is known about the dynamical aspects of oxygen chemisorption on copper surfaces, but the kinetics of reaction and the resulting structures have been revealed. Recently, hyperthermal O2 molecular beam (HOMB)-induced oxidation of Cu(001),11,12 Cu(111),13,14 and Cu(110)15 surfaces at room temperature has been reported. In these studies, we demonstrated that oxygen molecules with a hyperthermal incident energy (Ei) can induce the effective dissociative chemisorption and the collision-induced absorption, suggesting that highly energetic O2 molecules can efficiently control copper oxidation. Oxygen chemisorbs dissociatively on copper at 300 K. It occurs irreversibly, thus the adsorbed oxygen cannot be easily removed at this temperature. Experimental studies using O2 molecular beams estimate that the activation energy for the dissociation of oxygen is around 0.2 eV on Cu(110)4 and around 0.1-0.2 eV on Cu(001).5,6 Experimental4-7 and theoretical8 studies indicate that the dissociation of O2 on Cu(001) and (110) surfaces is a direct process at higher energies, while a precursormediated mechanism plays a role at low energies around several * Corresponding author. E-mail:
[email protected]. Present address: Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2201. † Japan Atomic Energy Agency. ‡ Graduate School of Engineering, Osaka University. § Graduate School of Science, Osaka University. | PRESTO, Japan Science and Technology Agency.
tens meV. The closely packed Cu(111) is stable due to higher coordination numbers of a surface atom, suggesting a higher activation barrier on a (111) surface than on (001) and (110) surfaces. A theoretical study9 has estimated the activation barrier at the most favorable site and direction of dissociation on Cu(111) is 0.20 eV, but there is very little experimental information about the adsorption dynamics on Cu(111). In these activated adsorption systems, the translational energy of O2 can help overcome the activation barrier. Furthermore, the vibrationally excited states probably assist the dissociation in the direct process because a theoretical study using empirical potential surfaces on Cu(001) and Cu(110) indicates that the activation barrier exists as an exit channel (late barrier).8 Moreover, the rotational states may influence the dissociation process of O2 on the surface. It has been reported extensively that the vibrational excitations of an incident molecule play an important role at dissociative adsorption process.16-18 On the other hand, the role of rotational motion of a molecule has been studied in a few systems of H2 on Cu(111),19,20 C2H4, C3H6 on Ag(001),21-23 and O2 on bare and CO-precovered Pd(001).24 In this paper, we study the dissociative adsorption of oxygen on Cu(111), (110), and (001) using an HOMB at Ei ) 0.5 eV and ab initio calculations based on the density functional theory (DFT). At Ei ) 0.5 eV, O2 adsorbs dissociatively on the surface with no oxidation into bulk. The reaction kinetics depend strongly on the nozzle temperatures (Tn), even compared at the same kinetic energy. The efficiency of dissociative adsorption of O2 at Ei ) 0.5 eV is enhanced with elevating nozzle temperatures up to ∼1000 K in the O2 incidence along the surface normal. Further elevating Tn to 1400 K, however, suppresses the dissociative sticking. These experimental results can be interpreted by the vibrationally assisted and rotationally hindered adsorption processes. In addition, these results can be understood qualitatively based on the calculated two-dimensional (2D) potential energy surfaces (PESs), possessing the activation
10.1021/jp0718496 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007
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Figure 1. O-coverage dependence of the O-1s XPS spectra for the 0.5 eV HOMB incidence along the surface normal on Cu(001) (a), Cu(110) (b), and Cu(111) (c) at ∼300 K. O-1s spectra in (a) and (b) have a single Voigt component in all coverage regions, while the spectra in (c) have other components. The peak component at the lower binding energy side above Θ)0.27 ML corresponds to the 3 2 structure. (See text.) -1 2
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Figure 2. Nozzle temperature dependence of the O-uptake curves for the 0.5 eV HOMB incidence along the surface normal on Cu(001) (a), Cu(110) (b), and (111) (c). Uptakes are determined by the integration of O-1s XPS spectra. ML unit represents the number of atoms per surface Cu atom. Calculated kinetic energy of 0.5 eV is kept constant by changing the seeding ratio of He and/or Ar and the nozzle temperature.
barrier on the exit channel and a strong anisotropy depending on the O2 alignment. Experimental Section All experiments were performed with a surface reaction analysis apparatus (SUREAC 2000) constructed as an endstation of a soft-X-ray beam line, BL23SU in the SPring-8.25 Briefly, the surface reaction analysis chamber was equipped with an electron energy analyzer (OMICRON EA125) and a Mg/AlKR twin-anode X-ray source (OMICRON DAR400). A differentially pumped quadrupole mass spectrometer (QMS), which was used to analyze the molecular species in the HOMB, was equipped on the line of the molecular beam axis. A supersonic molecular beam apparatus was in conjunction with the soft-Xray synchrotron beamline. The details about the experimental apparatus were described elsewhere.26 A variable temperature nozzle, which consisted of pyrolytic boron nitride (PBN) and had φ0.1 mm aperture, was attached in the molecular beam apparatus. Nozzle temperatures of up to 1400 K were attained by resistive heating of the graphite ribbon
heater. Even at Tn ) 1400 K, there was no increase of mass signal at m/e ) 16 corresponding to the dissociated O atoms in the HOMB in the mass spectra measured by QMS. An oxygen molecular beam was seeded by He and/or Ar gas. Changing the seeding ratio and the nozzle temperature from room temperature to 1400 K controlled the kinetic energy of incident O2. The beam diameter at the sample position was ∼4 mm. The flux density of O2 molecules at the sample position was experimentally estimated at each experimental condition using an estimation method described elsewhere.27 Under our operating conditions, the typical flux density of O2 molecules at the sample surface was 3 × 1014, 8 × 1013, 5 × 1013 and 3 × 1013 molecules cm-2 s-1 at Tn ) 300, 940, 1200, and 1400 K, respectively. The background pressure and parasite oxygen pressure during the HOMB incidence were ∼5 × 10-6 and ∼5 × 10-8 Pa, respectively. The base pressure of the surface reaction chamber was below 3 × 10-8 Pa. The 15 × 15 mm2 copper samples were cleaned by repeatedly sputtering with 1 keV Ar+ and annealing at 870 K until impurities were not detected by X-ray photoelectron
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Figure 3. Nozzle temperature dependence of the sticking probability of oxygen for the 0.5 eV HOMB incidence along the surface normal on Cu(001) (a), Cu(110) (b), and (111) (c). Sticking probability is calculated from the oxygen uptake curves.
spectroscopy (XPS), and the low-energy electron diffraction (LEED) showed a sharp (1 × 1) pattern with low backgrounds. After irradiating with a proper amount of HOMB, highresolution O-1s XPS spectra were measured using synchrotron radiation with a photon energy of 886.8 eV. All experiments were performed at a sample temperature of ∼300 K. Theoretical Calculations We performed calculations on the basis of the DFT28,29 with the Becke-Perdew-Wang (B3PW91) exchange-correlation functional30,31 and the Dunning-Hay-Wadt basis sets,32-35 as implemented in the Gaussian 03 suite of programs.36 The substrate was constructed using 10 Cu atoms where seven and three atoms were set in the first and second layers, respectively. The Cu-Cu nearest neighbor distance in bulk Cu was fixed to 2.556 Å. To calculate the reaction path, the O-O distance r and the O2 center of mass distance z from the surface were varied, but the O-O axis was fixed parallel to the surface (sideon collision) or perpendicular to the surface (end-on collision). The reaction coordinate s was defined as the measuring distance from starting point along the path of the minimum potential energies on the 2D PESs. s began at a distance of 3.0 Å from the surface (s ) 0.0 Å, r ) 1.26 Å, and z ) 3.0 Å). Each of the PESs was relative to the total energy of the triplet O2 + Cu10. Results Figure 1a-c shows the oxygen-coverage (Θ) dependence of the representative O-1s XPS spectra in the 0.5 eV HOMB incidence with Tn ) 300 K on Cu(001), (110), and (111) surfaces at ∼300 K, respectively. The HOMB was irradiated along the surface normal. The O coverage (Θ) was determined by integrating the O-1s XPS peak. An experimentally determined Θ was calibrated using the 0.5 ML O atoms on Cu(100) with a sharp LEED pattern of Cu(100)-(2x2 × x2)R45°-O. On the Cu(100) surface, the O-1s peak, which was fitted well with a symmetric single component of Voigt function, shifted from a binding energy of 528.7 to 529.6 eV by 0.9 eV as the HOMB dose increased, while its full width at half-maximum (fwhm) decreased as Θ increased. The surface was saturated at Θ ) 0.5 ML where a sharp LEED pattern of the (2x2 × x2)R45° structure was observed. The shifting and narrowing of the peak corresponded to the transition from a poorly ordered phase to the highly ordered phase of the (2x2 × x2)R45°, which has a
Figure 4. Nozzle temperature dependence of the O-uptake curves for the 0.5 eV HOMB incidence along an incident angle of 40° from the surface normal on Cu(111). The inset is the sticking probabilities calculated from the uptake curves.
missing row structure.12 On the Cu(110) surface, the intensity of the O-1s peak, which is also fitted well with a symmetric single component of Voigt function, increased without any shifts in the binding energy. At Θ ∼ 0.5 ML, a sharp LEED pattern of a (2 × 1) added row structure was observed. On the (2 × 1) surface, the -Cu-O-Cu- chains along the direction were grown spontaneously during the dissociative adsorption of O2 and were completed at Θ ∼ 0.5 ML. On the Cu(111) surface, the intensity of the symmetric single O-1s peak increased without shifting the binding energy below Θ ∼ 0.27 ML at 529.4 eV. Further irradiating of O2 above Θ ∼ 0.27 ML induced another component at 528.9 eV. The O-1s component 3 2 structure.37 The at 528.9 eV corresponds to the -1 2 surface was saturated at Θ ∼ 0.4 ML using HOMB, where a sharp LEED pattern of the 3 2 structure was observed, -1 2 while the saturation coverage was Θ ∼ 0.3 ML with no ordered LEED patterns for a thermal O2 gas atmosphere.13 O2 is
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Figure 5. (a) Schematic representation of the model used to investigate O2 dissociative adsorption on Cu(111). (b) Calculated 2D PESs for triplet and quintet states of side-on O2/Cu(111) as a function of r and z. Contour spacing is 0.1 eV. (c) Calculated potential energy curves (PECs) as functions of reaction coordinate s.
chemisorbed dissociatively on all Cu faces, because no traces of O-1s peaks, corresponding to the surface O2 and the bulk-O components in XPS spectra, were observed for the 0.5 eV O2 incidence.38,39 Figure 2a-c shows the nozzle temperature dependence of the O-uptake curves during the incidence of the 0.5 eV HOMB on the Cu(001), (110), and (111) surfaces, respectively. Each uptake curve was produced from the integration of the O-1s XPS peaks. The HOMBs were irradiated along the surface normal direction. The kinetic energy of 0.5 eV has been kept constant by changing the seeding ratio of O2 in He and/or Ar at the different nozzle temperatures. The O-uptake curves in the dependence of the nozzle temperature on the Cu(001), (110), and (111) surfaces showed similar tendencies. Figure 3a-c shows the nozzle temperature dependence of the sticking probability, which is calculated from the oxygen uptake curves. Increasing the nozzle temperature from 300 to ∼1000 K enhanced the sticking of oxygen. However, further increasing the temperature up to 1400 K suppressed the sticking compared to 940 K, although it is higher than that at Tn ) 300 K. These results suggest that the vibrational and rotational states of an oxygen molecule, which is excited by the high-temperature nozzle, contribute strongly to the dissociative adsorption process on copper surfaces. Figure 4 shows the O-uptake curves and calculated sticking probability for 0.5 eV HOMB generated at Tn ) 940 and 1400 K on Cu(111) surface. The HOMB was irradiated along an incidence angle of 40° from the surface normal. As shown in Figure 3c, heating at Tn ) 940 K enhanced the dissociative
adsorption of oxygen, but further heating at Tn ) 1400 K inhibited it to less than that at 940 and 1200 K with the HOMB incidence along the surface normal direction. On the other hand, in the case of the exposure along the incidence angle of 40°, the uptake curve for Tn ) 1400 K did not exhibit a hindering effect compared to that for Tn ) 940 K. To describe the PESs, which dominated the reaction, we performed calculations on the basis of DFT. Figure 5 shows the calculated 2D PESs where O2 approached along the surface normal direction with the molecular axis parallel to the surface (side-on collision) and the two constituent atoms of O2 dissociatively adsorb on Cu(111) surface. A side-on collision was the most favorable alignment of O2 in our calculation. The activation barrier at the crossing point between the triplet and quintet states existed in the exit channel for adsorption (late barrier). Thus, O2 had an elongated interatomic distance in the transition state (TS). The characteristic PES shapes of a late barrier system have also been reported on the O2/Cu(001) and (110).8 The calculated value of the activation barrier was ∼0.8 eV, but the absolute value should be lower in a real system because our calculation model for O2 dissociation on a 2D plane on Cu(111) did not consider the surface relaxation, the dissociation site, or the direction of dissociation, which are important factors for estimating the absolute value in the dissociation via a sideon collision. We also calculated the alternative PESs of O2 on Cu(111) where O2 approached along the surface normal direction with the molecular axis perpendicular to the surface (end-on colli-
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Figure 6. (a) Schematic representation of the model used to investigate O2 dissociative adsorption on Cu(111). (b) Calculated 2D PESs for triplet and quintet states of end-on O2/Cu(111) as a function of r and z. Contour spacing is 0.1 eV. (c) Calculated potential energy curves (PECs) as functions of reaction coordinate s.
sion). Figure 6 shows the calculated PES of an end-on collision where one constituent oxygen atom is adsorbed and the other is ejected into vacuum. In this reaction channel, a higher activation barrier of ∼1.2 eV existed in the crossing point between the triplet and quintet. O2 cannot overcome this activation barrier in the region of translational energy of ∼0.5 eV. Hence, the end-on scheme can play a role only at a higher kinetic energy of ∼2 eV due to the high activation barrier.12.14 In our calculation, the reaction channel where O2 approached with an end-on orientation on the surface and two constituent atoms of O2 dissociatively adsorb was impossible due to the high activation barrier. The result of the calculated PES obviously indicates that the O2 approaching with an end-on orientation is quite unfavorable in the dissociative adsorption on Cu(111). Such anisotropy of PESs by molecular alignment has also been reported on (001) and (110) surfaces.8,12 Discussion Dynamics calculations using empirical parameters provide an estimated activation barrier of dissociation on Cu(001) and (110) surfaces as 0.08 and 0.1 eV, respectively.8 The experimental results also indicate a direct process for the O2 molecular beam at a higher translational energy on Cu(110) and (001).4,7 The lowest activation barrier on Cu(111) is also calculated to be 0.20 eV at the most favorable site by a DFT calculation study where the surface relaxation and dissociative direction are considered.9 Thus, O2 molecules, which have kinetic energy above ∼0.2 eV, can directly dissociate on Cu(001), (110), and (111) surfaces.
To understand the effect of the nozzle temperature, the internal excited states of the O2 molecular beam generated by the high-temperature nozzle must be considered. During the free jet expansion where binary collisions among the molecules occur about 102∼103 times, the internal energy relaxation is due to molecular collisions, which lead to an energy exchange to the translational modes. Generally, vibrational modes cannot relax during expansion because the vibrational degrees of freedom of a diatomic molecule must be in excess of 104 collisions before adopting translational modes.40 On the other hand, the rotational modes can achieve equilibrium with a translational degree of freedom, which can cool down more easily than the vibrational mode during expansion. With the assumption that oxygen molecules maintain their vibrational temperature during a free jet expansion, the relative population in the ith vibration state is given by the Boltzmann distribution
PV)i ) exp(-EV)i/kTn)/ZV where EV)i is the energy of the ith vibrational states, k is Boltzmann’s constant, Tn is the nozzle temperature, and ZV is the vibrational contribution of the partition function. The relative populations of V ) 1 to V ) 0 are 5 × 10-4, 0.10, 0.15, and 0.19 at Tn ) 300, 940, 1200, and 1400 K, respectively, where E(V ) 1) ) 193 meV.41 The late barrier of PESs suggests that the vibration of O2 assists the dissociation because the vibration and the concomitant bond extension help achieve a transition state where O2 has an elongated interatomic distance and, consequently, overcomes the activation barrier. The enhanced
9966 J. Phys. Chem. C, Vol. 111, No. 27, 2007 sticking probability at a higher temperature up to ∼1000 K in our experimental data can be interpreted by the vibration-assisted adsorption. However, further heating up to 1400 K hinders the sticking. We interpret this behavior in terms of rotation of incident O2. Although the rotational temperature can be cooled more easily during a free jet expansion than the vibrational temperature, the population of the rotational excited molecules cannot be neglected as the nozzle temperature increases. Although there are detailed studies about the behavior of the rotational excited H2 on metal surfaces,16,17,42-44 information about an O2 chemisorption system has yet to be reported. The strong anisotropy of PES with a molecular alignment of side-on and end-on causes the rotational hindering in the dissociative adsorption. As shown in Figures 5 and 6, an O2 molecule, which aligns its molecular axis perpendicular to the surface, is quite unfavorable for dissociation due to the high activation barrier for an end-on collision. Therefore, O2 molecules can only dissociate when their molecular axis is parallel to the surface. A rapidly rotating molecule approaching the surface can be in favorable orientation only for a short time relative to the collision time, and then it will be scattered from the surface before it completely dissociates. On the other hand, the end-on orientation has a high activation barrier compared to that in a side-on orientation, so that rotating molecules have a higher probability to face the higher activation barrier of potential. Thus, the rotational excitation causes an enhanced reflectivity, which consequently suppresses the sticking. At a higher nozzle temperature, the vibration-assisted adsorption and rotational hindering occur competitively. Consequently, the sticking of O2 at Tn ) 1400 K is suppressed compared to that at 940 K but is enhanced compared to that at 300 K. This scenario can be applicable to the dissociation on (001) and (110) surfaces where the PESs show a strong anisotropy with O2 alignment.8,12 As shown in Figure 4, the rotational hindering is suppressed on Cu(111) when the angle of incidence is moved to 40°. The off-normal incidence decreases the normal component of the incident energy compared to the normal incidence at the same total energy. Hence, the time that an O2 molecule spends in the interacting region is elongated. Consequently, O2 can encounter the lower activation barrier with a sufficient collision time. And also, the steering forces can turn the incoming molecule into the most favorable configuration during the elongated interaction time. Therefore, in the O2 incidence at an off-normal direction, the rotational hindering is smeared. Such stereo insensitivity is also reported for O2 on bare Pd(001).24 This reaction model of dissociative adsorption of O2 is valid below 0.27 ML on a Cu(111) surface. However, above 0.27 ML, the dissociative adsorption of O2 is accompanied with the surface reconstructions. In this region, the reaction model is not valid probably due to the reconstruction 3 2 structure, which is induced only by the to the -1 2 energetic O2 molecules.13 This reconstruction process on Cu(111) differs from those on (001) and (110), which are caused spontaneously even by the dissociative chemisorption of thermal O2 on the surfaces. In the adsorption process accompanied with energetic O2-induced reconstruction at Θ > 0.27 ML on (111), the kinetic energy is probably transferred to the surface and as a result, enhances the mobility of the Cu atoms. The uptake curves at Tn ) 1400 and 940 K in Figure 4 exhibit different kinetics Θ > 0.27 ML for the same reason. For the energetic beam-induced surface process, more complicated adsorption
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Moritani et al. dynamics, including the energy relaxation of O2 and the local heating of atoms, must be considered. Conclusion We have demonstrated that the oxygen adsorption process on low-index copper surfaces is dependent on the nozzle temperature. The enhanced sticking at a high nozzle temperature can be interpreted by the vibrational-assisted adsorption, which is due to the activation barrier on the exit side in the PES. The suppression of the sticking at a higher nozzle temperature of 1400 K indicates a rotational hindering, which is due to the anisotropy of the PESs in the molecular alignment. However, the molecular beam incidence along the off-normal angle can suppress the rotational hindering. Acknowledgment. All of the experiments were performed using SUREAC2000 in BL23SU at SPring-8. The authors are thankful to Dr. Y. Saitoh and Dr. S. Fujimori for their help with the operation of the monochromatic system at the beamline. The group at Osaka University is financially supported by the 21st Century COE Program. M.T. was supported by a JSPS Research Fellowship for Young Scientists. All the computations were performed using the facilities of JAEA. M.O. gratefully acknowledges the Hyogo Science and Technology Association and MEXT for a Grant-in-Aid for Scientific Research (No. 17550011). References and Notes (1) Zhou, G.-W.; Wang, L.; Birtcher, R. C.; Baldo, P. M.; Pearson, J. E.; Yang, J. C.; Eastman, J. A. Phys. ReV. Lett. 2006, 96, 226108. (2) Hammer, B.; Nørskov, J. K. In Impact of Surface Science on Catalysis; Gates, B. C., Kno¨zinger, H., Eds.; Academic Press: New York, 2000. (3) Over, H.; Seitsonen, A. P. Science 2002, 297, 2003. (4) Hodgson, A.; Levin, A. K.; Nesbit, A. Surf. Sci. 1993, 293, 211. (5) Pudney, P.; Bowker, M. Chem. Phys. Lett. 1990, 171, 373. (6) Balkenende, A. R.; den Daas, H.; Huisman, M.; Gijzeman, O. L.; Geus, J. W. Appl. Surf. Sci. 1991, 47, 341. (7) Hall, J.; Saksager, O.; Chorkendorff, I. Chem. Phys. Lett. 1993, 216, 413. (8) Ge, J.-Y.; Dai, J.; Zhang, J. Z. H. J. Phys. Chem. 1996, 100, 11432. (9) Xu, Y.; Mavrikakis, M. Surf. Sci. 2001, 494, 131. (10) Liem, S. Y.; Kresse, G.; Clark, J. H. R. Surf. Sci. 1998, 415, 194. (11) Okada, M.; Moritani, K.; Goto, S.; Kasai, T.; Yoshigoe, A.; Tearaoka, Y. J. Chem. Phys. 2003, 119, 6994. (12) Okada, M.; Moritani, K.; Yoshigoe, A.; Teraoka, Y.; Nakanishi, H.; Dino, W. A.; Kasai, H.; Kasai, T. Chem. Phys. 2004, 301, 315. (13) Moritani, K.; Okada, M.; Sato, S.; Goto, S.; Kasai, T.; Yoshigoe, A.; Teraoka, Y. Thin Solid Films 2004, 48, 464-465. (14) Moritani, K.; Okada, M.; Sato, S.; Goto, S.; Kasai, T.; Yoshigoe, A.; Teraoka, Y. J. Vac. Sci. Technol., A 2004, 22, 1625. (15) Moritani, K.; Okada, M.; Fukuyama, T.; Teraoka, Y.; Yoshigoe, A.; Kasai, T. Eur. Phys. J. D 2006, 38, 111. (16) Rettner, C. T.; Auerbach, D. J.; Michelson, H. A. Phys. ReV. Lett. 1992, 68, 1164. (17) Michelsen, H. A.; Rettner, C. T.; Auerbach, D. J. J. Chem. Phys. 1993, 98, 8294. (18) Rettner, C. T.; Michelson, H. A.; Auerbach, D. J. J. Chem. Phys. 1995, 102, 4625. (19) Darling, G. R.; Holloway, S. J. Chem. Phys. 1994, 101, 3268. (20) Darling, G. R.; Holloway, S. Surf. Sci. Lett. 1994, 304, L461. (21) Vattuone, L.; Valbusa, U.; Rocca, M. Phys. ReV. Lett. 1999, 82, 4878. (22) Vattuone, L.; Gerbi, A.; Rocca, M.; Valbusa, U.; Pirani, F.; Vecchiocattivi, F.; Cappelletti, D. Angew. Chem., Int Ed. 2004, 43, 5200. (23) Gerbi, A.; Vattuone, L.; Rocca, M.; Pirani, F.; Valbusa, U.; Cappelletti, D.; Vecchiocattivi, F. J. Phys. Chem. B 2005, 109, 22884. (24) Gerbi, A.; Savio, L.; Vattuone, L.; Pirani, F.; Cappelletti, D.; Rocca, M. Angew. Chem., Int. Ed. 2006, 45, 6655. (25) Yokoya, A.; Sekiguchi, T.; Saitoh, Y.; Okane, T.; Nakatani, T.; Shimada, T.; Kobayashi, H.; Takao, M.; Teraoka, Y.; Hayashi, Y.; Saaki, S.; Miyahara, Y.; Harami, T.; Saaki, T. A. J. Synchrotron Radiat. 1998, 5, 10. Nakatani, T.; Saitoh, Y.; Teraoka, Y.; Okane, T.; Yokoya, A. J. Syncrotron Rad. 1998, 5, 536.
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