J. Phys. Chem. 1996, 100, 8011-8014
8011
Generation of Radicals on a Metal Surface from Photoinduced Dissociation of Physisorbed Molecules: CH2 from H2CO on Ag(111) Louise E. Fleck, Pui-Teng Howe, Jung-Soo Kim, and Hai-Lung Dai* Laboratory for Research on the Structure of Matter, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6202 ReceiVed: February 13, 1996; In Final Form: March 6, 1996X
Submonolayer formaldehyde (H2CO) has been photodissociated on Ag(111) at 35 K using nanosecond 287 nm laser pulses. The low temperature prohibits further bimolecular reactions of the dissociation products and allows their analysis by electron energy loss spectroscopy. One of the dissociation products has been identified as the radical species methylene (CH2). The dissociation is induced by photoexcited substrate electrons attached to formaldehyde, forming a transient formaldehyde negative ion that dissociates. Due to the influence of adsorbate-substrate bonding on reaction energetics, the dissociation channel of H2CO- on Ag appears to be different from that of the gaseous negative ion.
I. Introduction In the study of photochemistry at metal surfaces in the past decade, photoinduced desorption and dissociation of molecular adsorbates have been reported for a large number of systems.1,2 It has been established that there are several mechanisms by which the molecular adsorbates are energized. For example, excitation can be achieved by direct absorption of photons by the adsorbate or adsorbate-surface complex. However, due to rapid relaxation of molecular excitation on metals, photochemistry is often induced by indirect excitation involving absorption of photons by the metal substrate electrons. The photoexcited substrate electrons may subsequently attach onto the adsorbate, forming a transient negative ion, or scatter off of the adsorbate. Excitation of adsorbates via both photoemitted substrate electrons3-5 and subvacuum substrate electrons6-11 has been observed. Recently, the first observation of photoinduced, nonthermal polymerization on a metal surface was reported.12 Formaldehyde (H2CO) physisorbed on Ag(111) at coverages from 0.1 to >1 monolayer is observed to polymerize upon irradiation with nanosecond UV pulses. It has been shown that the H2CO molecule undergoes excitation by subvacuum substrate electron attachment. The transient H2CO- ion so formed dissociates into radical species which initiate the polymerization reaction. The gas phase photochemistry of H2CO is well established.13,14 The lowest molecular dissociation channel, with a 80 kcal/mol barrier, is to form H2 and CO. The reaction channel to form H + HCO is about 6 kcal/mol higher. In the gas phase these radical species may polymerize H2CO. However, it has been established that the molecular dissociation channels are quenched on Ag.15 On the other hand, even though it has been shown that the unstable gaseous H2CO- would dissociate into H- and HCO,16 it is not clear whether the negative ion adsorbed on the Ag surface would dissociate into the same species as in the gas phase to initiate H2CO polymerization on Ag. In this paper we will establish that the radicals generated from photoinduced dissociation of H2CO on the surface are methylene (CH2) and atomic oxygen. Oxygen-covered Ag(110) was known to polymerize H2CO, and it was suggested that atomic oxygen initiates H2CO polymerization.17 Although there has not been any direct study, it is reasonable to speculate that the CH2 radicals on Ag may also initiate H2CO polymerization. X
Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(96)00449-2 CCC: $12.00
In order to identify the initiators in the polymerization reaction, which are the transient products of the photodissociation reaction, we need to stabilize the short-lived radical species on the surface. This is accomplished by performing the experiments at low temperatures (T ) 35 K). At temperatures below 60 K, it has been shown that the dissociation products remain adsorbed on the Ag surface and are preserved for a long period of time (>1 h).18 This is because the low temperature prohibits diffusion and thus bimolecular reactions. This makes it possible to characterize the radical species by surface analysis techniques such as electron energy loss spectroscopy (EELS). This method of producing and preserving radical species on a metal surface is useful for identifying reaction intermediates and should be generally applicable for many molecular adsorbates. The radical species CH2 and O resulting from the dissociation of H2CO, through the unstable H2CO- ion, on Ag are in total contrast to the gas phase H2CO- dissociation. On the surface, the bonding of Ag with CH2 and O may be stronger than with H and HCO, thereby lowering the energy barrier of this reaction channel. Atomic oxygen is known to be strongly adsorbed on Ag.17 On the basis of the known energy of the formaldehyde negative ion state and the electron affinity of oxygen and the estimated H2CO dissociation energy and surface-bonding energies of the dissociation fragments, we have found that the dissociation reaction of the H2CO- ion on Ag is energetically plausible. II. Experimental Section The UHV conditions, sample mounting, H2CO and D2CO preparation, and dosing procedure have been described elsewhere.12 The experiments were conducted at a sample temperature of 35 K, which was achieved by liquid helium cooling. The commercial continuous flow cryotstat (Hansen), compatible for use with either liquid helium or liquid nitrogen, consists of a 0.5 in. inner diameter stainless steel tube which was electronbeam-welded to the cold end. The latter is made of oxygenfree high-conductivity copper. A polished sapphire washer of 1 mm thickness provides good thermal contact as well as electrical insulation between the cold end and the sample support.19 In order to detect the EEL spectra of the dissociation products, it is necessary to generate a sufficiently high concentration of © 1996 American Chemical Society
8012 J. Phys. Chem., Vol. 100, No. 19, 1996
Figure 1. Vibrational EEL spectrum of 2 langmuir H2CO on Ag(111): (a) at 35 K and after irradiation with 287 nm photons (0.7 mJ per pulse, 36 × 103 pulses, 1.5 × 1019 photons/cm2), (b) at 85 K after irradiation with 355 nm photons (1.2 × 1019 photons/cm2), and (c) without irradiation. The spectrum in c is from the formaldehyde monomer submonolayer. In b the monomer has been fully converted to polymer after irradiation at this elevated temperature.
these species, which may require a high photon fluence. However, extensive UV irradiation degrades the signal-to-noise ratio of the EEL spectra, presumably due to the degradation of the Ag surface quality. It has been shown that polymerization occurs for λ e 355 nm,19 with the cross section increasing with photon energy. We have chosen to use 287 nm because of the high polymerization cross section of (42 ( 9) × 10-19 cm2,14 and thus a lower photon fluence can be used. The UV source was a frequency-doubled, pulsed dye laser (Quanta-Ray) pumped by a Nd:YAG laser (Continuum; 20 Hz repetition rate, 14 ns pulse width). The p-polarized beam was expanded to a diameter of 15 mm to cover the entire crystal surface and was incident on the surface at a 45° angle. With a laser power of 0.7 mJ per pulse and an irradiation time of 30 min, the photon fluence is on the order of 1019 per cm2, which should be sufficient to achieve near complete dissociation of the H2CO submonolayer. The electron energy loss spectrometer is a commercial instrument (McAllister) having a 127° cylindrical deflector monochromator and an identical energy analyzer. In the specular scattering geometry, which was used in the present experiments, the incident and scattering angles are both 65°. The resolution was determined from the elastic peak as 12 meV (FWHM). III. Results and Analysis Figure 1a shows the EEL spectrum of a 2 langmuir exposure of H2CO on Ag(111) after irradiation with 287 nm photons at 35 K. This exposure corresponds to 0.6 of the saturation coverage. The spectrum shows peaks at 146, 182, and 358 meV, as well as a shoulder at about 120 meV and an even smaller peak at about 210 meV. (The small peak at about 266 meV is most likely due to CO adsorbed from the background.) Figure 1b shows the EEL spectrum after irradiation at 85 K. Here at T > 60 K, polymerization has occurred, resulting in a very different spectrum than that in Figure 1a. For comparison, Figure 1c shows the EEL spectrum of H2CO monomer without irradiation. Although most of the peaks identified in Figure 1a are close in position to those polymer peaks appearing in Figure 1b and those formaldehyde monomer peaks in Figure 1c, an
Fleck et al. examination of their intensities shows that they do not originate from either the formaldehyde polymer or monomer. The strongest monomer peak is at 210 meV. So the small peak at this energy in Figure 1a indicates the maximal possible amount of monomer after irradiation at 35 K. From the intensity ratios of the monomer peaks, one can easily see that the 146 and 182 meV peaks are not from the monomer. The strongest polymer peak is at 119 meV. Even if the 120 meV feature is from the polymer, again, the intensity ratios of the polymer peaks depict that the 146 and 182 meV peaks are not from the polymer. Furthermore, previous temperature dependent experiments have shown that the H2CO/Ag samples at 35 K following irradiation did not show any trace of polymer for hours.18 It is rather unlikely that the 120 meV feature in Figure 1a is from the polymer. The EELS peaks observed for the H2CO and D2CO overlayer after irradiation at 35 K are summarized in Table 1. The likely candidates for the new species in Figure 1a are HCO + H or CH2 + O. On Ag(111), the measured EELS peak energy for the Ag-H stretch was around 100 meV.19,20 This peak is absent in Figure 1a. The IR spectrum of HCO in matrix indicates peaks to appear at 230 meV (CO stretch), 308 meV (CH stretch), and 135 meV (bend).21 These peak energies do not match those in the spectrum shown in Figure 1a. There has been only one report of EEL spectra of HCO on a metal surface: HCO and DCO on Ru(001).22 The most prominent feature in both the HCO and DCO on Ru spectra is the CO stretch at 146 meV (HCO) and 144 meV (DCO). The much weaker bending modes were assigned as 174 and 132 meV for HCO and 122 and 102 meV for DCO. The moderately intense CH stretch of HCO was identified at 360 meV. A comparison with the EEL spectra observed in our experiments shows a complete mismatch of the intensity pattern. Specifically, the strong CO stretch mode cannot be identified in either spectrum. Since the spectrum is not consistent with either HCO or H on a metal surface, it is concluded that the dissociation products H + HCO are not generated by 287 nm irradiation of formaldehyde on Ag(111). We now consider the other likely pair of dissociation products, CH2 + O. The EELS vibrational peak for atomic oxygen on Ag(110) is located at 39-40 meV.23,24 The corresponding peak on Cu(111) is located at 29-30 meV.25 Peaks at such low energy cannot be resolved from the elastic peak in the spectrum shown in Figure 1a. Hence, the presence of oxygen atoms cannot be confirmed or ruled out. EEL spectra for the other dissociation product, methylene (CH2), have been measured on Fe(110)26 and Ru(001).27,28 As shown in Table 1, there is a good correspondence, in terms of both frequency and intensity, with the spectrum in Figure 1a. The conclusion is therefore that the new species present on Ag after extensive UV irradiation of H2CO at 35 K is methylene. The presence of atomic oxygen is inferred from mass balance of the dissociation reaction. IV. Discussion A. Dissociation of H2CO- to (CH2 + O)- on Ag. Light polarization and angle of incidence experiments have shown that the photoinduced products from H2CO/Ag are caused by photoexcited electrons in Ag.19 In addition, irradiation with near zero kinetic energy electrons from the vacuum side was found to also induce polymerization, supporting the electron-attachment excitation mechanism.29 Dissociation of H2CO into H2 + CO or H + HCO via direct excitation, though energetically accessible by 287 nm photons, is insignificant because the cross section of the molecular dissociation is much smaller than that of the photoexcited substrate electron-induced dissociation
CH2 from H2CO on Ag(111)
J. Phys. Chem., Vol. 100, No. 19, 1996 8013
TABLE 1: Assignments of the Vibrational EELS Peaks (meV) for 2 langmuir H2CO on Ag(111) after 287 nm Irradiation at 35 Kg CD2f
CH2 assignment
Ag(111)a
CH2 rock CH2 twist CH2 wag CH2 scissor CH2 sym str CH2 asym str
120 (w) 146 (ms) 182 (ms) 358e (ms) 358e (ms)
Fe(110)b
Ru(001)c
Ru(001)d
98 (w) 115 (w) 126 (w) 177 (s) 368 (ms)
96 (w) 112 (ms) 141 (ms) 180 (ms) 364 (ms) 378 (m)
110 (ms) 132 (ms) 160 (ms) 356 (ms) 365 (m)
Ag(111)a
Ru(001)c
130e 130e 266e 266e
84 104 150 274 284
Ru(001)d 88 113 139 266 273
a This work. b Reference 26. c Reference 27. d Reference 28. e Unresolved. f The intensity information was not listed in the earlier references and thus is not listed here. g The D2CO peaks were observed under similar conditions but with 355 nm irradiation and 1 langmuir exposure. Comparison to three previous studies is made. The relative intensity is shown in parentheses.
channel. At 287 nm, σ for H2CO + hν f H + HCO in the gas phase is 0.19 × 10-19 cm2 whereas σ for H2CO (ad) + hν f CH2 (ad) + O(ad) on the Ag surface is (42 ( 9) × 10-19 cm2.19 The molecular excitation channel is even less likely to contribute as one considers the rapid relaxation of molecular excitation on metal. Consideration of the energetics of H2CO- on Ag shows the plausibility of the generation of adsorbed H2CO- through electron attachment by subvacuum photoexcited electrons. The electron affinity of H2CO has not been measured experimentally, but a negative ion resonance of formaldehyde in the gas phase has been observed at about 0.9 eV above the vacuum level.30 Assuming an adsorbate-substrate distance of ∼2-3 Å for a physisorbed molecule, the image charge stabilization would lower the energy of the ion resonance state by ∼2 eV31 to about ∼1 eV below the vacuum level. This is consistent with the 3.1 ( 0.2 eV photon threshold energy experimentally determined for H2CO polymerization on the Ag surface.19 The work function of Ag(111) exposed to 2 langmuir H2CO was measured to be 4.4 eV. Hence, the 3.1 eV photon would excite the Ag electrons to 1.3 eV below the vacuum level. At this energy, the photoexcited electrons in Ag may tunnel to the formaldehyde negative ion resonance level. If the formaldehyde negative ion on Ag would dissociate, radicals species as dissociation products of H2CO- may be generated. It was previously observed that as gaseous H2CO was irradiated with 5.8 eV electrons, H2CO- dissociated to form H-.16 On the surface, however, it appears that H2CO- follows a different dissociation pathway and dissociates into CH2 and O-. Both the stability and the preferred dissociation channel of H2CO- on Ag can be examined through the energetics considerations. In addition to the bond-breaking energy, several factors may influence the energetics of the dissociation pathway of H2CO- adsorbed on a metal surface: potential energy lowering on the surface due to either image charge stabilization, if the species is charged, and/or chemical bonding with surface. The relevant processes and their associated energy changes for consideration are listed as eqs 1-6. The 2.2 eV adsorption
H2CO- (ad) f H2CO- (g)
∆E ) 2.2 eV
(1)
H2CO- (g) f H2CO (g) + e
∆E ) -0.9 eV
(2)
H2CO (g) f CH2 (g) + O (g)
∆E ) 179 kcal/mol (3)
CH2 (g) f CH2 (ad)
∆E e -30 kcal/mol (4)
O (g) + e f O- (g)
∆E ) -1.46 eV
(5)
O- (g) f O- (ad)
∆E ∼ -7.0 eV
(6)
energy of H2
CO-
in eq 1 lowers the negative ion resonance
state from the gaseous value of 0.9 eV above vacuum30 to 1.3 eV below vacuum on the surface.19 The dissociation energy of gaseous formaldehyde into CH2 and O in eq 3 can only be estimated from the formation enthalpies.32 The adsorption energy of methylene on Ag has not been measured but can be safely estimated to be stronger than 30 kcal/mol.33 The electron affinity of O has been measured.34 The adsorption energy of O- in eq 6 is estimated as the sum of the image charge stabilization of 3.6 eV (assuming a 1 Å Ag-O bond distance) and the Ag-O bond energy. The Ag-O bond is expected to be very strong. Oxygen atoms adsorbed on Ag(110) are known to desorb as O2 with an activation energy of 35.9 kcal/mol.35 Noting that the dissociation energy for O2 is 119.1 kcal/mol, one obtains an estimate of the Ag-O bond as 77.5 kcal/mol. The sum of eqs 1-6 would result in the process
H2CO- (ad) f CH2 (ad) + O- (ad) ∆E ∼ -15 kcal/mol (7) demonstrating the plausibility of negative formaldehyde ion dissociation on Ag. Next, we consider the question as to which dissociation channel is preferred for H2CO-(ad) through examining the energies associated with the different dissociation fragments. The image charge stabilization energy is dependent only on the charge on the fragment and its distance from the metal surface. Hence, this quantity should be similar for H-, CH2-, or O-. Radical species are expected to form strong bonds with the Ag surface. The Ag-CH2 bond is expected to be at least as strong as or stronger than the Ag-HCO bond, since CH2 is able to form a double bond while the HCO can form only a single bond. Arguments in the above paragraph have led to a 77.5 kcal/mol bond energy for Ag-O. On the other hand, an upper bound estimate of the Ag-H bond energy has been obtained at 52 kcal/mol.36 Hence, we see that on the Ag surface, H2COdissociation into CH2- + O or CH2 + O- is energetically more favorable than dissociation into H- + HCO by several tens of kilocalories per mole. There is no experimental evidence to indicate which of the CH2- + O or CH2 + O- channels is preferred. However, on the basis of gas phase electron affinities of CH2 and O we speculate that the production of CH2 + Ois favored. The electron affinity of O is more than twice the electron affinity of CH2, which is 0.652 eV.37 Thus, the formation of O- is more likely than the formation of CH2-. Once the transient H2CO- dissociates on the Ag surface, the negative charge on the products will be neutralized, leaving neutral CH2 and O on the Ag surface. The dissociation of H2CO- on Ag into CH2 + O- but not HCO + H- may also be rationalized from the dynamics aspects. Ab initio calculations show that the LUMO of H2CO is π* antibonding with respect to the CdO bond.38 The lowest electronic state of H2CO- should have a weakened CO bond.
8014 J. Phys. Chem., Vol. 100, No. 19, 1996 Dissociation along coordinates other than the CO bond, such as dissociation into H- + HCO, requires the accumulation of vibrational energy through internal conversion into these coordinates. Dissociation for a small molecule like formaldehyde induced by vibrational excitation occurs on a time scale much slower than that of electronic dissociation.13 A previous EELS study on H2CO/Ag has indicated a 10-14 s time scale for relaxation of the excited vibrational levels of the first electronic excited state.15 Hence, dissociation into H- + HCO may occur for the gaseous H2CO-, but this channel may not compete effectively with quenching on a metal surface. B. Generation of Radical Species from Photoinduced Dissociation of Physisorbed Molecules. Our results show that a highly reactive radical species can be generated from photoinduced dissociation of a physisorbed molecule and can be preserved on the surface for an indefinite amount of time for spectroscopic characterization. This observation suggests a general scheme for generating radical species on surfaces based on photoinduced dissociation of physisorbed molecules. In this case, the radical methylene is a reaction intermediate in the polymerization reactions of formaldehyde. For a physisorbed molecule, the molecular properties of the adsorbate may be largely retained, which provide a basis for understanding the mechanism of its photochemistry. On the other hand, the unique surface influences on the dynamics and energetics of the molecular dissociation channels have to be considered. These surface influences include image charge stabilization, rapid quenching of molecular excitation, and different surface-bonding strengths for different products. The most important excitation mechanism on metals appears to be caused by tunneling of photoexcited substrate electrons to the adsorbates. This calls for a better understanding of the reaction pathways of the negative ion rather than of the neutral molecule itself. Following dissociation of the adsorbates, the radical species can only be preserved at a temperature low enough to prevent further reactions. The low temperature may prohibit diffusion of reactants or activation of a bimolecular reaction. In the case of formaldehyde polymerization, the critical temperature of 65 K is associated with a barrier of about 2 kcal/mol,18 which is most likely due to diffusion of the formaldehyde molecules adsorbed on the Ag(111) surface with a 6 kcal/mol energy.15 V. Conclusion One of the products from photoinduced dissociation of formaldehyde on Ag(111) has been identified as methylene. The existence of O as a product is implied from mass balance. These radical species are reaction intermediates in the photoinduced polymerization of submonolayer formaldehyde on Ag. The experiment was performed at 35 K, a temperature low enough to prevent further reactions of the radical species and allow spectroscopic characterization by EELS. This observation suggests a generally applicable method for producing and identifying reactive species on a metal surface. The photodissociation of formaldehyde on Ag appears to go through the unstable formaldehyde negative ion, generated from attachment of photoexcited substrate electrons to the adsorbates. However, the dissociation channel of the H2CO- adsorbed on Ag appears totally different from that of the gaseous molecular ion, which dissociates into H- and HCO. This can be understood from the much stronger O/Ag and CH2/Ag bonds than H/Ag and HCO/Ag bonds.
Fleck et al. Acknowledgment. This work is supported in part by the National Science Foundation MRL Program under Grant No. DMR91-20668. Acknowledgement is made to the donors of the Petroleum Research Fund of the American Chemical Society for the initiation of this project. References and Notes (1) Zhou, X. L.; Zhu, X. Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73 and references therein. (2) See chapters in: Laser Spectroscopy and Photochemistry on Metal Surfaces; Dai, H. L., Ho, W., Eds.; Advanced Series in Physical Chemistry 5; World Scientific: River Edge, NJ, 1995. (3) Cho, C. C.; Collings, B. A.; Hammer, R. E.; Polanyi, J. C.; Stanners, C. D.; Wang, J. H.; Xu, G. Q. J. Phys. Chem. 1989, 93, 7761. (4) Wolf, M.; Nettesheim, S.; White, J. M.; Hasselbrink, E.; Ertl, G. J. Chem. Phys. 1990, 92, 1509; 1990, 93, 5327. (5) (a) Jo, S. K.; White, J. M. J. Phys. Chem. 1990, 94, 6852. (b) Jo, S. K.; Zhu, X. Y.; Lennon, D.; White, J. M. Surf. Sci. 1991, 241, 231. (6) Buntin, S. A.; Richter, L. J.; King, D. S.; Cavanaugh, R. R. J. Chem. Phys. 1989, 91, 6429. (7) Hasselbrink, E.; Jakubith, S.; Nettesheim, S.; Wolf, M.; Cassuto, A.; Ertl, G. J. Chem. Phys. 1990, 92, 3154. (8) Wolf, M.; Hasselbrink, E.; Ertl, G.; Zhu, X. Y.; White, J. M. Surf. Sci. Lett. 1991, 248, L235. (9) Hatch, S.; Zhu, X. Y.; White, J. M.; Campion, A. J. Chem. Phys. 1990, 91, 2681; J. Phys. Chem. 1991, 95, 1759. (10) Ying, Z. C.; Ho, W. J. Chem. Phys. 1990, 93, 9077; 1991, 94, 5701. (11) Ying, Z. C.; Ho, W. Phys. ReV. Lett. 1990, 65, 741. (12) Fleck, L. E.; Feehery, W. F.; Plummer, E. W.; Ying, Z. C.; Dai, H. L. J. Phys. Chem. 1991, 95, 8428. (13) Moore, C. B.; Weisshaar, J. C. Annu. ReV. Phys. Chem. 1983, 34, 525 and references therein. (14) Dai, H. L.; Field, R. W.; Kinsey, J. L. J. Chem. Phys. 1985, 82, 1606. (15) Fleck, L. E.; Ying, Z. C.; Feehery, M.; Dai, H. L. Surf. Sci. 1993, 296, 400. (16) Azria, R. Ph.D. Thesis, University of Orsay, Orsay, France 1972. (17) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1982, 119, 279. (18) Fleck, L. E.; Ying, Z. C.; Dai, H. L. J. Vac. Sci. Technol. 1993, A11, 1942. (19) Fleck, L. E., Ph.D. Thesis, University of Pennsylvania, 1994. (20) Sprunger, P. T.; Plummer, E. W. Phys. ReV. B 1993, 48, 14436. (21) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1964, 41, 3032. (22) Anton, A. B.; Parmeter, J. E.; Weinberg, W. H. J. Am. Chem. Soc. 1986, 108, 1823. (23) Backx, C.; de Groot, C. P. M.; Biloen, P. Surf. Sci. 1981, 104, 300. (24) Sexton, B. A.; Madix, R. J. Chem. Phys. Lett. 1980, 76, 294. (25) Dubois, L. H. Surf. Sci. 1982, 119, 399. (26) McBreen, P. H.; Erley, W.; Ibach, H. Surf. Sci. 1984, 148, 292. (27) George, P. M.; Avery, N. R.; Weinberg, W. H.; Tebbe, F. N. J. Am. Chem. Soc. 1983, 105, 1393. (28) Henderson, M. A.; Radloff, P. L.; White, J. M.; Mims, C. A. J. Phys. Chem. 1988, 92, 4111. (29) Fleck, L. E.; Kim, J. S.; Dai, H. L. Submitted for publication in Surf. Sci. Lett. (30) Van Veen, E. H.; Van Dijk, W. L.; Brongersma, H. H. Chem. Phys. 1976, 16, 337. (31) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cambridge University Press: Cambridge, England, 1986; Chapter 7. (32) JANAF Thermochemical Tables, 3rd ed.; Chase, M. W., Jr., et al., Eds.; AIP, New York, 1986. (33) Chiang, C. M.; Wentzlaff, T. H.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836. (34) Neumark, D. M.; Lykke, K. R.; Anderson, T.; Lineberger, W. C. Phys. ReV. A 1985, 32, 1890. (35) Madix, R. J. Crit. ReV. Solid State Mater. Sci. 1978, 7, 143. (36) Zhou, X. L.; White, J. M. Surf. Sci. 1989, 218, 201. (37) Leopold, D. G.; Murray, K. K.; Miller, A. E. S.; Lineberger, W. C. J. Chem. Phys. 1985, 83, 4849. (38) Davis, T. D.; Maggiora, G. M.; Christoffersen, R. E. J. Am. Chem. Soc. 1974, 96 (26), 7878.
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