J. Phys. Chem. 1994, 98, 46414646
4641
Wavelength-Dependent Photochemical Pathways of SO2 Adsorbed on Ag( 111) Z.-J. Sun and J. M. White' Department of Chemistry and Biochemistry, University of Texas Austin, Austin, Texas 78712 Received: October 21, 1993; In Final Form: February 15, 1994'
Using pulsed laser excitation, and focusing on multilayers, the photochemistry of SO2 adsorbed on Ag( 1 11) has been studied by time-of-flight mass spectrometry, complemented by temperature-programmed desorption and Auger electron spectroscopy. At 193,248, and 351 nm, irradiation of multilayers, but not of monolayers, leads to reactions that break sulfur-oxygen bonds. Photochemistry in the multilayer tracks the SO2 absorption. At 437 and SO0 nm, where SO2 does not absorb, no dissociation products (0,SO,or SOs) were detected during (time-of-flight) or after (temperature-programmed desorption and Auger electron spectroscopy) irradiation. Only 193-nm photons-induce unimolecular bond dissociation, whereas parent desorption and a bimolecular photoreaction are evident a t three wavelengths (193,248, and 351 nm). The translational energy distributions of the photochemical products vary with wavelength, in general, shifting to higher energies with increasing photon energy. The excitation mechanism and possible primary photochemical processes are discussed.
1. Introduction The work described here is part of our continuing effort to understand the UV photochemistry of SO2 on Ag( 111); earlier, we showed that parent photodesorption, attributed to activation by substrate hot carriers, is the only photochemical process for coverages up to 1.35 ML (ML = monolayer).' This paper focuses on adsorbate excitation mechanisms and reaction pathways involving photodissociation,photoreactions, and photodesorption processes in physisorbed layers (>1.35 ML). The rich photochemistryand photophysics of gaseous SO2 have been studied extensively, and its UV oxidation in the atmosphere is environmentally significant.2 In the gas phase, unimolecular photodissociation occurs when the photon energy exceeds the S-0 bond dissociation energy of 5.65 eV.3-7 Extending to much lower photon energies (3.2 eV), and particularly evident at high pressures, there is a bimolecular photoreaction involving electronically excited SO2 and ground-state S O Z . ~ -The ~ ~ excited SO2 molecules can be efficiently quenched by collision with ground-state SO2 molecules8J2-14(self-quenching)or with foreign gases.8.15 The present work focuses on the UV photochemistry of multilayer SO2 adsorbed on Ag(ll1). We find that direct photoexcitation of the adsorbate accounts for the observed photochemical processes. The photolysis proceeds through several paths leading to unimolecular photodissociation, bimolecular photoreaction, and parent molecule photodesorption. To explore this, we used pulsed laser excitation and time-of-flight mass spectrometry (TOF-MS), complemented by temperatureprogrammed desorption (TPD) and Auger electron spectroscopy (AES). The key independent variables were wavelength, SO2 coverage, and laser pulse energy.
2. Experimental Section The experiments were carried out using a standard ultrahighvacuum chamber described elsewhere.' The chamber is equipped with a single pass cylindrical mirror analyzer and a coaxial electron gun for AES; a quadrupole mass spectrometer for TPD, TOF, and residual gas analysis (RGA); an ion gun for sputtering; and a pinhole (10 ctm in diameter), which terminated about 1 mm above the surface, for dosing. The working base pressure was 3 X 1O-I'J Torr. A verifiably clean Ag( 1 1 1) surface was prepared by Ar+ ion sputtering and subsequent annealing at 690 K. The temperature Abstract published in Advance ACS Absrracrs, April 1, 1994.
was monitored by a chromel-alumel thermocouple spot-welded to a tantalum loop that was pressed into a hole in the edge of the Ag. The crystal could be cooled to as low as 89 K by thermal contact with a liquid nitrogen reservoir and resistively heated to the melting point of Ag, a limit we carefully avoided. Sulfur dioxide (Matheson, 99%) was degassed at 77 K for several cycles before dosing. TPD spectra (multiplexed) were gathered using a linear temperature ramp (5 K s-1). An excimer laser (Questek 2000) provided pulses (1 5 ns) of 193-, 248-, and 351-nm photons that were incident at 60° off the surface normal. The irradiated area, contrllled by a circular iris set parallel to the sample surface, is circular with a diameter of 1.4 cm. Although varied, typical energies per unit area were 0.75 mJ cm-2 for 193 nm and 2.5 mJ cm-2 for 248 and 351 nm. A Lambda Physik GL2002 dye laser, pumped at 308 nm by a Lambda Physik EMG201 excimer laser, provided the 437- and 500-nm photons (3 mJ cm-2). The beam output from the dye laser was expanded and spatially filtered by an iris to pass only the central portion of the Gaussian beam, -68% of the total pulse energy. The transient temperature rise due to laser irradiation was less than 5 K.16 The TOF spectra, sampled over 128 laser shots, removed no more than 5% of the initial SO2 coverage (verified by postirradiation TPD). TOF signals were collected along the surface normal. Ions corresponding to SOz, SO3, SO, and 0 were detected. The SO+ (mass 48) signal came from two sources, SO itself and the fragmentation of SO2. A correction was made by subtracting parent fragmentation; this correction was based on the measured 48/64 amu ratio for SO2 photodesorption at 1 ML SO2 coverage, a coverage where only parent desorption occurs.1 From the flux-weightedTOF spectra," the mean translational energy ( ( E , ) )of the desorbing species was obtained by fitting to a modified Maxwell-Boltzmann distribution. The maximum translational energy, E M ,of the desorbing species was estimated from the high-energy cutoff of the energy distribution. 3. Results and Discussion
Section 3.1 describes what was detected on the surface by AES and TPD after photon irradiation. TOF results are presented in section 3.2. The reaction products and their dependencies on photon flux, coverage, and wavelength are described. The excitation mechanism and possible primary photoreaction pathways are discussed. 3.1. Surface Species Present after Photon Irradiation. As demonstrated in earlier work, the thermal adsorption and
0022-3654/94/2098-4641%04.50/0 0 1994 American Chemical Society
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Temperature (K) Figure 1. Postirradiation TPD of 3.5 ML S02/Ag( 111) after irradiation at 89 K with -5 X lOI9 photons cm-2 at 193 nm. T 2 250 K is shown. Between 89 and 250 K,only SO2 desorbs and the spectra resembles the TPD without photoirradiation.I desorption of SO2 on Ag( 111) are completely reversible; Le., only SO2 desorbs, and after flashing to 300 K, TPD and AES indicate that the surface is clean.lJ8 Moreover, unless multilayers are present, there are no photon-driven reactions that dissociate S02.l Only parent photodesorption was detected by TOF.1 The desorption is substrate mediated,lJ8 and it diminishes sharply for coverages just above 1 ML. For multilayers the outcome is different, depending on wavelength. At 437 and 500 nm, there is evidence for parent desorption but no photodissociation. After irradiating 3.5 ML with 193-, 248-, and 351-nm photons and flashing to 500 K, AES shows that sulfur and a small amount of 0 are left on the surface. Postirradiation TPD provides additional evidence for photodissociation (Figure 1). In the absence of photons, there is product desorption below, but not above, 250 K. After photon irradiation, ion signals at 64, 48, and 32 amu were detected at higher temperatures. Mass 64 had a small peak at 380 K and a more intense and broader peak at 780 K. While the 48 and 64 amu signals followed each other and their intensity ratio is characteristic of S02, the 32 amu signal is different, and the peak appears only above 600 K, where it follows the 48 and 64 amu signals. We attribute 32 amu to dioxygen formed in a surface reaction that simultaneously leads to SO2 desorption. The above agrees qualitatively with a previous study on Ag(1 10) involving SO2 thermal reactions with predosed O.I9 Based on evidence from low-energy electron diffraction, TPD, and X-ray and ultraviolet photoelectron spectroscopy, SO2 reacts with predosed 0 to form adsorbed sO3.l9 When heated, the SO3 decomposes around 650 K and leads to gaseous SO2 and 02, but no SO3. By analogy, we attribute the peaks around 780 K to the decomposition of photochemically produced SO3. The desorption at 380 K, which is not accompanied by O2production (consistent with the cited Ag(ll0) work), is tentatively ascribed to the desorption of SO2 stabilized by O(a). Residual S and 0, detected by AES, could not be completely removed by flashing to 900 K. These residues may indicate the existence of some SO1 and subsurface 0,as proposed for SO2 on Ag( 110).19 Ar+ sputtering successfully removed these residuals. 3.2. Photochemistry Revealed by TOF. In this section, we report the photochemistry, revealed by TOF results, of multilayer S02/Ag(ll1). On the basisoftheresults,wediscusstheexcitation mechanism and possible primary product channels. We demonstrate that, due to the formation and the subsequent relaxation
0
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Time (ps) Figure 2. TOF of the desorbing species at 193 nm for 3.5 ML S02/ Ag(ll1). The pulse energy per cm2during the experiments was 0.75 mJ cm-2. The contributionof SO2 fragmentation to the SO+TOF spectrum has been subtracted as described in section 2. The dashed lines are modified Maxwell-Boltzmann fits to the TOF signals. of the excited states, photodissociation, photoreaction, and photodesorption occur. 3.2.1. An Overview of Photochemical Products and Their Energy Distribution at 193 nm. Using 193-nm (6.4-eV) photons, T O F spectra of 16,48,64, and 80 amu ions were recorded. Figure 2 shows the TOF spectra for 3.5 ML S O ~ / A g ( l l l ) . The production of 0 and SO is consistent with gas-phase photochemistry,3-7 which leads exclusively to these two products at low pressures and 193 nm, Le.,
SO,+ 1 9 3 n m + S O + O
(1)
The mass 16 signal is too fast to be a fragment, formed in the ionizer, from the parent molecule or other sulfur oxide species. From the TOF, we obtained mean translational energies for 0, ( E ) = 0.15 f 0.01 eV,2O and SO, ( E ) = 0.33 f 0.09 eV. Interestingly, mass 80 was also detected. The mean translational energy is ( E ) = 0.18 f 0.03 eV. SO3, not S20,is favored since the former, but not the latter, is found in gas-phase photochemical s t ~ d i e s . ~ -Since l ~ 0 is produced at 193 nm,one possible way of making SO3is through collisional reaction between a recoiling 0 photofragment and a ground-state SO5
-
0 + SO,
SO3
(2)
This type of reaction has been demonstrated in a nice set of experiments involving ph ysisorbed molecules and has been termed “surface aligned photoreaction” or Photoreactions between two surface species have been detected in other cases as well.22 Another pathway, involving an excited SO2 and a ground state S02, is also likely:
-
+ 193 nm SO,(C’B,) + SO, SO,
SO,(C’B,)
SO + SO,
(3) (4)
For simplicity, in the remainder of this paper, reactions 3 and 4 will be referred to as the bimolecular photoreaction. As shown in section 3.2.4, the bimolecular photoreaction is the sole reaction pathway for SO and SO3 production at longer wavelengths (248 and 351 nm). In high-pressure gases, an excited state can be
Photochemical Pathways of SO2 Adsorbed on Ag( 111) I
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Pulse energy/an2 (mJ/cm*) Figure 3. Desorption yield per pulse (TOF area) vs excitation pulse energy per cmz at 193 nm for 3.5 ML SOz/Ag(I 11) at 89 K. The total photon fluence was fixed at 9.3 X 10l6photon c r 2 by changing the number of laser shots. The yields for 0, SO,SO2, and SO3 measured at 0.25 mJ are normalized to unit values. We estimate *lo% error in the measurement of yields. The lines are fits to the data. efficiently quenched by intermolecular interactions. One result is decreased fluorescence inten~ity.23.2~Another radiationless quenching path is photorea~tion,~-l3.~5 e.g., analogous to reaction 4. We suppose both are important under our conditions. Supporting the proposed reaction path, photolysiss-ll of highpressure gas-phase SO2 shows that SO3 is produced exclusively through reactions 3 and 4. The 64 amu TOF signal is ascribed to parent desorption. The mean translational energy, ( E ) ,is 0.17 f 0.05 eV. The maximum translationalenergy, measured for 3.5 ML SOZ/Ag( 11l ) , is0.31 f 0.02 eV. As discussed later, the pathway probably involves the combined effects of electronic transitions and transient heating. 3.2.2. Dependenceon Pulse Energyper cm2at 193 nm. Figure 3 shows the pulse energy per cm2 dependence of the 0,SO, SO3, and SO2 yields at 193 nm for 3.5 ML S02/Ag(lll). The total photon fluence was fixed at 9.3 X 10I6photons cm-2 by varying the number of laser shots. In order to comapre the variations, the yields per pulse measured at 0.25 mJ cm-2 pulse-' were normalized to unity. While the yields per pulse for 0, SO, and SO3 are linearly dependent on pulse energy, that for the parent is nonlinear, particularly above 1.5 mJ cm-2 pulse-'. Moreover, except for a slight increase in the population of the fastest desorption fragments, the translational energy distributions of 0,SO, and SO3 do not change up to 5 mJ cm-2 pulse-'. These results point to a nonthermal mechanism for the production of desorbing 0,SO, and SO3but suggest contributions from thermal desorption for S02, at least above 1.5 mJ cm-? pulse-'. 3.2.3. Coverage Dependence at 193 nm. Figure 4 shows atomic oxygen TOF and yields (inset) at 193 nm. Up to 1.35 ML, no 0 desorbs; evidently, there is a threshold between 1.35 and 1.6 ML. After an initial increase from 1.6 to 3 ML, the yields remain constant. Characteristic energies (e.g., the mean translational energy (E) and the maximum translational energy E-) are (E) = 0.15 f 0.02 eV and E,, = 0.62 f 0.01 eV, with no measurable coverage dependence. E, is comparable with the gas-phase 0 translational energy at 193 nm (-0.58 eV6). Figure 5 shows the SO TOF spectra at 193 nm. The inset shows the SO yields as a function of coverage; although following the same trend as 0,they become constant only above 6.5 ML. The SO mean translational energy and maximum translational energy are ( E ) = 0.33 f 0.09 eV and E, = 0.53 k 0.07 eV.
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I1.kn---
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Time (ps) Figure 4. For 193 nm and 0.75 mJ pulses, the 0 TOF and yields (shown in the inset) for several coverages. The solid line in the inset is to guide the eye.
0
Time (ps) Figure 5. For 193 nm and 0.75 mJ pulses, the SO TOF and yields (shown in the inset) for several coverages. The solid line in the inset is to guide the eye. The maximum translational energy is much higher than that for 193-nm gas-phase photodissociation (-0.2 eV).6 Interestingly, while the E, of 0 is approximately the gasphase value, the E- of SO is much larger than its gas-phase counterpart. While linear momentum conservation calculations may involve more than the two photofragments (0and SO) of reaction 1, in many physisorbed systems the maximum translational energies of photoproducts are close to their gas-phase counterparts.26 We believe it more likely that the SO maximum translational energy is determined by the bimolecular photoreaction. The threshold photon energy (3.2 eV)z7for the bimolecular reaction is lower than that for the unimolecular reaction (5.65 eV). By simple reaction enthalpy analysis, we estimate that, in producing SO, reaction pathway 3 and 4 is 348.06 kJ/mol more exothermic than pathway 1.28 While the partitioning of the available energy is not known, the translational energy of exiting SO could be very different for the bimolecular and unimolecular paths.
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Time (11s) Figure 7. SO2 TOF spectra of 1 and 3.5 ML S02/Ag( 111) excited by 193-nm photons (0.75 mJ c m 2 pulse-1). The solid lines are modified Maxwell-Boltzmann fits to the TOF signals.
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Parent desorption follows a different trend (Figure 6). Unlike the other species, SO2 desorbs even at submonolayer coverages. For all five wavelengths used in this experiment (193, 248, 351, 437, and 500 nm) and low coverages (c2.7 ML for 248 nm and 10 mJ cm-2) and high SO2 coverages (>6 ML). This minor desorption channel at high coverages is not due to substrate electron driven processes, which we observed but only for coverages less than 3.5 ML.' TheSOz mean translational energy corresponds to about 150 K,close to the peak temperature (130 K) observed in TPD of SO2 multilayers. The yield also increased exponentially with laser excitation power. These facts point to a thermal origin. Since this channel is important at high coverages where substrate heating should be minimal, we propose that desorption is due to weak local heating of the SO2 film associated with absorption of light at impurities or other imperfections in the SO2 thin film. Similar processes are known: e.g., color centers in a transparent substrate (e.g., LiF) can lead to adorbate desorption.30 The 0 signal measured a t 193 nm is absent at longer wavelengths, but the other reaction products appear a t 248 and 351 nm (Figure 8 and Table 1). Except for SO, and SO (248 and 35 1 nm only), the product translational energy distributions vary with wavelength but not with pulse energy. This evidence
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4645
Photochemical Pathways of SO2 Adsorbed on Ag( 11 1)
TABLE 1: Mean Translational Energies at 193, 248, and 351 nm ( E ) (ev) A, nm
193 248 351 437,500
0 0.15f0.02 no product no product no product
so
so2
so3
0.33f0.09 0.2f 0.05 0.15i 0.05 no product
0.17f0.05 0.13 f 0.04 0.09 f 0.03 no product'
0.18f0.03 0.17 f 0.03 0.16 i 0.03 no product
Small amounts of S02, attributed to transient thermal heating,desorb at high laser energy per cm2 (>lo mJ cm-2). (I
further supports direct electronic excitation in the physisorbed layer as driving the observed photochemical processes. As outlined below, we attribute the SO and SO3formed at 248 and 351 nm solely to bimolecular photoreactions like (3) and (4). To begin, we exclude two other possibilities. (1) A two-photon excitation process is unlikely because the pulse power density used here is about lo3 smaller than used in two-photon experim e n t ~ .Moreover, ~~ the 0 signal, which is the main product of the two-photon dissociation process in the gas phase,31 is absent. (2) With 248- and 351-nm photon excitation, reaction pathway 2 is not accessible, since no 0 is produced. We turn to the bimolecular photoreaction which involves a photoexcited SO2 reacting with a ground-state SOz. Following gas-phase r e s u l t ~ , ~we + ~propose ~ 3 ~ that, at 248 rim, the molecules are excited to the lowest singlet excited state, BIB1:
SO, + 248 nm -,SO,(B'B,)
(5)
S02(B'B1)+ SO, -,SO + SO,
(6)
At 351 nm, we propose that the molecules are excited to the lowest triplet states, P B 1 or ,A2:
SO,
+ 351 nm
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S0,(B3B1 or 3A,)
S0,(B3Bl or 3A,) + SO, -,SO + SO,
(7)
(8)
Many gas-phase experiments have shown that these paths are important a t high pressures and are the only photochemical channel at long wavelengths (hv < Edissociation).8-9-32 Regarding the condensed phase as an extremely high-density gas,33 various energy-transfer pathways (with and without orbitaloverlap) could function within the short intermolecular distances in physisorbed l a y e r ~ . ~The ~ 3 molecular ~ orbitals of excited SO2 may overlap with a nearby ground-state SO2 molecule and facilitate reaction. The gas-phase reaction proceeds with high probability (at high pressure, the quantum yields for SO3are as high as 0.5),10 and we suppose the same is true qualitatively for multilayers. Significantly, the distribution of 0 and SO is quite nonBoltzmann at 193nm, with a sharp rising edge in the TOF spectra. In fact, 0 and SO F93-nm TOF data can be adequately fit by Gaussian distributions, while other masses and/or SO a t other wavelengths are only adequately fit by a modified Boltzmann di~tribution.'~This indicates that, upon photon absorption, a significant fraction of the dissociation and desorption events are prompt and occur with quite specific dynamics within a limited set of adsorbate configurations. Fluorescence lifetime and quantum yield measurements from excited SO2 single vibronic states in CIBI demonstrate that energy randomization is not complete before photodissociati~n.~~ Contrasting with these 193nm results, the 248- and 351-nm T O F data are adquately fit by modified Boltzmann distributions, indicating significant relaxation during the time interval between excitation and reaction. When excited to the BIBI (248 nm) state, the bimolecular photoreactions probably involve crossover to the lowest triplet states (83B1 or 3A2).8 Significant thermalization could occur in the long-lived triplet state before the photoreaction occurs. Comparing the product mean translational energy in Table 1, we note that the
bimolecular photoreaction products SO and SO3 have similar mean translational energies a t 248 and 35 1 nm. This agrees with the notion that bimolecular reaction at 248 and 351 nm occurs through a common low-lying, long-lived triplet state, independent initial excitation energy. Further support comes from the fact that the 0 yields saturate sharply when 3 ML coverage is reached (Figure 4), while the saturation of SO (Figure 5) and SO3 (not shown) occurs a t higher coverages. While transient heating may become influential at high ~overages,3~-38 the above coverageeffect is mainly attributable to a substrate quenching effect. In other experiments, where we kept SO2 coverage constant and varied the spacer layer thickness, we showed that substrate quenching of the bimolecular reaction extended to much thicker spacer layers than the unimolecular photodisso~iation.~~ These effects reflect the different quenching effect of the substrate on the molecular excited states (singlet or triplet) from which the photoreactions originate. To identify the desorption mechanism@) for parent molecules in multilayers is moredifficult. One possibility involves substrate hot carrier and/or photoelectron mediated processes. But, as demonstrated in previous studies,lJ8J9 the substrate-mediated processes diminish rapidly in multilayers; thus, other paths must operate to account for the high coverage results shown in Figures 3 and 6. Various systems show parent photodesorption from physisorbed layers falling into two categories, both initiated by electronic transitions within the adsorbate: (1) nonthermal energy transfer and desorption40 and (2) transient heating accompanied by d e ~ o r p t i o n .Here, ~ ~ ~ ~the ~ SO2 T O F distributions depend on wavelength. At 437 and 500 nm, where SO2 is transparent, we see no parent desorption below 10 mJ cm-2. However, a t higher excitation powers the yield rises exponentially as expected for a thermally driven process. At shorter wavelengths where SO2 absorbs photons, the parent desorption yield/pulse increases supralinearly at high pulse energies ( 1 2 mJ cm-2 a t 193 nm in Figure 3), also indicating the importance of thermal effects. But at lower pulse energies, 0.25-1.3 mJ cm-2 at 193 nm, the SO2 yield per pulse is linear (Figure 3). We propose a desorption mechanism induced by electronic transitions that leads to both nonthermal and thermal parent desorption. Overall, this kind of desorption can be described by the following sequence: photons electronic excitation nonthermal excitation of modes of atom/molecule motion -,thermalization thermal desorption. The bimolecular reaction (5-8) and nonthermal desorption (9, 10) occur in the second step. The transient temperature rise depends on the thermal conductivity, and since the adsorbate's thermal conductivity is poor compared to the substrate, energy deposited in multilayers will lead to a relatively large, but very local, temperature increase and enhanced thermal des~rption.~l While it is evident that SO2 desorption is initiated by electronic transitions, the dynamics have yet to be explored. We are interested in how large amounts of electronic energy be converted into heavy-particle translational motion. One possible pathway may be similar to the "caging" observed in liquids, where dissociation of thesolute is prevented by energy-transfer collisions with nearby solvent molecules.42 In our experiments, directly analogous unimolecular dissociation of SO2 is possible only at 193 nm. The energy transfer required to initiate desorption a t longer wavelengths is likely analogous to the self-quenching process found in gas-phase reactions:I2-I4
-
-
-
SO, + hv -,*SO,
-
*so*+ so* so, + so,
(9) (10)
where *SO2 is an electronically excited adsorbate. The bimolecular process 10 converts electronic energy to molecular translational and internal energies. Considering reaction 10 more deeply, nonzero intermolecular interaction is the result of
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molecular orbital overlap between an excited SO2 and a groundstate SOz. Recent experimentaland theoretical studies of excitedstate relaxation in fluids indicate that even a nonpolar solvent can induce significant modification of the solute potential energy surface, especially when the solute is electronically During quenching, nonadiabatic coupling can produce solvent fragments and/or vibrationally excited solutes in their ground electronic state. Multiple solute-solvent bond breaking has been observed, and mediation of intermediate states has been proposed.43~44 In the case of physisorbed S02, several neighboring SO2 molecules may be dislodged. In summary, we have demonstrated that the photochemistry of S02/Ag( 111) depends strongly on wavelength and coverage. No photochemistry involving S-0 bond breaking occurs below multilayer coverage. Various photochemical pathways have been found in physisorbed multilayers. While unimolecular photodissociation occurs only at 193 nm, bimolecular photoreaction and photodesorption occur when 193-, 248-, and 35 1-nm photons are used. Except for a small amount of parent desorption due tolaser heatingat high powers (> 10mJcm-2),nophotochemistry occurs at 437 and 500 nm, where SO2 is transparent.
4. Conclusions Based on AES, postirradiation TPD, and TOF-MS data, photochemistry, absent for monolayers, occurs for multilayers of SO2 adsorbed on Ag( 111). The multilayer photochemistry tracks theS02absorption. At 437 and 500 nm, whereSOz is transparent, no reactions involving S-O bond cleavage occur. At shorter wavelengths, the excitation mechanism for the observed multilayer photochemical processes is attributed to direct electronic transitions. Excited by 193-nm photons, 0, SO,S02, and SO3 are detected. Both unimolecular photodissociation, forming 0 and SO,and bimolecular photoreaction, forming SO and SO3, occur. When theexcitation photonenergies (248 and 351 nm) aresmaller than the S-O bond dissociation energy, photochemistry stilloccurs, but only through the bimolecular photoreaction, to form SO and SO3. At low laser excitation powers ( < l o mJ cm-2), SO2 desorption from physisorbed multilayers is observed at 193,248, and 351 nm and is mainly attributed to nonthermal processes that are initiated by electronic transitions in the SO2 and/or the substrate. At very high laser excitation (>lo mJ cm-2), a second parent desorption channel is observed, even at 437 and 500 nm, and is attributed to transient heating.
Acknowledgment. We thank Drs. S.Gravelle, R. S.Mackay, and X.-Y.Zhu for their helpful discussions. This work is supported by the National Science Foundation, Grant CHE93 19640. References and Notes (1) Sun, Z.-J.; Gravelle, S.;Mackay, R. S.;Zhu, X.-Y.; White, J. M. J . Chem. Phys. 1993, 99, 10021. (2) Ryan, R. R.; Kubas, G. J.; Moody, D. C.; Eller, P. G. In Structure and Bonding, Clarke, M. J., Goodenough, J. B., Hemmerich, J. A., Ibers, J. A., Jorgcns.cn, C. K., Nerlands, J. B., Reinen, D., Wciss, R., Williams, R. J. P., Us.; Springer-Verlag: Berlin, 1981; Vol. 46. (3) Huang, Y.-L.; Gordon, R. J. J . Chem. Phys. 1990, 93, 868. (4) Kawasaki, M.; Kasatani, K.; Sato, H. Chem. Phys. 1982, 73, 377. (5) Kawasaki, M.: Sato. H. Chem. Phys. Lett. 1987. 139, 585. (6) Felder, P.; Effenhauser, C. S.;Haas, B. M.; Huber, J. R. Chem. Phys. Lett. 1988, 148, 417. (7) Chen, X.; Federico, A,; Wang, H.; Weiner, B. R. J . Phys. Chem. 1991, 95, 6415. (8) Okuda, S.;Navaneeth Rao, T.; Slater, D. H.; Calvert, J. G. J. Phys. Chem. 1969, 73,4412. (9) Otsuka, K.; Calvert, J. G. J. Am. Chem. SOC.1971, 93, 2581. (IO) Driscoll, J. N.; Warneck, P. J. Phys. Chem. 1968, 72, 3736. (11) Lalo, C.; Vermeil, C. J . Photochem. 1974, 3, 441. (12) Mettee, H. D. J . Chem. Phys. 1968, 49, 1784. (13) Strickler, S.J.; Howell, D. B. J. Chem. Phys. 1968, 49, 1947. (14) Phillips, L. F.; Smith, J. J.; Meyer, B. J . Mol. Spectrosc. 1969,29, 230.
Sun and White (15) Cehelnik, E.; Spicer, C. W.; Heicklen, J. J. Am. Chem. Soc. 1971, 93, 5373. (16) (a) Brand, J. L.; George, S.M.Surf.Sci. 1986, 167,341. (b) Hall, R. B. J. Phys. Chem. 1987, 91, 1007. (17) Guthrie, W. L.; Lin, T.-H.; Ceyer, S.T.; Somorjai, G. A. J. Chem. Phys. 1982, 76,6398. (18) Castro, M. E.;White, J. M. J. Chem. Phys. 1991, 95, 6057. (19) Outka, D. A.; Madix, R. J. Surf.Sci. 1982,137, 242. (20) The error bars obtained in this paper are obtained from the standard
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