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Surface photochemistry: Monolayer versus multilayer for alkyl halides on Ag{111}
CHEMICAL PHYSICS LETTERS
Volume 167, number 3
23 March 1990
SURFACE PHOTOCHEMISTRY: MONOLAYER VERSUS MULTILAYER FOR ALKYL HALIDES ON Ag{ 111) X.-L. ZHOU
and J.M. WHITE
Department of Chemistry, University of Texas, Austin, TX 78 712, USA
Received 6 November 1989; in final form 8 January 1990
The UV photodissociation of CH3Br and CzHsCl on Ag{111) has been studied at 1 and 2 monolayer coverages. As compared to the tirsc monolayer, the second layer has much lower photodissociation rates and a different wavelength dependence.
1. Introduction Recently, the photodissociation of molecules adsorbed on and near metal surfaces has become a subject of considerable interest in the surface science community. Two kinds of experiments have been done: the first focuses on the desorbing fragments [ 11, the second on products retained at the surface [ 21. In this communication, we compare the photon energy dependence of the photodissociation of CHSBr and C,H,Cl on Ag{11 l} and demonstrate, by examining retained products, distinct differences for 1 and 2 monolayer (ML) coverages.
2. Experimental
The experimental practices have been outlined previously [ 31. Briefly, standard UHV procedures were followed as regards surface cleanliness, dosing controlled amounts of adsorbate through a directed tubular doser, temperature control at 100 K, and post irradiation analysis at temperature-programmed desorption (TPD), work function change (A@), and X-ray photoelectron spectroscopy (XPS). The adsorbates were reagent grade gases and no impurities were detected by mass spectrometry. Ethyl, rather than methyl, chloride was chosen because multilayers of methyl chloride cannot be prepared at 100 K. The UV light source was a focused 100 W Hg arc lamp and the radiation passed through a 1.2 cm di-
aperture and selected cut-off filters before entering the vacuum chamber through a fused silica window. With the full arc (no filter) the incident power flux was 75 mW cms2 and caused a bulk temperature rise of 5 K. The angle of incidence was 15” with respect to the surface normal. ameter
3. Results and discussion In the absence of illumination, there is no detectable thermal decomposition of the adsorbates either during adsorption or subsequent thermal desorption. Based on XPS, there is neither loss of adsorbate nor breakage of C-Cl or C-Br bonds when the system is held below 110 K for extended periods [ 41. In TPD, only monolayer and multilayer parent molecule peaks are found (145 and 120 K for CH,Br; 154 and 124 K for &H&l) and, after TPD, the sample is clean as judged by XPS and A@measurements. Illumination for 30 min with filtered UV light gives unambiguous photochemistry. XPS shows the timedependent loss of C-X (X =Cl, Br) bonds and the formation of Ag-X bonds. While the C( 1s) signal decays, the signal from X remains constant. Post irradiation TPD has only three contributions; residual parent, ethane (butane) from C&Br (&H&l) and AgX [ 5 1. The interpretation is as follows: UV irradiation activates the system and leads to C-X bond cleavage with simultaneous formation of strong AgX bonds and either adsorbed or gas phase CH3
0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland)
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(C,H,). During subsequent TPD, the adsorbed alkyl fragments recombine to give the desorbing hydrocarbons and the X desorbs as AgX. Fig. 1 summarizes results for both molecules as average (30 min photolysis) rates attributable to the first and second ML, the latter calculated as the difference between the measured 2 ML and 1 ML rates. These curves are based on the loss of parent molecule desorption. The TPD peak areas are calibrated based on thermal desorption data which contains the easily distinguished monolayer and multilayer peaks noted above [ 5 1, Lines A, B and C mark the measured work functions of 1 ML methyl bromide (3.7 eV), 1 ML ethyl chloride (3.75 eV), and 2 ML of either adsorbate (4.0 eV), respectively. Fig. 2 compares, in logarithmic form, the gas phase optical absorption cross sections with measured photodissociation cross sections for the 1 ML cases. The latter are estimated, from photodissociation rate and energy flux measurements, by calculating differences between measured initial rates and between measured energy fluxes for sequential cut-off filters [ 5 1. The optical absorption cross section for the chloride involves extrapolation of literature values [ 6 ] _ Directly from these two figures we learn: ( 1) As
0.4 ,'
Cut-off energy/eV
Fig. 1.Photochemical rates of parent molecule loss, based on TPD peak areas, as a function of cut-off energy (determined by selected cut-off filters). Photolysis of 1 ML of CHJBr (open squares), I ML of C2HJC1 (open triangles), 2 ML of CHsBr (closed squares), and 2 ML of CsHsCl (closed triangles). Note that the 2 ML curves are calculated from the difference in the measured rates for 2 ML and 1 ML coverages. A, B, and C are work functions (see text).
206
23 March i 990
2
Optical AbsorDtionCross section
(Q=phase) Photodissociation Cross Section l0 =
5 ._ P Ot P 0
Chloride
/ -1 -
/ -2 1 3
.
I 4
6
Energy /eV
Fig. 2. Comparison of gas phase optical absorption cross sections and photodissociation cross sections for 1 ML cases (logarithmic scale). The zero of the ordinate corresponds to a cross section of 1O-zocm’.
indicated by the different cut-off energies, the photochemistry is molecule specific [ 7 1. Based on the 1 ML data of fig. 1, we estimate a 3 eV cut-off for CH,Br and 3.3 eV for &H&l. For the second ML (plotted as the difference between the measured rates for 2 ML and 1 ML), these cut-offs move to 3.5 and 4.0 eV, respectively. (2 ) The chemisorbed first layers are unique and can be photolyzed at much lower, E 0.5 eV, photon energies than the physisorbed second layer. For a given wavelength, the photodissociation cross section for chemisorbed monolayer is significantly higher than in the gas phase. (3 ) The monolayer photochemistry is much faster than the multilayer. For the full arc where both layers are subject to photodissociation, the average rate in the first layer is 3 to 5 times faster. (4) For both the 1 and 2 ML curves, the photochemistry does not follow the optical absorption cross section for either of the isolated molecules. The onset energy is much lower, the energy dependence above the threshold is much weaker, and the chloride/bromide rate ratio at 4.9 eV is lo4 larger than predicted using gas phase absorption cross section estimates. (5) The photochemistry is not, in general, directly correlated with the work function. Only in the case of the second layer of &H&l does the onset correspond to the work function. In all other cases, the onset lies well below the work function threshold. From these five points, we draw the following four conclusions. First, and even though quenching should
Volume 167,number 3
CHEMICALPHYSICSLETTERS
be faster, adsorbate molecules in direct contact with the substrate are much more susceptible to bond cleavage than those that are not. Second, regardless of the wavelength, there is no correlation, even for the physisorbed second layer, of the photochemistry with the gas phase molecular absorption cross section of the adsorbate. Third, since fig. 1 shows clear evidence for photochemistry with photon energies below the work function threshold, photoelectrons are not required for C-X bond cleavage. Fourth, above the work function thresholds, none of the yield curves tracks the expected number of photoelectrons. Based on measurements in the CH,Br/ Pt ( 111) system [ 81, we expect the number of photoelectrons to increase by about lo4 within 0.2 eV of the work function threshold and then increase by less than a factor of 10 over the next 1 eV. None of the yield curves shows a sharp step at the work function threshold. The distinctive features of the 1 ML photochemistry lead us to consider it separately. Based on analogies with organometallic compounds, it is reasonable to model the first monolayer of adsorbate and the first layer of Ag as a l&and-metal species that is weakly linked in the ground electronic state but strongly interacting in the excited states. In line with this analogy, the optical excitation cross sections could easily be lo2 larger at their maxima and quite strong at wavelengths much longer than for the alkyl halides themselves [ 91. Such excitations would place excited, localized holes and electrons in the valence orbitals of the adsorbate-substrate complex. These would be stabilized by interactions with the substrate. Interactions with the substrate would also tend to delocalize and quench these excitations. Dissociation would occur either as the result of direct bond cleavage along repulsive excited state potential energy surfaces or, after quenching, through reactions of vibrationally activated ground state species with substrate atoms [ 2,7,10]. If the absorption cross section were as high as 1O-‘* cm*, then no more than 10m4 of the incident radiation would be absorbed in the adsorbate-substrate layer described above. The remainder would either be absorbed in the substrate or reflected. Thus, most of the absorption occurs in the metal. For C-X bond dissociation to follow from substrate excitation, the excited electrons and/or holes must, with measura-
23 March 1990
bIe probability,
be captured (localized) by the adsorbate-substrate complex. Even if only a small fraction of these substrate excitations become localized at the surface, their numbers could still compete with the small number of localized excitations formed by localized surface excitation. Capture of a hole and an electron would lead to a final state equivalent to that reached by the kind of adsorbate-substrate excitation described above. Electron (hole) capture alone would lead to different, temporary anion (cation), states. As in many other cases [ 2,3,10 J, we cannot resolve the contributions of direct excitation of the adsorbate-substrate complex and of indirect excitation via excited substrate electrons and holes formed in the substrate and transferred to (captured by) the adsorbate-substrate complex. Most likely, both of these make contributions, depending upon the photon energies involved. Turning to the second monolayer, we anticipate, because of slower quenching, that excited states, once formed, will dissociate with higher probability (faster rates) than in the first monolayer. To account for the observed slower rates in the second layer, we must turn then to excitation probabilities. Direct optical excitation of these physisorbed molecules is unlikely since, due to spatial separation, there is not strong coupling to the substrate, even in the excited state and since the absorption coefficients of the molecules themselves are negligibly small. Direct excitation of the adsorbate-substrate complex is not a relevant issue for the second monoiayer (the adsorbatesubstrate complex forms part of the overall substrate for the second monolayer). Thus, and particularly for the 2 ML C2H5Cl photochemistry, we are left with a process involving excited electrons and/or holes formed in the substrate. According to these ideas, the second layer photochemistry is much slower because direct excitation plays no role and because substrate excitations are strongly quenched before reaching the second layer. One may argue that the slower rate for the second layer is because the first layer uses up “hot” electrons. However, these low energy electrons have very large mean free paths and should be able to easily reach the second layer unless there is a large potential barrier at the first and the second layer interface which prevents them passing through. Regardless of the possibilities, the first monolayer is 207
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unique and has a higher photodissociation probability than the second layer. The results can be understood in terms of an electron attachment process leading to bond cleavage a process known for gas phase alkyl halides. As a function of kinetic energy, electron capture by gas phase &H&l has a cross section peaking at 0.13 eV with an amplitude of order lo-l9 cm2 and a half width of 0.05 eV [ 111. Assuming that this gas phase property applies for the second monolayer of &H&l, we predict a photon energy threshold slightly above (x 0.1 eV) the work function threshold (3.75 eV) of the first monolayer (second layer “detects” the electrons passing through the first layer) and a yield curve depending on the number of electrons in a 0.05 eV energy range. Qualitatively, this simplified model is consistent with our data (threshold between 3.9 and 4.1 eV and no stepped increase in the rate above the threshold). Qualitatively, the same ideas hold for CHaBr. A more realistic model, beyond the scope of this work, would account for the local potential at various sites rather than simply taking the average potential measured by the work function. Our results show that the total photodissociation cross section drops sharply in passing from 1 to 2 ML. On the other hand, dynamics measurements in the CH$l-Ni ( 111) system shows a rising cross section for CH, desorption over the first 4 ML [ 121. These two sets of results become qualitatively consistent when retained fragments are considered. Based on XPS and TPD results, we estimate that, for 1 ML of CH,Br on Ag{11 l}, the initial rate of CHJ desorption is G 0.33 of the total rate of dissociation, whereas in the second layer the desorption rate is nearly equal to the dissociation rate.
4. Summary For CH,Br and &H&l adsorbed on Ag{111) at 100 K, there is no thermal chemistry but unambiguous, wavelength and coverage-dependent, photochemical cleavage of the C-X bonds. Our results underscore the distinctive properties of the first and second monolayers, as follows: ( I ) The threshold photon energy for dissociation in the second monolayer is significantly higher than in the first. 208
23 March 1990
(2) For photon energies up to 5 eV, the total dissociation cross section of the first monolayer is more than 3 times of that in the second monolayer. We attribute the unique character of the first monolayer to interactions, particularly those in the excited state, that involve the metal substrate. The C-X bond is activated by optical absorption, in analogy with organometallic processes such as metal-ligand charge transfer, and/or by capturing substrate excitations.
Acknowledgement
This work supported by the US Department of Energy, Offlice of Basic Energy Sciences. We thank Dr. A. Cassuto for helpful comments.
References [ I ] D.S. King and R.R. Cavanagh, Advan. Chem. Phys. 76 (1989) 45.
[ 2) S.A. Costello, B. Roop, Z.-M. Liu and SM. White, J. Phys. Chem. 92 (1988) 1019: X. Guo, L. Hanley and J.T. Yates Jr., J. Chem Phys. 90 ( 1989) 5200; V.H. Grassian and G.C. Pimentel, J. Chem. Phys. 88 ( 1988) 4484. [ 3 ] KG. Lloyd, A. Campion and J.M. White, Catal. Letters 2 (1989) 105; K.G. Lloyd, B. Roop, A. Campion and J.M. White, Surface Sci. 214 (1989) 227. [ 41 X.-L. Zhou, F. Solymosi, P.M. Blass, KC. Cannon and J.M. White, Surface Sci. 2 19 ( 1989) 294. [5] X.-L. Zhou and J.M. White, Surface Sci., submitted for publication. [6] J.C. Calvert and J.N. Pitts Jr., Photochemistry (Wiley, New York, 1966) p. 523. [ 71 B. Roop, KG. Lloyd, S.A. Costello, A. Campion and J.M. White, J. Chem. Phys. 91 ( 1989) 5103. [ 8 ] SK. Jo and J.M. White, to be published. 191 G.L. Geoffroy and M.S. Wrigbton, Organometallic photochemistry (Academic Press, New York, 1979). [IO] Z.-M. Liu, S.A. Costello, B. Roop, S. Coon, S. Akhter and J.M. White, J. Phys. Chem. 93 (1989) 7681. [ 111 Z. Lj. Pettrovic, WC. Wang and L.C. Lee, J. Chem. Phys. 90 (1989) 3145. [ 121 E.P. Marsh, T.L. Gilton, W. Meier, M.R. Schneider and J.P. Cowin, Phys. Rev. Letters 61 (1988) 2725; T.L. Gilton, C.P. Dehnbostel and J.P. Cowin, J. Chem. Phys. 91 (1989) 1937.
Volume 167, number 3
23 March 1990
SURFACE PHOTOCHEMISTRY: MONOLAYER VERSUS MULTILAYER FOR ALKYL HALIDES ON Ag{ 111) X.-L. ZHOU
and J.M. WHITE
Department of Chemistry, University of Texas, Austin, TX 78 712, USA
Received 6 November 1989; in final form 8 January 1990
The UV photodissociation of CH3Br and CzHsCl on Ag{111) has been studied at 1 and 2 monolayer coverages. As compared to the tirsc monolayer, the second layer has much lower photodissociation rates and a different wavelength dependence.
1. Introduction Recently, the photodissociation of molecules adsorbed on and near metal surfaces has become a subject of considerable interest in the surface science community. Two kinds of experiments have been done: the first focuses on the desorbing fragments [ 11, the second on products retained at the surface [ 21. In this communication, we compare the photon energy dependence of the photodissociation of CHSBr and C,H,Cl on Ag{11 l} and demonstrate, by examining retained products, distinct differences for 1 and 2 monolayer (ML) coverages.
2. Experimental
The experimental practices have been outlined previously [ 31. Briefly, standard UHV procedures were followed as regards surface cleanliness, dosing controlled amounts of adsorbate through a directed tubular doser, temperature control at 100 K, and post irradiation analysis at temperature-programmed desorption (TPD), work function change (A@), and X-ray photoelectron spectroscopy (XPS). The adsorbates were reagent grade gases and no impurities were detected by mass spectrometry. Ethyl, rather than methyl, chloride was chosen because multilayers of methyl chloride cannot be prepared at 100 K. The UV light source was a focused 100 W Hg arc lamp and the radiation passed through a 1.2 cm di-
aperture and selected cut-off filters before entering the vacuum chamber through a fused silica window. With the full arc (no filter) the incident power flux was 75 mW cms2 and caused a bulk temperature rise of 5 K. The angle of incidence was 15” with respect to the surface normal. ameter
3. Results and discussion In the absence of illumination, there is no detectable thermal decomposition of the adsorbates either during adsorption or subsequent thermal desorption. Based on XPS, there is neither loss of adsorbate nor breakage of C-Cl or C-Br bonds when the system is held below 110 K for extended periods [ 41. In TPD, only monolayer and multilayer parent molecule peaks are found (145 and 120 K for CH,Br; 154 and 124 K for &H&l) and, after TPD, the sample is clean as judged by XPS and A@measurements. Illumination for 30 min with filtered UV light gives unambiguous photochemistry. XPS shows the timedependent loss of C-X (X =Cl, Br) bonds and the formation of Ag-X bonds. While the C( 1s) signal decays, the signal from X remains constant. Post irradiation TPD has only three contributions; residual parent, ethane (butane) from C&Br (&H&l) and AgX [ 5 1. The interpretation is as follows: UV irradiation activates the system and leads to C-X bond cleavage with simultaneous formation of strong AgX bonds and either adsorbed or gas phase CH3
0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland)
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(C,H,). During subsequent TPD, the adsorbed alkyl fragments recombine to give the desorbing hydrocarbons and the X desorbs as AgX. Fig. 1 summarizes results for both molecules as average (30 min photolysis) rates attributable to the first and second ML, the latter calculated as the difference between the measured 2 ML and 1 ML rates. These curves are based on the loss of parent molecule desorption. The TPD peak areas are calibrated based on thermal desorption data which contains the easily distinguished monolayer and multilayer peaks noted above [ 5 1, Lines A, B and C mark the measured work functions of 1 ML methyl bromide (3.7 eV), 1 ML ethyl chloride (3.75 eV), and 2 ML of either adsorbate (4.0 eV), respectively. Fig. 2 compares, in logarithmic form, the gas phase optical absorption cross sections with measured photodissociation cross sections for the 1 ML cases. The latter are estimated, from photodissociation rate and energy flux measurements, by calculating differences between measured initial rates and between measured energy fluxes for sequential cut-off filters [ 5 1. The optical absorption cross section for the chloride involves extrapolation of literature values [ 6 ] _ Directly from these two figures we learn: ( 1) As
0.4 ,'
Cut-off energy/eV
Fig. 1.Photochemical rates of parent molecule loss, based on TPD peak areas, as a function of cut-off energy (determined by selected cut-off filters). Photolysis of 1 ML of CHJBr (open squares), I ML of C2HJC1 (open triangles), 2 ML of CHsBr (closed squares), and 2 ML of CsHsCl (closed triangles). Note that the 2 ML curves are calculated from the difference in the measured rates for 2 ML and 1 ML coverages. A, B, and C are work functions (see text).
206
23 March i 990
2
Optical AbsorDtionCross section
(Q=phase) Photodissociation Cross Section l0 =
5 ._ P Ot P 0
Chloride
/ -1 -
/ -2 1 3
.
I 4
6
Energy /eV
Fig. 2. Comparison of gas phase optical absorption cross sections and photodissociation cross sections for 1 ML cases (logarithmic scale). The zero of the ordinate corresponds to a cross section of 1O-zocm’.
indicated by the different cut-off energies, the photochemistry is molecule specific [ 7 1. Based on the 1 ML data of fig. 1, we estimate a 3 eV cut-off for CH,Br and 3.3 eV for &H&l. For the second ML (plotted as the difference between the measured rates for 2 ML and 1 ML), these cut-offs move to 3.5 and 4.0 eV, respectively. (2 ) The chemisorbed first layers are unique and can be photolyzed at much lower, E 0.5 eV, photon energies than the physisorbed second layer. For a given wavelength, the photodissociation cross section for chemisorbed monolayer is significantly higher than in the gas phase. (3 ) The monolayer photochemistry is much faster than the multilayer. For the full arc where both layers are subject to photodissociation, the average rate in the first layer is 3 to 5 times faster. (4) For both the 1 and 2 ML curves, the photochemistry does not follow the optical absorption cross section for either of the isolated molecules. The onset energy is much lower, the energy dependence above the threshold is much weaker, and the chloride/bromide rate ratio at 4.9 eV is lo4 larger than predicted using gas phase absorption cross section estimates. (5) The photochemistry is not, in general, directly correlated with the work function. Only in the case of the second layer of &H&l does the onset correspond to the work function. In all other cases, the onset lies well below the work function threshold. From these five points, we draw the following four conclusions. First, and even though quenching should
Volume 167,number 3
CHEMICALPHYSICSLETTERS
be faster, adsorbate molecules in direct contact with the substrate are much more susceptible to bond cleavage than those that are not. Second, regardless of the wavelength, there is no correlation, even for the physisorbed second layer, of the photochemistry with the gas phase molecular absorption cross section of the adsorbate. Third, since fig. 1 shows clear evidence for photochemistry with photon energies below the work function threshold, photoelectrons are not required for C-X bond cleavage. Fourth, above the work function thresholds, none of the yield curves tracks the expected number of photoelectrons. Based on measurements in the CH,Br/ Pt ( 111) system [ 81, we expect the number of photoelectrons to increase by about lo4 within 0.2 eV of the work function threshold and then increase by less than a factor of 10 over the next 1 eV. None of the yield curves shows a sharp step at the work function threshold. The distinctive features of the 1 ML photochemistry lead us to consider it separately. Based on analogies with organometallic compounds, it is reasonable to model the first monolayer of adsorbate and the first layer of Ag as a l&and-metal species that is weakly linked in the ground electronic state but strongly interacting in the excited states. In line with this analogy, the optical excitation cross sections could easily be lo2 larger at their maxima and quite strong at wavelengths much longer than for the alkyl halides themselves [ 91. Such excitations would place excited, localized holes and electrons in the valence orbitals of the adsorbate-substrate complex. These would be stabilized by interactions with the substrate. Interactions with the substrate would also tend to delocalize and quench these excitations. Dissociation would occur either as the result of direct bond cleavage along repulsive excited state potential energy surfaces or, after quenching, through reactions of vibrationally activated ground state species with substrate atoms [ 2,7,10]. If the absorption cross section were as high as 1O-‘* cm*, then no more than 10m4 of the incident radiation would be absorbed in the adsorbate-substrate layer described above. The remainder would either be absorbed in the substrate or reflected. Thus, most of the absorption occurs in the metal. For C-X bond dissociation to follow from substrate excitation, the excited electrons and/or holes must, with measura-
23 March 1990
bIe probability,
be captured (localized) by the adsorbate-substrate complex. Even if only a small fraction of these substrate excitations become localized at the surface, their numbers could still compete with the small number of localized excitations formed by localized surface excitation. Capture of a hole and an electron would lead to a final state equivalent to that reached by the kind of adsorbate-substrate excitation described above. Electron (hole) capture alone would lead to different, temporary anion (cation), states. As in many other cases [ 2,3,10 J, we cannot resolve the contributions of direct excitation of the adsorbate-substrate complex and of indirect excitation via excited substrate electrons and holes formed in the substrate and transferred to (captured by) the adsorbate-substrate complex. Most likely, both of these make contributions, depending upon the photon energies involved. Turning to the second monolayer, we anticipate, because of slower quenching, that excited states, once formed, will dissociate with higher probability (faster rates) than in the first monolayer. To account for the observed slower rates in the second layer, we must turn then to excitation probabilities. Direct optical excitation of these physisorbed molecules is unlikely since, due to spatial separation, there is not strong coupling to the substrate, even in the excited state and since the absorption coefficients of the molecules themselves are negligibly small. Direct excitation of the adsorbate-substrate complex is not a relevant issue for the second monoiayer (the adsorbatesubstrate complex forms part of the overall substrate for the second monolayer). Thus, and particularly for the 2 ML C2H5Cl photochemistry, we are left with a process involving excited electrons and/or holes formed in the substrate. According to these ideas, the second layer photochemistry is much slower because direct excitation plays no role and because substrate excitations are strongly quenched before reaching the second layer. One may argue that the slower rate for the second layer is because the first layer uses up “hot” electrons. However, these low energy electrons have very large mean free paths and should be able to easily reach the second layer unless there is a large potential barrier at the first and the second layer interface which prevents them passing through. Regardless of the possibilities, the first monolayer is 207
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unique and has a higher photodissociation probability than the second layer. The results can be understood in terms of an electron attachment process leading to bond cleavage a process known for gas phase alkyl halides. As a function of kinetic energy, electron capture by gas phase &H&l has a cross section peaking at 0.13 eV with an amplitude of order lo-l9 cm2 and a half width of 0.05 eV [ 111. Assuming that this gas phase property applies for the second monolayer of &H&l, we predict a photon energy threshold slightly above (x 0.1 eV) the work function threshold (3.75 eV) of the first monolayer (second layer “detects” the electrons passing through the first layer) and a yield curve depending on the number of electrons in a 0.05 eV energy range. Qualitatively, this simplified model is consistent with our data (threshold between 3.9 and 4.1 eV and no stepped increase in the rate above the threshold). Qualitatively, the same ideas hold for CHaBr. A more realistic model, beyond the scope of this work, would account for the local potential at various sites rather than simply taking the average potential measured by the work function. Our results show that the total photodissociation cross section drops sharply in passing from 1 to 2 ML. On the other hand, dynamics measurements in the CH$l-Ni ( 111) system shows a rising cross section for CH, desorption over the first 4 ML [ 121. These two sets of results become qualitatively consistent when retained fragments are considered. Based on XPS and TPD results, we estimate that, for 1 ML of CH,Br on Ag{11 l}, the initial rate of CHJ desorption is G 0.33 of the total rate of dissociation, whereas in the second layer the desorption rate is nearly equal to the dissociation rate.
4. Summary For CH,Br and &H&l adsorbed on Ag{111) at 100 K, there is no thermal chemistry but unambiguous, wavelength and coverage-dependent, photochemical cleavage of the C-X bonds. Our results underscore the distinctive properties of the first and second monolayers, as follows: ( I ) The threshold photon energy for dissociation in the second monolayer is significantly higher than in the first. 208
23 March 1990
(2) For photon energies up to 5 eV, the total dissociation cross section of the first monolayer is more than 3 times of that in the second monolayer. We attribute the unique character of the first monolayer to interactions, particularly those in the excited state, that involve the metal substrate. The C-X bond is activated by optical absorption, in analogy with organometallic processes such as metal-ligand charge transfer, and/or by capturing substrate excitations.
Acknowledgement
This work supported by the US Department of Energy, Offlice of Basic Energy Sciences. We thank Dr. A. Cassuto for helpful comments.
References [ I ] D.S. King and R.R. Cavanagh, Advan. Chem. Phys. 76 (1989) 45.
[ 2) S.A. Costello, B. Roop, Z.-M. Liu and SM. White, J. Phys. Chem. 92 (1988) 1019: X. Guo, L. Hanley and J.T. Yates Jr., J. Chem Phys. 90 ( 1989) 5200; V.H. Grassian and G.C. Pimentel, J. Chem. Phys. 88 ( 1988) 4484. [ 3 ] KG. Lloyd, A. Campion and J.M. White, Catal. Letters 2 (1989) 105; K.G. Lloyd, B. Roop, A. Campion and J.M. White, Surface Sci. 214 (1989) 227. [ 41 X.-L. Zhou, F. Solymosi, P.M. Blass, KC. Cannon and J.M. White, Surface Sci. 2 19 ( 1989) 294. [5] X.-L. Zhou and J.M. White, Surface Sci., submitted for publication. [6] J.C. Calvert and J.N. Pitts Jr., Photochemistry (Wiley, New York, 1966) p. 523. [ 71 B. Roop, KG. Lloyd, S.A. Costello, A. Campion and J.M. White, J. Chem. Phys. 91 ( 1989) 5103. [ 8 ] SK. Jo and J.M. White, to be published. 191 G.L. Geoffroy and M.S. Wrigbton, Organometallic photochemistry (Academic Press, New York, 1979). [IO] Z.-M. Liu, S.A. Costello, B. Roop, S. Coon, S. Akhter and J.M. White, J. Phys. Chem. 93 (1989) 7681. [ 111 Z. Lj. Pettrovic, WC. Wang and L.C. Lee, J. Chem. Phys. 90 (1989) 3145. [ 121 E.P. Marsh, T.L. Gilton, W. Meier, M.R. Schneider and J.P. Cowin, Phys. Rev. Letters 61 (1988) 2725; T.L. Gilton, C.P. Dehnbostel and J.P. Cowin, J. Chem. Phys. 91 (1989) 1937.