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The photodissociation of methyl bromide chemisorbed on Cu{111}: influence of pre-adsorbed bromine
.
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Surface Science 287/288 (1993) 169-174 North-Holland
surface science h.: .. ‘.‘A ,:,: ;:;:::::: .i. “A:.: .,....., “-“:‘” vi............... *x.: .,.,.. :.:.:.:.: ,,:.:.:,:, :::,:.:. ,:,:,.
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The photodissociation of methyl bromide chemisorbed influence of pre-adsorbed bromine
on Cu{ 111):
C.L.A. Lamont, H. Conrad and A.M. Bradshaw Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, Germany
Received 27 August 1992; accepted for publication 19 November 1992
The photodissociation cross-section of CH,Br chemisorbed on a Cu(lll) surface pre-covered with Br has been measured as a function of wavelength. The onset for the reaction shifts to higher energy with increasing Br coverage and can be correlated with the increase in work function. The data are compatible with the charge-transfer mechanism involving excitation of an electron from the d-band in the substrate directly into the u * C-Br orbital.
1. Introduction
The photodissociation of chemisorbed CH,Br has been studied on a variety of metal surfaces [l-8]. In every case the primary products of the photodissociation process are CH, and Br, as in the free molecule [9]. The energetics of the fragmentation process and the subsequent thermal behaviour are, however, substrate-dependent. With a view to determining the dissociation mechanism we have previously measured the reaction cross-section as a detailed function of the photon energy for the CH,Br/Pt{lll} [4,5] and Cu(lll] [S] systems using a Xe arc lamp as the photon source and a series of cut-off filters in the energy range 2.6-6.0 eV to give the energy dependence. The dissociation cross-section curves as a function of the photon energy for CH,Br adsorbed on both Cu{lll} and Pt{lll] each showed an onset and a maximum. The presence of the maximum pointed to a discrete electronic excitation and it was suggested that an electron was excited from a d-band state in the substrate directly into the u* C-Br antibonding orbital. In order for the transition-matrix element to be non-zero there must be some form of hybridisation between the (T* orbital and the unoccupied substrate states. Evidence for such coupling is the well known broadening of molecular orbitals which occurs on chemisorption. This means that 0039-6028/93/$06.00
the broad antibonding level, as generally visible in inverse photoemission, can be considered as an adsorbate-induced surface resonance. The onset was attributed to excitations from the top of the sp-band in the substrate into the u* C-Br orbital. (White and co-workers have also carried out a photon-energy-dependence measurement of the dissociation cross-section for CH,Br on Ag{lll] 161. An onset to dissociation was observed but the narrow energy range investigated (3.0-4.5 eV) prohibited the possible detection of a maximum.) On changing the metal substrate, the d-bands, the work function and, possibly, the position of the U* C-Br orbital are all altered, which means that although the above explanation fits both systems, the assignments are not completely unambiguous. In order to reduce the number of uncertainties, the experiments have been repeated on a Cu(lll] surface which was pre-dosed with Br. On Br adsorption, the work function of the substrate is altered, which should manifest itself as a corresponding shift in the onset - if the above mechanism is correct. 2. Experimental
All experiments were performed under ultrahigh vacuum conditions (routine pressure 2 x 10-i’ mbar). The CuIlll] crystal was mounted
0 1993 - Elsevier Science Publishers B.V. All rights reserved
170
C.L.A. Lamont et al. / Photodissociation of methyl bromide chemisorbed on Cu{lll}
between two tungsten wires which were connected directly to a liquid-nitrogen Dewar by copper wires. It was cleaned by 30 min argon-ion bombardment at 670 K followed by 30 min annealing at 670 K. Cleanliness was checked by LEED and high-resolution electron energy-loss spectroscopy (HREELS). HREEL spectra were recorded in both the specular and - 10” off-specular directions using a primary energy of 1.8 eV and an incidence angle of 55”. The Br-covered surface was prepared by electron-beam-induced decomposition of adsorbed CH,Br. The crystal was cooled to 110 K and positioned directly in front of the defocused LEED gun at a CH,Br pressure of 1 X 10e7 mbar (Linde 95%). The typical beam current was 1.9 FA at an energy of 130 eV. During the exposure the formation of a sharp (6 x &)R30 LEED pattern was observed after 2 min as in the case of Cl adsorption on Cu(lll} [lo]. With further exposure the pattern became fuzzier and each l/a order spot changed into a smeared triangular shape as sketched in fig. 1. After 5 min these triangular shapes were resolved into three sharp spots in a triangular configuration. A similar pattern was observed for Cl on Cu{lll} and was designated (66 x 6fi)R30”. After 10 min these spots became weaker. The crystal was then flashed to 800 K to remove any CH, produced by this process and re-cooled. The Br coverage was estimated from CuBr thermal desorption peak areas and was found to increase approximately linearly with CH,Br exposure up to saturation at 8 X 10P5 mbar . s. 8 = 0.45 k 0.025 and 0.7 f 0.05 of the saturation coverage were used as Br precoverages for the photodissociation experiments. Only those measurements with CuBr areas within the stated error limits have been used in the evaluation of the cross-sections. These areas correspond to the onset and the decay of the observation of the higher-coverage ordered LEED structure. A monolayer of CH,Br was formed on this surface by dosing 5.5 x 10e6 mbar . s at 110 K. For each cross-section measurement the Br surface was freshly prepared. The 1000 W (LTI 1000X) Xe arc lamp, used as the photon source, was focused so that the entire sample was irradi-
(1x1)
2 minutes I :e ,’
cl3 x$3) R30°
/,
3 minutes I I/i
intermediate f> ; .i ,;!
+
phase
0 5 minutes e 1,> :;
I @ : ‘6
ordered phase
,’
Fig. 1. Schematic LEED patterns observed during exposure of Cu(ll1) crystal to 1 X lo-’ mbar of CH,Br in the presence of the LEED gun.
ated. A series of cut-off filters (Schott WG 225GG 475) was used to measure the cross-section as a function of the photon energy. A Balzers (QMS 4201 quadrupole mass spectrometer was used to follow the thermal desorption with and without irradiation. Typical heating rates were 5 K s-‘.
3. Results and discussion The HREEL spectrum of a monolayer of CH,Br adsorbed on a Br-covered Cu{lll) surface, measured in the specular direction, showed losses at 71 (Br-CH, stretch), 116 (CH, rock),
C.L.A. Lamont et al. / Photodissociation of methyl bromide chemisorbed on Cu(lll}
(a)
171
(b)
400 500 surface temperature (K)
06
600
400
600’ ’
600
surface temperature (K)
Fig. 2. CH, thermal desorption spectra for different cut-off filters for (a) 13= 0.45 and (b) 0 = 0.70 initial Br coverage. The exposure time was 18 min at 920 W lamp power.
adsorbed intact on the surface through the Br atom with the C-Br axis tilted with respect to the surface normal as described in refs. [4,5,8,11].
160 (CH, s-deform), 177 (CH, d-deform), 369 (CH, s-stretch) and 372 meV (CH, d-stretch). These are characteristic of a CH,Br molecule
*
.
280
.
.
320
.
360
cut-off wavelength
400
(nm)
Fig. 3. CH, thermal desorption peak areas for different filters for the clean surface and for initial Br coverages of 0.45 and 0.70. The x-axis represents the label for the cut-off filter.
172
C. L.A. Lamont et al. / Photodissociation of methyl bromide chemisorbed on Cu {11l}
The molecule desorbed at 150 K which is close to the desorption temperature on the clean surface, indicating that the bonding interaction is similar on both surfaces. The primary products of photodissociation were identified by HREELS as CH, adsorbed in a C,, configuration (losses at 146 (CH, s-deform) and 347 (CH, s-stretch) in the specular direction and 90 (CH, rock), 177 (CH, d-deform) and 342 meV (CH, d-stretch) in the off-specular direction) and Br (shoulder at 20 meV). The Cu-CH, stretch was not resolved from the Cu-Br stretch. These results are similar to those for clean Cu(ll1). CH, was the main product in the subsequently measured thermal desorption spectra, desorbing at a temperature of - 450 K which is similar to the clean surface. The amount of CH, produced was taken as proportional to the number of photodissociation events which had occurred. This was measured for each filter using a fixed exposure time of 18 min and a lamp power of 920 W for the two different initial Br coverages of 0.45 and 0.70 (figs. 2a and 2b, respectively). The frac-
tion of the monolayer which had reacted in each case is plotted in fig. 3 together with the values for the clean surface. The error bars represent the uncertainty due to the slight differences in initial Br coverage. It can be seen that with increasing Br coverage, the amount of product is reduced most significantly for the longer wavelength (lower energy) cut-off filters, which is in qualitative agreement with the notion that the onset should be shifted to higher energies with increasing work function. The possible role of site blocking in causing the different results for the three surfaces should be discussed. There are two possible ways in which Br could block the reaction. The first is by blocking the initial CH,Br adsorption. One would then expect all three “curves” to have exactly the same shape, scaled down by an appropriate factor for the higher Br coverages, rather than the differently shaped “curves” obtained. Pre-adsorbed Br may also block the dissociation process. On the clean surface, the reaction was a linear function of time up to 0.2 ML and then it started to
photon energy (eV) 60 10'
'
200
50 I
240
30 I
40 I
280
320
wavelength Fig. 4. Photodissociation
cross-sections
as a function
360
400
440
480
(nm)
of the photon wavelength 0.45 and 0.70.
for the clean surface
and for initial Br coverages
of
C.L.A. Lamont et al. / Photodissociationof methylbromide chemisorbedon Cu{Ill}
decrease. This may have been due to a reduction in the number of available sites, a change in the work function, or build up of contaminants during the long exposure time required. If the dissociation pathway was blocked, the CH, thermal desorption peak areas would be reduced, most likely in a non-linear fashion, whereby the smallest areas (longer wavelength cut-off filters) would be little affected and the larger ones (shorter wavelength cut-off filters) would be more severely reduced. This may play a role in the large uncertainties in the results for the 8 = 0.70 surface measured using the lower wavelength cut-off filters, but it cannot explain the different curves in fig. 3. The photodissociation cross-section can be calculated using Cp= (number of reacted photons) / [(incoming x
photon flux)
(number of target particles)] .
In our case, the number of reacted photons is proportional to the number of CH, molecules desorbed and the number of target molecules is proportional to the number of CH,Br molecules adsorbed. The incoming photon flux from the lamp has been measured as a function of wavelength [4]. For a detailed description of the method of calculation of the cross-section the reader is referred to refs. [4,5,8]. The reaction cross-sections have been calculated for both the clean surface and the 0 = 0.45 Br-covered surface (fig. 4). For the latter, an onset is observed at 2.80 eV and a maximum at 3.65 eV, compared to 2.55 and 3.55 eV, respectively, for the clean surface. The measurements at the higher Br coverage gave unpredictable results at lower wavelength cut-off filters, possibly due to a competitive reaction pathway at these energies. It was, however, possible to calculate cross-sections for lower energies using the areas measured for the 320-385 nm cut-off filters. This gave an onset at 3.05 eV. The absolute intensities are of the same order of magnitude for all three surfaces. To test if these results are in agreement with the proposed model we have to be able to relate
173
the positions of the onsets and the maxima to the energies of the substrate sp- and d-bands, respectively. Considering first of all the onsets, on the clean surface there is a relative band gap at the surface and the sp band begins about 0.9 eV below the Fermi level (E,) 1121.(Note, only the I-L direction of the Brillouin zone, which corresponds to the (111) direction in real space, has been considered since the inclusion of other directions would lead to broader features which is incompatible with the narrow width of the maximum.) On the clean surface this gave an onset at 2.55 eV. On pre-adsorption of 0 = 0.45 Br, the onset increased to 2.80 eV, a shift of 0.25 eV. Bange et al. [13] measured an increase in work function of 0.8 eV on adsorption of a monolayer of Br. Using this data, we have estimated an increase in work function of 0.35 eV for 0 = 0.45 and 0.55 eV for 0 = 0.70. Therefore, one would expect a shift in the onset of 0.35 eV for the 8 = 0.45 surface, which is slightly larger than that found experimentally. With an initial Br coverage of 0.70, the onset was shifted to 3.05 eV, a change of 0.50 eV which is close to the value of 0.55 eV anticipated. The maximum occurred at approximately 0.1 eV higher on the 8 = 0.45 Br surface than on the clean surface. This is a slightly smaller shift than expected, but it is in the right direction. There also appears to be a shoulder on the maximum at around 3.40 eV which may be due to a Br-induced feature in the d-states. On the clean surface, the d-band is 2.2 eV below E, 1121. On Br adsorption on Cu{lOO) [14], a new feature appears in the d-band structure at about 0.4 eV above the highest d-band. Adsorption of Cl on Cu{lll} [153 also induces a new feature at 0.5 eV above the highest d-band state. Therefore, it is possible that adsorbate-induced localised surface states will be present at 1.7-1.8 eV below E, on adsorption of Br on Cu{llll and could explain the presence of the shoulder in the cross-section curve. The results presented here thus support the charge-transfer mechanism previously suggested to explain the cross-section curves obtained for the CH,Br/Pt(lll} and Cullll) systems. By increasing the work function of the substrate the whole curve shifts to higher energy.
174
C. L.A. Lamont et al. / Photodissociation of methyl bromide chemisorbed on Cu{ll I}
Although there is only qualitative agreement as to the extent of shift, these observations highlight the role of the substrate in the photo-excitation process.
Acknowledgement
This work has been supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 6-81.
References [l] S.A. Costello, B. Roop, Y.M. Liu and J.M. White, J. Chem. Phys. 92 (1988) 1019. [2] E.P. Marsh, M.R. Schneider, T.L. Gilton, F.L. Tabares, W. Meier and J.P. Cowin, Phys. Rev. Lett. 60 (1988) 2551.
K.G. [31 B. Roop, Y. Zhou, Z.-M. Liu, M.A. Henderson, Lloyd, A. Campion and J.M. White, J. Vat. Sci. Technol. A 7 (1989) 2121. W. Stenzel, H. Conrad and A.M. [41 G. Radhakrishnan, Bradshaw, Appl. Surf. Sci. 46 (1990) 36. W. Stenzel, R. Hemmen, H. Conrad [51 G. Radhakrishnan, and A.M. Bradshaw, J. Chem. Phys. 95 (1991) 3930. [61 X.-L. Zhou and J.M. White, Surf. Sci. 241 (1991) 259. T.J. Long and I. Harrison, J. Chem. [71 V.A. Ukraintser, Phys. 96 (1992) 3957. H. Conrad, A.M. Bradshaw, Surf. Sci. Bl C.L.A. Lamont, 280 (1993) 79. [91 G.N.A. van Veen, T. Bailer and A.E. de Vries, Chem. Phys. 92 (1985) 59. [lOI P.J. Goddard and R.M. Lambert, Surf. Sci. 67 (1977) 180. 1111 S.K. Jo, X.-Y. Zhu, D. Lennon and J.M. White, Surf. Sci. 67 (1991) 231. [121 G.A. Burdick, Phys. Rev. 129 (1963) 138. [131 K. Bange, R. Dohl, D.E. Grider and J.K. Sass, Vacuum 33 (1983) 757. and J.K. Sass, Surf. Sci. 103 (1981) 496. [141 N.V. Richardson and A. Goldmann, Surf. Sci. 154 (1985) [151 K.K. Kleinherbers 489.
..A..
. ........+........:g.:.:.:q .~,:,~.,.,., _, .(,
I:~:~:::::::::::::::~:::::::::::::.:.:::::::::::
.:.:.x.:.x.,...,:.:.:.:.............n....
.,.,...........,
:,:,:+,(
.::,:,,
,,..,.,.,.,.,v.....
p::::.:.““‘
Surface Science 287/288 (1993) 169-174 North-Holland
surface science h.: .. ‘.‘A ,:,: ;:;:::::: .i. “A:.: .,....., “-“:‘” vi............... *x.: .,.,.. :.:.:.:.: ,,:.:.:,:, :::,:.:. ,:,:,.
.::~~::.:.~.~.~.~~:~:::::.:.::~:::::::::::~,~ :::::::::: 5::,:.:,,. -‘.‘A’... .......__.......,,.__,,,~,, ‘.‘i.:.:::::~:~::::::$$~:~::::$::$ :!:.:.,,) ....(...
The photodissociation of methyl bromide chemisorbed influence of pre-adsorbed bromine
on Cu{ 111):
C.L.A. Lamont, H. Conrad and A.M. Bradshaw Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, Germany
Received 27 August 1992; accepted for publication 19 November 1992
The photodissociation cross-section of CH,Br chemisorbed on a Cu(lll) surface pre-covered with Br has been measured as a function of wavelength. The onset for the reaction shifts to higher energy with increasing Br coverage and can be correlated with the increase in work function. The data are compatible with the charge-transfer mechanism involving excitation of an electron from the d-band in the substrate directly into the u * C-Br orbital.
1. Introduction
The photodissociation of chemisorbed CH,Br has been studied on a variety of metal surfaces [l-8]. In every case the primary products of the photodissociation process are CH, and Br, as in the free molecule [9]. The energetics of the fragmentation process and the subsequent thermal behaviour are, however, substrate-dependent. With a view to determining the dissociation mechanism we have previously measured the reaction cross-section as a detailed function of the photon energy for the CH,Br/Pt{lll} [4,5] and Cu(lll] [S] systems using a Xe arc lamp as the photon source and a series of cut-off filters in the energy range 2.6-6.0 eV to give the energy dependence. The dissociation cross-section curves as a function of the photon energy for CH,Br adsorbed on both Cu{lll} and Pt{lll] each showed an onset and a maximum. The presence of the maximum pointed to a discrete electronic excitation and it was suggested that an electron was excited from a d-band state in the substrate directly into the u* C-Br antibonding orbital. In order for the transition-matrix element to be non-zero there must be some form of hybridisation between the (T* orbital and the unoccupied substrate states. Evidence for such coupling is the well known broadening of molecular orbitals which occurs on chemisorption. This means that 0039-6028/93/$06.00
the broad antibonding level, as generally visible in inverse photoemission, can be considered as an adsorbate-induced surface resonance. The onset was attributed to excitations from the top of the sp-band in the substrate into the u* C-Br orbital. (White and co-workers have also carried out a photon-energy-dependence measurement of the dissociation cross-section for CH,Br on Ag{lll] 161. An onset to dissociation was observed but the narrow energy range investigated (3.0-4.5 eV) prohibited the possible detection of a maximum.) On changing the metal substrate, the d-bands, the work function and, possibly, the position of the U* C-Br orbital are all altered, which means that although the above explanation fits both systems, the assignments are not completely unambiguous. In order to reduce the number of uncertainties, the experiments have been repeated on a Cu(lll] surface which was pre-dosed with Br. On Br adsorption, the work function of the substrate is altered, which should manifest itself as a corresponding shift in the onset - if the above mechanism is correct. 2. Experimental
All experiments were performed under ultrahigh vacuum conditions (routine pressure 2 x 10-i’ mbar). The CuIlll] crystal was mounted
0 1993 - Elsevier Science Publishers B.V. All rights reserved
170
C.L.A. Lamont et al. / Photodissociation of methyl bromide chemisorbed on Cu{lll}
between two tungsten wires which were connected directly to a liquid-nitrogen Dewar by copper wires. It was cleaned by 30 min argon-ion bombardment at 670 K followed by 30 min annealing at 670 K. Cleanliness was checked by LEED and high-resolution electron energy-loss spectroscopy (HREELS). HREEL spectra were recorded in both the specular and - 10” off-specular directions using a primary energy of 1.8 eV and an incidence angle of 55”. The Br-covered surface was prepared by electron-beam-induced decomposition of adsorbed CH,Br. The crystal was cooled to 110 K and positioned directly in front of the defocused LEED gun at a CH,Br pressure of 1 X 10e7 mbar (Linde 95%). The typical beam current was 1.9 FA at an energy of 130 eV. During the exposure the formation of a sharp (6 x &)R30 LEED pattern was observed after 2 min as in the case of Cl adsorption on Cu(lll} [lo]. With further exposure the pattern became fuzzier and each l/a order spot changed into a smeared triangular shape as sketched in fig. 1. After 5 min these triangular shapes were resolved into three sharp spots in a triangular configuration. A similar pattern was observed for Cl on Cu{lll} and was designated (66 x 6fi)R30”. After 10 min these spots became weaker. The crystal was then flashed to 800 K to remove any CH, produced by this process and re-cooled. The Br coverage was estimated from CuBr thermal desorption peak areas and was found to increase approximately linearly with CH,Br exposure up to saturation at 8 X 10P5 mbar . s. 8 = 0.45 k 0.025 and 0.7 f 0.05 of the saturation coverage were used as Br precoverages for the photodissociation experiments. Only those measurements with CuBr areas within the stated error limits have been used in the evaluation of the cross-sections. These areas correspond to the onset and the decay of the observation of the higher-coverage ordered LEED structure. A monolayer of CH,Br was formed on this surface by dosing 5.5 x 10e6 mbar . s at 110 K. For each cross-section measurement the Br surface was freshly prepared. The 1000 W (LTI 1000X) Xe arc lamp, used as the photon source, was focused so that the entire sample was irradi-
(1x1)
2 minutes I :e ,’
cl3 x$3) R30°
/,
3 minutes I I/i
intermediate f> ; .i ,;!
+
phase
0 5 minutes e 1,> :;
I @ : ‘6
ordered phase
,’
Fig. 1. Schematic LEED patterns observed during exposure of Cu(ll1) crystal to 1 X lo-’ mbar of CH,Br in the presence of the LEED gun.
ated. A series of cut-off filters (Schott WG 225GG 475) was used to measure the cross-section as a function of the photon energy. A Balzers (QMS 4201 quadrupole mass spectrometer was used to follow the thermal desorption with and without irradiation. Typical heating rates were 5 K s-‘.
3. Results and discussion The HREEL spectrum of a monolayer of CH,Br adsorbed on a Br-covered Cu{lll) surface, measured in the specular direction, showed losses at 71 (Br-CH, stretch), 116 (CH, rock),
C.L.A. Lamont et al. / Photodissociation of methyl bromide chemisorbed on Cu(lll}
(a)
171
(b)
400 500 surface temperature (K)
06
600
400
600’ ’
600
surface temperature (K)
Fig. 2. CH, thermal desorption spectra for different cut-off filters for (a) 13= 0.45 and (b) 0 = 0.70 initial Br coverage. The exposure time was 18 min at 920 W lamp power.
adsorbed intact on the surface through the Br atom with the C-Br axis tilted with respect to the surface normal as described in refs. [4,5,8,11].
160 (CH, s-deform), 177 (CH, d-deform), 369 (CH, s-stretch) and 372 meV (CH, d-stretch). These are characteristic of a CH,Br molecule
*
.
280
.
.
320
.
360
cut-off wavelength
400
(nm)
Fig. 3. CH, thermal desorption peak areas for different filters for the clean surface and for initial Br coverages of 0.45 and 0.70. The x-axis represents the label for the cut-off filter.
172
C. L.A. Lamont et al. / Photodissociation of methyl bromide chemisorbed on Cu {11l}
The molecule desorbed at 150 K which is close to the desorption temperature on the clean surface, indicating that the bonding interaction is similar on both surfaces. The primary products of photodissociation were identified by HREELS as CH, adsorbed in a C,, configuration (losses at 146 (CH, s-deform) and 347 (CH, s-stretch) in the specular direction and 90 (CH, rock), 177 (CH, d-deform) and 342 meV (CH, d-stretch) in the off-specular direction) and Br (shoulder at 20 meV). The Cu-CH, stretch was not resolved from the Cu-Br stretch. These results are similar to those for clean Cu(ll1). CH, was the main product in the subsequently measured thermal desorption spectra, desorbing at a temperature of - 450 K which is similar to the clean surface. The amount of CH, produced was taken as proportional to the number of photodissociation events which had occurred. This was measured for each filter using a fixed exposure time of 18 min and a lamp power of 920 W for the two different initial Br coverages of 0.45 and 0.70 (figs. 2a and 2b, respectively). The frac-
tion of the monolayer which had reacted in each case is plotted in fig. 3 together with the values for the clean surface. The error bars represent the uncertainty due to the slight differences in initial Br coverage. It can be seen that with increasing Br coverage, the amount of product is reduced most significantly for the longer wavelength (lower energy) cut-off filters, which is in qualitative agreement with the notion that the onset should be shifted to higher energies with increasing work function. The possible role of site blocking in causing the different results for the three surfaces should be discussed. There are two possible ways in which Br could block the reaction. The first is by blocking the initial CH,Br adsorption. One would then expect all three “curves” to have exactly the same shape, scaled down by an appropriate factor for the higher Br coverages, rather than the differently shaped “curves” obtained. Pre-adsorbed Br may also block the dissociation process. On the clean surface, the reaction was a linear function of time up to 0.2 ML and then it started to
photon energy (eV) 60 10'
'
200
50 I
240
30 I
40 I
280
320
wavelength Fig. 4. Photodissociation
cross-sections
as a function
360
400
440
480
(nm)
of the photon wavelength 0.45 and 0.70.
for the clean surface
and for initial Br coverages
of
C.L.A. Lamont et al. / Photodissociationof methylbromide chemisorbedon Cu{Ill}
decrease. This may have been due to a reduction in the number of available sites, a change in the work function, or build up of contaminants during the long exposure time required. If the dissociation pathway was blocked, the CH, thermal desorption peak areas would be reduced, most likely in a non-linear fashion, whereby the smallest areas (longer wavelength cut-off filters) would be little affected and the larger ones (shorter wavelength cut-off filters) would be more severely reduced. This may play a role in the large uncertainties in the results for the 8 = 0.70 surface measured using the lower wavelength cut-off filters, but it cannot explain the different curves in fig. 3. The photodissociation cross-section can be calculated using Cp= (number of reacted photons) / [(incoming x
photon flux)
(number of target particles)] .
In our case, the number of reacted photons is proportional to the number of CH, molecules desorbed and the number of target molecules is proportional to the number of CH,Br molecules adsorbed. The incoming photon flux from the lamp has been measured as a function of wavelength [4]. For a detailed description of the method of calculation of the cross-section the reader is referred to refs. [4,5,8]. The reaction cross-sections have been calculated for both the clean surface and the 0 = 0.45 Br-covered surface (fig. 4). For the latter, an onset is observed at 2.80 eV and a maximum at 3.65 eV, compared to 2.55 and 3.55 eV, respectively, for the clean surface. The measurements at the higher Br coverage gave unpredictable results at lower wavelength cut-off filters, possibly due to a competitive reaction pathway at these energies. It was, however, possible to calculate cross-sections for lower energies using the areas measured for the 320-385 nm cut-off filters. This gave an onset at 3.05 eV. The absolute intensities are of the same order of magnitude for all three surfaces. To test if these results are in agreement with the proposed model we have to be able to relate
173
the positions of the onsets and the maxima to the energies of the substrate sp- and d-bands, respectively. Considering first of all the onsets, on the clean surface there is a relative band gap at the surface and the sp band begins about 0.9 eV below the Fermi level (E,) 1121.(Note, only the I-L direction of the Brillouin zone, which corresponds to the (111) direction in real space, has been considered since the inclusion of other directions would lead to broader features which is incompatible with the narrow width of the maximum.) On the clean surface this gave an onset at 2.55 eV. On pre-adsorption of 0 = 0.45 Br, the onset increased to 2.80 eV, a shift of 0.25 eV. Bange et al. [13] measured an increase in work function of 0.8 eV on adsorption of a monolayer of Br. Using this data, we have estimated an increase in work function of 0.35 eV for 0 = 0.45 and 0.55 eV for 0 = 0.70. Therefore, one would expect a shift in the onset of 0.35 eV for the 8 = 0.45 surface, which is slightly larger than that found experimentally. With an initial Br coverage of 0.70, the onset was shifted to 3.05 eV, a change of 0.50 eV which is close to the value of 0.55 eV anticipated. The maximum occurred at approximately 0.1 eV higher on the 8 = 0.45 Br surface than on the clean surface. This is a slightly smaller shift than expected, but it is in the right direction. There also appears to be a shoulder on the maximum at around 3.40 eV which may be due to a Br-induced feature in the d-states. On the clean surface, the d-band is 2.2 eV below E, 1121. On Br adsorption on Cu{lOO) [14], a new feature appears in the d-band structure at about 0.4 eV above the highest d-band. Adsorption of Cl on Cu{lll} [153 also induces a new feature at 0.5 eV above the highest d-band state. Therefore, it is possible that adsorbate-induced localised surface states will be present at 1.7-1.8 eV below E, on adsorption of Br on Cu{llll and could explain the presence of the shoulder in the cross-section curve. The results presented here thus support the charge-transfer mechanism previously suggested to explain the cross-section curves obtained for the CH,Br/Pt(lll} and Cullll) systems. By increasing the work function of the substrate the whole curve shifts to higher energy.
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Although there is only qualitative agreement as to the extent of shift, these observations highlight the role of the substrate in the photo-excitation process.
Acknowledgement
This work has been supported by the Deutsche Forschungsgemeinschaft through the Sonderforschungsbereich 6-81.
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