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Comment on ‘The precursor state to dissociation of CO adsorbed on the W(110) surface by J.E. Houston’
Surface Science Letters 276 (1992) LI2-LI4 North-Holland
surface s c i e n c e letters
Surface Science Letters
Comment on 'The precursor state to dissociation of CO adsorbed on the W(ll0) surface by J.E. Houston' Some aspects of CO adsorption on W ( l l 0 ) at low coverage
Y.B. Z h a o and R. G o m e r Department of Chemistry and James Franck Institute, The University of Chicago, Chicago, IL 60637, USA Received 20 November 1991; accepted for publication 30 June 1992
The work function change versus exposure for CO on W(ll0) at various temperatures has been measured via the Kelvin method. We find essentially linear rise with coverage at Ts _< 200 K in agreement with a previous determination by Wang and Gomer [C. Wang and R. Gomer, Surf. Sci. 74 (1978) 389] but in disagreement with a recent measurement by Houston [J.E. Houston, Surf. Sci. 225 (1991) 303] based on changes in secondary electron emission with CO coverage for 2000 eV incident electrons. We have also measured the ratio of 4tr to (5~r + 17r) peaks in UPS at 21,2 and 40.8 eV photon energies at relative CO coverages of 1 of 0.3-0.25. For 21.2 eV photons the 4tr peak virtually disappears at 0 = 0.3, but at 40.8 eV 4~r/(5~r + l~r) is greater at 0 = 0.25 than at 0 = 1. The present results do not contradict the tilted CO configuration at low coverage suggested by Houston's C - O stretch results, but do not lend it direct support.
I. Introduction
2. Experimental
In a recent publication [1], Houston reports very low vibrational C - O stretch frequencies for CO on W(ll0) at relative coverages of 0 < 0.5 which he attributes to a bent configuration, analogous to semi-bridge bonded CO which has been seen in some carbonyl structures [2]. He also finds the work function change A~b versus coverage curve has very low slope initially, suggesting a small dipole moment normal to the surface in the bent configuration. A~b values were deduced from changes in the onset of the secondary electron spectrum in AES for an incident electron energy of 2000 eV. These work function results disagree with a previous measurement by Wang and Gomer [3] who used a Kelvin probe and found, at 100 K, a linear rise in A~b versus coverage to near saturation. We have repeated A~b versus exposure measurements in a different apparatus from that used by Wang and Gomer for various surface temperatures also via a Kelvin probe. We also report here some UPS results at full and low coverages.
The apparatus used in the present work has been described previously [4]. The Kelvin probe [5] consisted of passivated Mo Lektromesh screen of 70% transparency. In the present set of measurements the crystal was kept in front of the probe and the apparatus backfilled with CO to a desired pressure, as indicated by a Varian UHV24 ion gauge, so that continuous A~b versus time curves were obtained. UPS measurements were made with a He resonance lamp, the photon beam hitting the crystal at an angle of 80° from the crystal normal. Electrons were energy analyzed by a double pass CMA which accepted a cone of electrons making an angle of 42° with the crystal normal.
3. Results and discussion Fig. 1 shows A~b versus time curves for adsorption at crystal temperatures TS= 25, 90, 120, 200 and 300 K, the gas temperature being ~ 300 K in
0039-6028/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
Y.B. Zhao, R. Gomer / Comment
5250
,,,,,,,,I
.........
~,,,,,,,,,i,,,,,,,,,i,,,,,,,,,I,,,,,,,,,I
........
co/w(11 o)
3000 2750
20K
//
2500 2250
;;> ~
2000
1750 1500
<3
,I,,,,,,,,
1250 1000 75O 300K
5OO 25O 0
0
100
200
500
400
500
600
700
800
TIME (see) Fig. I. Work function increments, A ~ versus exposure times for various surface temperatures, indicated on the figure. The dashed vertical line indicates the approximate time at which the desired C O pressures wcrc established in the system. The truc starting times varied slightly from that shown by the dashed line and wcrc established individually on an expanded scalc for each curve. The actual C O pressurcs varied from 1.0 to 1.4XI0 -s Torr, as read by a Varian U H V - 2 4 ion gauge and arc shown hcrc as if they had bccn 1.0)< I0 -s Torr in all cases. Curves arc displaced verticallyfor case of viewing.
all cases. The curves were obtained at CO pressures of 1.0 to 1.4 x 10 -8 Torr, as read by the UHV-24 ion gauge. For CO on W(110) Kohrt and Gomer [6] found sticking coefficients s to be constant to high coverage. This is borne out by the data of fig. 1, which show nearly linear plots of A~ versus exposure time with the exception of the T~ = 300 K curve. It is possible by extrapolating the initial segments of the curves to be the times which would have been required to reach maximum A~ if s had remained constant to calculate either the CO coverage for s assumed unity, or the initial sticking coefficient s o if the coverage data of Wang and Gomer [3] are used. For T, -- 20 K s is known to be unity [3,6] and the CO coverage is found to be 9.7x1014 CO
L13
molecules/cm 2, assuming the ion gauge reading to be accurate. The latter had in fact been found to be quite accurate by comparison with effusion source results for CO on Pdl/W(110) [7] This CO coverage is in very good agreement with the Wang and Gomer result, 1.0 x 1015 molecules/ c m 2 [3]. For Ts -- 90, 120, and 200 K respectively the s o values calculated on the basis of a true coverage of 1 X 1015 molecules/cm 2 are s o = 0.91 at 90 K and s o = 0.81 at 120 and 200 K. These values are slightly higher than those extrapolated from the data of Kohrt and Gomer [8], namely 0.82 at 90 K, 0.75 at 120 K, and 0.6 at 200 K for normal gas incidence. The estimates of s o in the present work also assume constant dipole moment from zero to maximum coverage. If the dipole moment were actually slightly larger at low than at high coverage, the A~b value at full coverage required for our extrapolation would be larger than the actual one, thus increasing the (hypothetical) time required to reach maximum coverage if s were constant at s 0. Thus our estimates of s o may be too high. However it is clear that the present coverage and s o values are reasonably consistent with previous ones. The A4, values for Ts < 200 K obtained here are slightly higher, with the exception of the 25 K data, than those of Wang and Gomer [3], namely At~max = 1000 meV for 1015 CO molecules/cm 2, i.e., for C O / W = 0.70. The present results are probably somewhat more accurate that the Wang and Gomer value, 880 meV. In any case, there is no sign at all of the initially small slope seen by Houston, except for T~ = 300 K. In that case the small initial slope is almost certainly the result of conversion of/3-CO, i.e., dissociated CO which is known to have a small dipole moment [8]. The small overall A~b value at T~= 300 K is also in agreement with the fact that the maximum coverage for adsorption at T~= 300 K is much less than at T~< 200 K [6], presumably because a mixture of fl-plus virgin-CO adsorption. The high dipole moment at low 0 does not contradict Houston's interpretation of the low C - O stretch frequency as corresponding to semi-bridge bonded CO: The substantial decrease in frequency implies considerable electron transfer to CO and this will counteract the decrease in dipole moment normal to
L14
Y.B. Zhao, R. Gorner
the surface resulting from the bent CO configuration. It is not obvious of course why the net dipole moment (allowing also for depolarization within the layer) should be nearly constant with coverage. We also have no explanation for the work function results of Houston at low 0. It is conceivable that they correspond to rapid electron induced conversion of virgin-CO to dissociated/3-CO which has small A&. The cross section for virgin to beta conversion (at least at 150 eV electron energy) increases markedly with decreasing CO coverage [4]. Fig. 2 shows He I (21.2 eV photon energy) UPS spectra at coverages of 0 = 1.0 and 0.3 relative to C O / W = 0.7. At the lower coverage the 40- peak has virtually disappeared. Fig. 3 shows He II (40.8 eV photon energy) spectra at relative 0 = 1.0 and 0.2. In this case the ratio of peak areas, 4o'/(5~r + l r r ) = 0.64 at 0 = 1 and 0.74 at 0 = 0.2. The ratio of peak heights only is 4~r/(5o" 50000
llll,'i~,l,l,llllllllUlllll,~,,lllll,llll
....... I . . . . . . .
l,ll,l,llllllll,},lI,llll,Illl,l
I ........
CO/W(110)
Comment
CO/W(1 10) 40"
3ooo ~-
A C0o7/W(I10)
or)
Z o r~
i -2
i 0
2
4.
6
8
10
12
14
16
18
BINDING ENERGY (eV) Fig. 3. HeII (40.8 eV photon energy) spectra for high and low coverages of CO on W(ll0). The ratio of 4~r/(5cr + 17r) actually increases at low coverage.
+ 17r)= 1.3 at 0 = 1 and 1.14 at 0 = 0.21. Thus the variation of relative peak strengths with coverage does not give unequivocal results which could be related to CO tilting. Such information could be obtained only from angle resolved UPS. In summary the results presented here do not argue either for or against tilted CO on W(ll0) at low coverage but do indicate that the component of dipole moment normal to the surface for virgin-CO is not less at low than at high coverage.
30000
c%/w(11 o)-w(1 lo) Z; 20000'
2 q~
CO0.z3/W( 1 1 0)
10000
o
10000 -
-2
Acknowledgements This work was supported in part by NSF Grant DMR 9005201. We have also benefited from the Materials Research Laboratory of the National Science Foundation at the University of Chicago.
References ........ i ......... , ......... , ......... ~......... ~. . . . . . . . . L......... , ......... *......... ~........ 0 2 4 6 8 10 12 14 16 18
BINDING ENERGY (eV) Fig. 2. He I (21.2 eV photon energy) UPS spectra for CO/W(110). The lower panel shows a spectrum for clean W(ll0), and for a relative CO coverage of 8 -~ 0.3, as well as a difference spectrum. The upper curve shows a difference spectrum for CO0.7/W(110)-W(l10), i.e, for the/9 = 1 case. At low coverage the 4 ~r peak has virtually disappeared.
[1] J.E. Houston, Surf. Sci. 255 (1991) 303. [2] W.A. Herrmann, H. Biersack, M.L. Ziegler, K. Weidenhammer, R. Siegel and D. Rehder, J. Am. Chem. Soc. 103 (1981) 1692. [3] C. Wang and R. Gomer, Surf. Sci. 74 (1978) 389. [4] J.C. Lin and R. Gomer, Surf. Sci. 218 (1989) 406. [5] M. Chelvayohan and R. Gomer, Surf. Sci. 172 (1986) 337. [6] C. Kohrt and R. Gomer, Surf. Sci. 40 (1973) 71. [7] Y.B. Zhao and R. Gomer, Surf. Sci. 239 (1990) 189. [8] C. Leung, M. Vass and R. Gomer, Surf. Sci. 66 (1977) 67.
surface s c i e n c e letters
Surface Science Letters
Comment on 'The precursor state to dissociation of CO adsorbed on the W(ll0) surface by J.E. Houston' Some aspects of CO adsorption on W ( l l 0 ) at low coverage
Y.B. Z h a o and R. G o m e r Department of Chemistry and James Franck Institute, The University of Chicago, Chicago, IL 60637, USA Received 20 November 1991; accepted for publication 30 June 1992
The work function change versus exposure for CO on W(ll0) at various temperatures has been measured via the Kelvin method. We find essentially linear rise with coverage at Ts _< 200 K in agreement with a previous determination by Wang and Gomer [C. Wang and R. Gomer, Surf. Sci. 74 (1978) 389] but in disagreement with a recent measurement by Houston [J.E. Houston, Surf. Sci. 225 (1991) 303] based on changes in secondary electron emission with CO coverage for 2000 eV incident electrons. We have also measured the ratio of 4tr to (5~r + 17r) peaks in UPS at 21,2 and 40.8 eV photon energies at relative CO coverages of 1 of 0.3-0.25. For 21.2 eV photons the 4tr peak virtually disappears at 0 = 0.3, but at 40.8 eV 4~r/(5~r + l~r) is greater at 0 = 0.25 than at 0 = 1. The present results do not contradict the tilted CO configuration at low coverage suggested by Houston's C - O stretch results, but do not lend it direct support.
I. Introduction
2. Experimental
In a recent publication [1], Houston reports very low vibrational C - O stretch frequencies for CO on W(ll0) at relative coverages of 0 < 0.5 which he attributes to a bent configuration, analogous to semi-bridge bonded CO which has been seen in some carbonyl structures [2]. He also finds the work function change A~b versus coverage curve has very low slope initially, suggesting a small dipole moment normal to the surface in the bent configuration. A~b values were deduced from changes in the onset of the secondary electron spectrum in AES for an incident electron energy of 2000 eV. These work function results disagree with a previous measurement by Wang and Gomer [3] who used a Kelvin probe and found, at 100 K, a linear rise in A~b versus coverage to near saturation. We have repeated A~b versus exposure measurements in a different apparatus from that used by Wang and Gomer for various surface temperatures also via a Kelvin probe. We also report here some UPS results at full and low coverages.
The apparatus used in the present work has been described previously [4]. The Kelvin probe [5] consisted of passivated Mo Lektromesh screen of 70% transparency. In the present set of measurements the crystal was kept in front of the probe and the apparatus backfilled with CO to a desired pressure, as indicated by a Varian UHV24 ion gauge, so that continuous A~b versus time curves were obtained. UPS measurements were made with a He resonance lamp, the photon beam hitting the crystal at an angle of 80° from the crystal normal. Electrons were energy analyzed by a double pass CMA which accepted a cone of electrons making an angle of 42° with the crystal normal.
3. Results and discussion Fig. 1 shows A~b versus time curves for adsorption at crystal temperatures TS= 25, 90, 120, 200 and 300 K, the gas temperature being ~ 300 K in
0039-6028/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
Y.B. Zhao, R. Gomer / Comment
5250
,,,,,,,,I
.........
~,,,,,,,,,i,,,,,,,,,i,,,,,,,,,I,,,,,,,,,I
........
co/w(11 o)
3000 2750
20K
//
2500 2250
;;> ~
2000
1750 1500
<3
,I,,,,,,,,
1250 1000 75O 300K
5OO 25O 0
0
100
200
500
400
500
600
700
800
TIME (see) Fig. I. Work function increments, A ~ versus exposure times for various surface temperatures, indicated on the figure. The dashed vertical line indicates the approximate time at which the desired C O pressures wcrc established in the system. The truc starting times varied slightly from that shown by the dashed line and wcrc established individually on an expanded scalc for each curve. The actual C O pressurcs varied from 1.0 to 1.4XI0 -s Torr, as read by a Varian U H V - 2 4 ion gauge and arc shown hcrc as if they had bccn 1.0)< I0 -s Torr in all cases. Curves arc displaced verticallyfor case of viewing.
all cases. The curves were obtained at CO pressures of 1.0 to 1.4 x 10 -8 Torr, as read by the UHV-24 ion gauge. For CO on W(110) Kohrt and Gomer [6] found sticking coefficients s to be constant to high coverage. This is borne out by the data of fig. 1, which show nearly linear plots of A~ versus exposure time with the exception of the T~ = 300 K curve. It is possible by extrapolating the initial segments of the curves to be the times which would have been required to reach maximum A~ if s had remained constant to calculate either the CO coverage for s assumed unity, or the initial sticking coefficient s o if the coverage data of Wang and Gomer [3] are used. For T, -- 20 K s is known to be unity [3,6] and the CO coverage is found to be 9.7x1014 CO
L13
molecules/cm 2, assuming the ion gauge reading to be accurate. The latter had in fact been found to be quite accurate by comparison with effusion source results for CO on Pdl/W(110) [7] This CO coverage is in very good agreement with the Wang and Gomer result, 1.0 x 1015 molecules/ c m 2 [3]. For Ts -- 90, 120, and 200 K respectively the s o values calculated on the basis of a true coverage of 1 X 1015 molecules/cm 2 are s o = 0.91 at 90 K and s o = 0.81 at 120 and 200 K. These values are slightly higher than those extrapolated from the data of Kohrt and Gomer [8], namely 0.82 at 90 K, 0.75 at 120 K, and 0.6 at 200 K for normal gas incidence. The estimates of s o in the present work also assume constant dipole moment from zero to maximum coverage. If the dipole moment were actually slightly larger at low than at high coverage, the A~b value at full coverage required for our extrapolation would be larger than the actual one, thus increasing the (hypothetical) time required to reach maximum coverage if s were constant at s 0. Thus our estimates of s o may be too high. However it is clear that the present coverage and s o values are reasonably consistent with previous ones. The A4, values for Ts < 200 K obtained here are slightly higher, with the exception of the 25 K data, than those of Wang and Gomer [3], namely At~max = 1000 meV for 1015 CO molecules/cm 2, i.e., for C O / W = 0.70. The present results are probably somewhat more accurate that the Wang and Gomer value, 880 meV. In any case, there is no sign at all of the initially small slope seen by Houston, except for T~ = 300 K. In that case the small initial slope is almost certainly the result of conversion of/3-CO, i.e., dissociated CO which is known to have a small dipole moment [8]. The small overall A~b value at T~= 300 K is also in agreement with the fact that the maximum coverage for adsorption at T~= 300 K is much less than at T~< 200 K [6], presumably because a mixture of fl-plus virgin-CO adsorption. The high dipole moment at low 0 does not contradict Houston's interpretation of the low C - O stretch frequency as corresponding to semi-bridge bonded CO: The substantial decrease in frequency implies considerable electron transfer to CO and this will counteract the decrease in dipole moment normal to
L14
Y.B. Zhao, R. Gorner
the surface resulting from the bent CO configuration. It is not obvious of course why the net dipole moment (allowing also for depolarization within the layer) should be nearly constant with coverage. We also have no explanation for the work function results of Houston at low 0. It is conceivable that they correspond to rapid electron induced conversion of virgin-CO to dissociated/3-CO which has small A&. The cross section for virgin to beta conversion (at least at 150 eV electron energy) increases markedly with decreasing CO coverage [4]. Fig. 2 shows He I (21.2 eV photon energy) UPS spectra at coverages of 0 = 1.0 and 0.3 relative to C O / W = 0.7. At the lower coverage the 40- peak has virtually disappeared. Fig. 3 shows He II (40.8 eV photon energy) spectra at relative 0 = 1.0 and 0.2. In this case the ratio of peak areas, 4o'/(5~r + l r r ) = 0.64 at 0 = 1 and 0.74 at 0 = 0.2. The ratio of peak heights only is 4~r/(5o" 50000
llll,'i~,l,l,llllllllUlllll,~,,lllll,llll
....... I . . . . . . .
l,ll,l,llllllll,},lI,llll,Illl,l
I ........
CO/W(110)
Comment
CO/W(1 10) 40"
3ooo ~-
A C0o7/W(I10)
or)
Z o r~
i -2
i 0
2
4.
6
8
10
12
14
16
18
BINDING ENERGY (eV) Fig. 3. HeII (40.8 eV photon energy) spectra for high and low coverages of CO on W(ll0). The ratio of 4~r/(5cr + 17r) actually increases at low coverage.
+ 17r)= 1.3 at 0 = 1 and 1.14 at 0 = 0.21. Thus the variation of relative peak strengths with coverage does not give unequivocal results which could be related to CO tilting. Such information could be obtained only from angle resolved UPS. In summary the results presented here do not argue either for or against tilted CO on W(ll0) at low coverage but do indicate that the component of dipole moment normal to the surface for virgin-CO is not less at low than at high coverage.
30000
c%/w(11 o)-w(1 lo) Z; 20000'
2 q~
CO0.z3/W( 1 1 0)
10000
o
10000 -
-2
Acknowledgements This work was supported in part by NSF Grant DMR 9005201. We have also benefited from the Materials Research Laboratory of the National Science Foundation at the University of Chicago.
References ........ i ......... , ......... , ......... ~......... ~. . . . . . . . . L......... , ......... *......... ~........ 0 2 4 6 8 10 12 14 16 18
BINDING ENERGY (eV) Fig. 2. He I (21.2 eV photon energy) UPS spectra for CO/W(110). The lower panel shows a spectrum for clean W(ll0), and for a relative CO coverage of 8 -~ 0.3, as well as a difference spectrum. The upper curve shows a difference spectrum for CO0.7/W(110)-W(l10), i.e, for the/9 = 1 case. At low coverage the 4 ~r peak has virtually disappeared.
[1] J.E. Houston, Surf. Sci. 255 (1991) 303. [2] W.A. Herrmann, H. Biersack, M.L. Ziegler, K. Weidenhammer, R. Siegel and D. Rehder, J. Am. Chem. Soc. 103 (1981) 1692. [3] C. Wang and R. Gomer, Surf. Sci. 74 (1978) 389. [4] J.C. Lin and R. Gomer, Surf. Sci. 218 (1989) 406. [5] M. Chelvayohan and R. Gomer, Surf. Sci. 172 (1986) 337. [6] C. Kohrt and R. Gomer, Surf. Sci. 40 (1973) 71. [7] Y.B. Zhao and R. Gomer, Surf. Sci. 239 (1990) 189. [8] C. Leung, M. Vass and R. Gomer, Surf. Sci. 66 (1977) 67.