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Adsorption of hydrogen on W (100)
SURFACE SCIENCE 23 (1970) 419-422 8 Noah-dolled
ADSORPTION
OF HYDROGEN
Publishin% Co.
ON W (100)
Received 29 June 1970
We have carried out a combined LEED/flash desorption study 1) of hydrogen adsorption upon W (IOO), which supplements reported findings by Estrup and Andersons), (EA), Tamm and Schmidts), (TS), and Madey and Yates *), (MY). Our experimental system and techniques have been described previously 5)_ A typical flash desorption spectrum from a surface that had been saturated with hydrogen is shown in fig. la. The amount of hydrogen desorbed in the /I1 peak is twice that desorbed in the & peak. Population of the two peaks occurs sequentially with exposure, with some overlap. Thus, the p1 peak is first observed at a coverage of p2=0.35 monolayer of atoms, although saturation coverage of /I2 =OS monolayer. This sequence is shown in fig. lb. A detailed analysis of the kinetics of adsorption has been made previouslys*4), and our results are in agreement with these studies. Exposure to hydrogen produces a C (2 x 2) LEED pattern. The intensity of the -&,+beam increases with exposure, reaching a maximum at the exposure of 0.44 L. This exposure is exactly that for which the j3r peak is first produced, as can be seen from fig. 2. Leggett and Armstrong6} have recently reported this result from a combined RHEED/flash desorption study. At exposures greater than 0.44 L, the intensity measured in the h -i-4, k -1-3 positions decreased, coincident with a splitting of the 3, + beam into four new beams, as described by EA. The coincidence of the maximum intensity of the +, 4 beam with the first appearance of & in the desorption spectra is of considerable interest. It provides direct evidence of an interrelation between adsorbate structure and binding state, and confirms the inference of TS from their flash desorption data and EA’s LEED results, that flz desorption occurs from hydrogen atoms in a C (2 x 2) array. Since the LEED observations are made at room temperature, we believe that this observation strongly suggests that the distribution of hydrogen between the binding states shown by flash desorption is that which is produced at room temperature. It is evident that the Br binding state coincides with the continuous change in adsorbate structure reported by EA, but a detailed understanding of the surface structures formed in this range of coverage has not been achieved. 419
420
D.L.ADAMS AND L.H.GBRMER
Exposure
(IO*
torr see)
Fig. 1. Desorption of hydrogen from a W(100) surface: (a) desorption spectrum after a saturation hydrogen exposure of 12 L (i.e. 12 x 1O-6 torr set). The crystal temperature was raised at 100” per sec. (b) Amounts of hydrogen desorbed in the /?I and /3a peaks plotted against hydrogen exposure. Saturation 82 coverage is assumed to be half a monolayer, based on the measurements of TS and MY. The hydrogen exposure Q may be converted to the number of atoms, n, incident per cm2 per set from the relation n= 7.40 x lo-r4 Q, where Q is in L units. The initial slope of the ~!?acurve then gives 0.29 for the initial sticking probability.
Exposure
Fig. 2. Change in intensity (in arbitrary units) of the +, 3 diffraction beam with hydrogen exposure, for comparison with the low exposure range of fig. lb. Note that the beginning of adsorption into the /3r state coincides with the maximum intensity of the 3, 4 beam, although the j&zstate is only two-thirds populated.
AJXORPTION
OF HYDRoGEN
ON w
(loo)
421
In the range of 0=0.35 to 0.8 the four new beams which originate from the +,& beam move apart continuously, with the separation in the (10) direo tions a linear function of coverage. Occupation of the pZ binding state is complete at 8=0.8, and this observation together with the fact that the maximum intensity of the 3, 3 beam occurs at 8 =0.35, might support an interpretation in terms of the formation of a superstructure of out-of-phase domains, with each domain having hydrogen atoms in a C (2 x 2) array’). This model, which is similar to a proposal we have made to explain our LEED observations for nitrogen adsorption on W(lOO)*), is difficult to reconcile with the observation that the four new beams move apart continuously with coverage, which implies that the domain size is decreasing with increasing coverage. The model proposed by EA for this coverage range is that (2xM) structures are formed, consisting of adsorption of hydrogen into the gaps of the C(2 x 2) array to form complete rows of hydrogen atoms in the {lo} directions, M unit cell spacings apart. (We have measured separations of the beams giving M= 12!.) This model satisfactorily explains the continuous decrease in M with coverage, but EA have shown that a kinematic analysis of these surface structures predicts extra beams, having intensities an order of magnitude less than for the four primary (2 x M) beams. We have taken photographs of (2 x M) patterns over-exposed by more than 10 x , and have failed to detect the extra beams. At coverage of 8=0.8, the (2 x M) beams degenerate into streaks in approximately one-third order positions, as described by EA. With increasing coverage the streaks weaken in intensity, and a complicated pattern of very weak streaks is formed, which includes the faint $ order spots described by EA. At saturation coverage, 8= 1.5, the diffraction pattern appears to have the (1 x 1) symmetry of the substrate, but photographic over-exposure reveals that a complex pattern of weak streak is still present. The complex nature of the LEED patterns in the coverage range 8=0.8 to 1.5 prevents a simple explanation of the surface structures. In particular we do not believe that the LEED observations can be unequivocally used to deduce the sites responsible for hydrogen adsorption, as has been suggested by EA. In summary, we believe that the & desorption peak is associated with the C(2 x 2) LEED pattern, which is evidence that this binding state consists of hydrogen atoms in a C(2 x 2) array. We believe that the surface structures responsible for the j?r peak have not been satisfactorily identified at the present time. We wish to thank R. A. Armstrong for a useful discussion. We thank also
422
D.L.ADAM.3
AND
L.H.GERMER
The Petroleum Research Fund and the American Iron and Steel Institute for their continued support. D. L. ADAMSand L. H. GERMER Cornell University, Ithaca, New York 14850, U.S.A. References 1) D. L. Adams and L. H. Germer, Abstracts of the 30th Physical Electronics Conference (Milwaukee, 1970). 2) P. J. Estrup and J. Anderson, J. Chem. Phys. 45 (1966) 2254. 3) P. W. Tamm and L. D. Schmidt, J. Chem. Phys. 51(1969) 5352. 4) T. E. Madey and J. T. Yates, in: Actes du Colloque Intern. sur la Structure et les PropriPt&s des Surfaces des Solides (Paris, July 1969). 5) D. L. Adams, L. H. Germer and J. W. May, Surface Sci. 22 (1970) 45. 6) M. Leggett and R. A. Armstrong, Abstracts of the 30th Physical Electronics Conference (Milwaukee, 1970). 7) R. A. Armstrong, private communication. 8) D. L. Adams and L. H. Germer, to be published. (A preliminary account of this work was presented at Milwaukee, 1970. See ref. 1.)
ADSORPTION
OF HYDROGEN
Publishin% Co.
ON W (100)
Received 29 June 1970
We have carried out a combined LEED/flash desorption study 1) of hydrogen adsorption upon W (IOO), which supplements reported findings by Estrup and Andersons), (EA), Tamm and Schmidts), (TS), and Madey and Yates *), (MY). Our experimental system and techniques have been described previously 5)_ A typical flash desorption spectrum from a surface that had been saturated with hydrogen is shown in fig. la. The amount of hydrogen desorbed in the /I1 peak is twice that desorbed in the & peak. Population of the two peaks occurs sequentially with exposure, with some overlap. Thus, the p1 peak is first observed at a coverage of p2=0.35 monolayer of atoms, although saturation coverage of /I2 =OS monolayer. This sequence is shown in fig. lb. A detailed analysis of the kinetics of adsorption has been made previouslys*4), and our results are in agreement with these studies. Exposure to hydrogen produces a C (2 x 2) LEED pattern. The intensity of the -&,+beam increases with exposure, reaching a maximum at the exposure of 0.44 L. This exposure is exactly that for which the j3r peak is first produced, as can be seen from fig. 2. Leggett and Armstrong6} have recently reported this result from a combined RHEED/flash desorption study. At exposures greater than 0.44 L, the intensity measured in the h -i-4, k -1-3 positions decreased, coincident with a splitting of the 3, + beam into four new beams, as described by EA. The coincidence of the maximum intensity of the +, 4 beam with the first appearance of & in the desorption spectra is of considerable interest. It provides direct evidence of an interrelation between adsorbate structure and binding state, and confirms the inference of TS from their flash desorption data and EA’s LEED results, that flz desorption occurs from hydrogen atoms in a C (2 x 2) array. Since the LEED observations are made at room temperature, we believe that this observation strongly suggests that the distribution of hydrogen between the binding states shown by flash desorption is that which is produced at room temperature. It is evident that the Br binding state coincides with the continuous change in adsorbate structure reported by EA, but a detailed understanding of the surface structures formed in this range of coverage has not been achieved. 419
420
D.L.ADAMS AND L.H.GBRMER
Exposure
(IO*
torr see)
Fig. 1. Desorption of hydrogen from a W(100) surface: (a) desorption spectrum after a saturation hydrogen exposure of 12 L (i.e. 12 x 1O-6 torr set). The crystal temperature was raised at 100” per sec. (b) Amounts of hydrogen desorbed in the /?I and /3a peaks plotted against hydrogen exposure. Saturation 82 coverage is assumed to be half a monolayer, based on the measurements of TS and MY. The hydrogen exposure Q may be converted to the number of atoms, n, incident per cm2 per set from the relation n= 7.40 x lo-r4 Q, where Q is in L units. The initial slope of the ~!?acurve then gives 0.29 for the initial sticking probability.
Exposure
Fig. 2. Change in intensity (in arbitrary units) of the +, 3 diffraction beam with hydrogen exposure, for comparison with the low exposure range of fig. lb. Note that the beginning of adsorption into the /3r state coincides with the maximum intensity of the 3, 4 beam, although the j&zstate is only two-thirds populated.
AJXORPTION
OF HYDRoGEN
ON w
(loo)
421
In the range of 0=0.35 to 0.8 the four new beams which originate from the +,& beam move apart continuously, with the separation in the (10) direo tions a linear function of coverage. Occupation of the pZ binding state is complete at 8=0.8, and this observation together with the fact that the maximum intensity of the 3, 3 beam occurs at 8 =0.35, might support an interpretation in terms of the formation of a superstructure of out-of-phase domains, with each domain having hydrogen atoms in a C (2 x 2) array’). This model, which is similar to a proposal we have made to explain our LEED observations for nitrogen adsorption on W(lOO)*), is difficult to reconcile with the observation that the four new beams move apart continuously with coverage, which implies that the domain size is decreasing with increasing coverage. The model proposed by EA for this coverage range is that (2xM) structures are formed, consisting of adsorption of hydrogen into the gaps of the C(2 x 2) array to form complete rows of hydrogen atoms in the {lo} directions, M unit cell spacings apart. (We have measured separations of the beams giving M= 12!.) This model satisfactorily explains the continuous decrease in M with coverage, but EA have shown that a kinematic analysis of these surface structures predicts extra beams, having intensities an order of magnitude less than for the four primary (2 x M) beams. We have taken photographs of (2 x M) patterns over-exposed by more than 10 x , and have failed to detect the extra beams. At coverage of 8=0.8, the (2 x M) beams degenerate into streaks in approximately one-third order positions, as described by EA. With increasing coverage the streaks weaken in intensity, and a complicated pattern of very weak streaks is formed, which includes the faint $ order spots described by EA. At saturation coverage, 8= 1.5, the diffraction pattern appears to have the (1 x 1) symmetry of the substrate, but photographic over-exposure reveals that a complex pattern of weak streak is still present. The complex nature of the LEED patterns in the coverage range 8=0.8 to 1.5 prevents a simple explanation of the surface structures. In particular we do not believe that the LEED observations can be unequivocally used to deduce the sites responsible for hydrogen adsorption, as has been suggested by EA. In summary, we believe that the & desorption peak is associated with the C(2 x 2) LEED pattern, which is evidence that this binding state consists of hydrogen atoms in a C(2 x 2) array. We believe that the surface structures responsible for the j?r peak have not been satisfactorily identified at the present time. We wish to thank R. A. Armstrong for a useful discussion. We thank also
422
D.L.ADAM.3
AND
L.H.GERMER
The Petroleum Research Fund and the American Iron and Steel Institute for their continued support. D. L. ADAMSand L. H. GERMER Cornell University, Ithaca, New York 14850, U.S.A. References 1) D. L. Adams and L. H. Germer, Abstracts of the 30th Physical Electronics Conference (Milwaukee, 1970). 2) P. J. Estrup and J. Anderson, J. Chem. Phys. 45 (1966) 2254. 3) P. W. Tamm and L. D. Schmidt, J. Chem. Phys. 51(1969) 5352. 4) T. E. Madey and J. T. Yates, in: Actes du Colloque Intern. sur la Structure et les PropriPt&s des Surfaces des Solides (Paris, July 1969). 5) D. L. Adams, L. H. Germer and J. W. May, Surface Sci. 22 (1970) 45. 6) M. Leggett and R. A. Armstrong, Abstracts of the 30th Physical Electronics Conference (Milwaukee, 1970). 7) R. A. Armstrong, private communication. 8) D. L. Adams and L. H. Germer, to be published. (A preliminary account of this work was presented at Milwaukee, 1970. See ref. 1.)