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Vibration states of hydrogen on tungsten
Volume 60A, number 2
PHYSICS LETTERS
7 February 1977
VIBRATION STATES OF HYDROGEN ON TUNGSTEN C. BACKX’, B. FEUERBACHER, B. FITTON and R.F. WILLIS Surface Physics Group, Astronomy Division, ESTEC, Noordwi/k, Holland Received 26 November 1976 The vibritional excitation of hydrogen adsorbed on the low-index faces of tungsten have been investigated using electron energy-loss spectroscopy. The results indicate the same vibrational force constant in “on-top” sites independent of substrate-atom geometrical environment.
Inelastic scattering of low energy electrons at surfaces has been developed into a powerful technique for studying surface-adsorbate interactions by means of vibrational excitation [1]. Recently it was demonstrated that the high-resolution electron energy loss spectra provide detailed information concerning the geometric site of the adsorbate relative to substrate atoms [2]. The coverage dependence of H2 chemisorbed on the (100) face of tungsten has been studied using this method and specific adsorption sites identifled [3]. The present letter compares the vibrational • frequencies observed and the crystallographic anisotropies encountered in hydrogen chemisorption on all three principal low-index faces of tungsten in terms of the different adsorbate sites. The results suggest that positions “on-top” of W substrate atoms are preferentially occupied by hydrogen chemisorbed on all three planes, showing the same force constant (i.e. vibrational frequency) independent of the substrate atom’s local geometric environment. New evidence is presented relating to the specific sites occupied during H2 chemisorption on the (110) face of tungsten. The experiments have been performed in a system consisting of a hemispherical monochromator and analyzer oriented at 45°to the crystal normal, with an overall resolution of 20 meV achievable. Tungsten single crystals were cut to the desired orientation to within 2°,polished, and cleaned in a vacuum of 10—10 torr. Hydrogen was adsorbed at room by ex7 temperature torr. All structures posure to has pressures around i0as being due to adsorbed discussed been identified hydrogen by observing the ~ isotope energy shift -
Koninklijke Shell Laboratorium Amsterdam B.V.), Amsterdam, The Netherlands.
(Shell Research
upon deuterium chemisorption. Typical results for vibration excitation of hydrogen chemisorbed on the three low-index faces of tungsten are shown in fig. 1. Each panel gives the normalized elastic peak on the left hand side, and data are shown for loss energies up to 280 meV energy. No losses have been found at higher vibration frequencies, e.g. near the H2 molecular vibration frequency at 545 meV, so dissociative chemisorption appears to occur at room temperature for all three faces. The spectra displayed for the (l10)*1 and (111) [4] face have been taken at saturation coverage. The lowest panel in fig. I shows the data reported by Froitzheim et al. [3] on the (100) face of tungsten for saturation (0 = 2) and at lower coverage (0 = 0.5). Here the single peaks at different frequencies for different coverages indicate a coverage. dependent adsorbate site change. A striking feature evident from fig. 1 is the fact that one loss peak near 160 meV is found on all three faces (vertical line), while a second peak occurs at various lower energies, dependent on the face, or is not observed at all in the case of the (Ill) face. For the (100) surface, Froitzheim et al. [3] deduce that the 155 meV loss corresponds to “on-top” site adsorption in the low coverage ~32 -phase which forms the observed c(2 X 2) superstructure [5]. A simple geometrical argument [2,3] indicates that the highest observed frequency should be due to a W—H vibration corresponding to adsorptionatom at a coordination. substrate site giving lowest adsorbate-substrate The The results on the (110) face have been obtained using electrons scattered 4°off the incidence plane and therefore show a strongly reduced background compared to the data taken in the specular beam for (111) and (100) planes.
145
Volume 60A, number 2
PHYSICS LETTERS
I
I
(110)
~--~
157
1JA~\A I
x20 I (111)
~
: ___________ z
x250
.
_____
•
J~i 155
I
~
x220 -
_________________________ 0 100 200 ENERGY LOSS (m.V)
Fig. 1. Energy loss spectra of H2 adsorbed on the three low index planes of tungsten. Top: (110) exposed to 10 L H2 impact energy is 4 eV and the detection angle is 4°out of the specular plane. Middle: (111) exposed to 20L H2 the impact energy is 5eV (ref. [4]). Bottom: the (100) spectra shown are taken at saturation coverage (0 = 2) and at 0 = 0.5. The impact energy is 5 eV (ref. [3]).
(111) surface of a bcc crystal is very open, such that all sites except the top site are equivalent to a large coordination number of tungsten atoms. This being the case, the 160 meV loss peak observed on the W(l 11) face is assigned to on-top site adsorption. For the (110) face, two spectral peaks are observed, and again the highest vibration frequency can be attributed to the on-top position. The conclusion to be drawn from this interpretation is that the vibration frequency, and therefore the 146
7 February 1977
.V~(
Fig. Models (100) proposed (110) for(111) the sites (111)at For saturation coverage for (110) and face tungsten. (100)2.face thethe sites occupied at 0adsorption = 0.5of (full circles) andthe 0= 2 (open circles) are given (ref. [31).
the on-top site on all three faces, independent of the metal relatedcrystallographic chemisorption bond environment. force, is This equivalent is supported for by the results of LCAO calculations for hydrogen on a variety in nickel faces [6], and on the W(l00) face in particular [7], allof which predict an energy of
approximately 170 meV for the on-top position irrefrom the present results that the top sites must be the energetically favourable and therefore represent those sites thesurface highest binding energy. This, in turn, would indicate thatones the order of equilispective of with crystal *2 relative Moreover, it follows brium binding energies for the various sites as obtained by LCAO calculations is indeed realistic Although adsorbate-adsorbate interaction at higher [6,7]. coverages does not appear to change the vibrational frequency, and hence the curvature of the associated potential energy well normal to the surface at equilibrium, it may well influence the relative magnitude of the minima as is suggested by the coverage dependent transition to different occupied sites on the (100) surface [3, 5].
The loss peak at 130 rneV on the (100) face has been assigned to hydrogen bonded in bridge sites [3] at full coverage, where 2 H atoms [8] have to be accommodated in equivalent sites within the surface unit cell. On the (110) face, only 1 H atom per W surface atom is adsorbed at saturation [8]. The observation of two peaks in the loss spectrum (top, fig. 1) indicates however that two separate sites are occupied, in agreement with conclusions reached from photoemission data [9], but in contrast to the currently accepted model *2
The vibration frequencies have been derived from the potential energy curves of refs. [6,7] by making a parabolic fit to their curves at the equilibrium distance.
Volume 60A, number 2
PHYSICS LETTERS
[10, 11] that proposes the H atoms to be adsorbed in the 5-fold coordination site where the W atom in the next plane would be located, forming a p(2 X 1) superstructure at half coverage [12]. The present results indicate that hydrogen is adsorbed in the top site, giving rise to 157 meV loss, while simultaneously a second different site is occupied. The associated low energy loss of 95 mey*3 is indicative of a higher coordination than twofold, based on the observation that the bridge site on W(100) gives rise to a 130 meV loss. The tungsten atoms forming a bridge site on W(l 10) are closer together than on the (100) face, which would produce a narrower W—H—W bond angle and therefore a higher vibrational frequency than 130 meV. The 5-fold coordination site, therefore, is a more likely position for the hydrogen atoms since this would account for the much lower 95 meV vibration frequency observed on W(l 10). A summary of these conclusions in terms of particular site models derived for hydrogen chemisorption on tungsten by energy loss spectroscopy is presented in fig. 2. The model for the (100) face is that described by Froitzheim et al. [3] including a shift from top sites at low coverage to bridge sites (open circles) at saturation. The structure proposed for the (110) face is consistent with the present coverage data, but not with RHEED results indicative of p(2 X 1) superstructure [12]. The room-temperature coverage results for the (Ill) face [8, 11] suggest that the presence of a second hydrogen atom on the surface is not observed in electron-energy4oss vibration spectroscopy. ThIS 15 due to the very open lattice structure of this surface, any “extra” chemisorbed atoms being “buried” within the surface with a high coordinate number of surroundbig W atoms such that the corresponding vibration fre*3
7 February 1977
quency normal to the (111) plane is very soft, too low in frequency in fact to be observed with the resolution achieved to data. In summary, two co-existing sites are found on the (1 10)W surface at all coverages in contrast to a coverage dependent site transition observed on (lOO)W. No coverage dependence of occupied sites has been observed on the (lll)W surface to date. The “on-top” site appears to be energetically the most favourable site on all three tungsten planes. For this site, the same vibrational frequency hw~b 157 ±3 meV is observed for chemisorbed hydrogen on the three different surfaces, independent of the local geometric spatial location of the crystal-field split d-band orbitals. This would indicate that strict applicability of an atomic bond orbital model in the prediction of likely adsorption sites [11] to be of very limited value.
References [1] F.M. Propst and T.C. Piper, J. Vac. Sci. Technol. 4 (1967) 53. [2] H. Froitzheim, H. Ibach and S. Lehwald, Phys. Rev. B14 [3] H. Froitzheim, H. Ibach and S. Lehwald, Phys. Rev. Lett. 36 (1976) 1549. [4] C. Backx, B. Feuerbacher, B. Fitton and R.F. Willis, Surface Sci. (in press). [5] For a review of the experimental situation see L.D. Schmidt, in: Interactions on metal surfaces, ed. by R. Gomer (Springer, Heidelberg, 1975) p. 64. [6] D.J.M. Fassaert and A. van der Avoird, Surface Sci. 55 (1976) 291; ibid., 55 (1976) 313. [7] L.W. Anders, R.S. Hansen and L.S. Bartell, J. Chem. Phys. 59 (1973) 5277. [8] T.E.Madey, Surface Sci. 36 (1973) 281; ibid., 29(1972)
This peak appears to be broader than the 157 meV struc-
[9] B. Feuerbacher and B. Fitton, Phys. Rev. B8 (1973) 4890. [10] E.W. Plummer, et al., Progr. Surface Sci. 7 (1976) 149.
ture. The larger width is due to a carbon peak at 73 meV arising from the carbon monoxide background pressure due to the high excitation cross section of this vibration.
[11] P.W. Tamm and L.D. Schmidt, J. Chem. Phys. 54 (1971) 4775. [12] K.J. Matysik, Surface Sci. 29(1972) 324.
147
PHYSICS LETTERS
7 February 1977
VIBRATION STATES OF HYDROGEN ON TUNGSTEN C. BACKX’, B. FEUERBACHER, B. FITTON and R.F. WILLIS Surface Physics Group, Astronomy Division, ESTEC, Noordwi/k, Holland Received 26 November 1976 The vibritional excitation of hydrogen adsorbed on the low-index faces of tungsten have been investigated using electron energy-loss spectroscopy. The results indicate the same vibrational force constant in “on-top” sites independent of substrate-atom geometrical environment.
Inelastic scattering of low energy electrons at surfaces has been developed into a powerful technique for studying surface-adsorbate interactions by means of vibrational excitation [1]. Recently it was demonstrated that the high-resolution electron energy loss spectra provide detailed information concerning the geometric site of the adsorbate relative to substrate atoms [2]. The coverage dependence of H2 chemisorbed on the (100) face of tungsten has been studied using this method and specific adsorption sites identifled [3]. The present letter compares the vibrational • frequencies observed and the crystallographic anisotropies encountered in hydrogen chemisorption on all three principal low-index faces of tungsten in terms of the different adsorbate sites. The results suggest that positions “on-top” of W substrate atoms are preferentially occupied by hydrogen chemisorbed on all three planes, showing the same force constant (i.e. vibrational frequency) independent of the substrate atom’s local geometric environment. New evidence is presented relating to the specific sites occupied during H2 chemisorption on the (110) face of tungsten. The experiments have been performed in a system consisting of a hemispherical monochromator and analyzer oriented at 45°to the crystal normal, with an overall resolution of 20 meV achievable. Tungsten single crystals were cut to the desired orientation to within 2°,polished, and cleaned in a vacuum of 10—10 torr. Hydrogen was adsorbed at room by ex7 temperature torr. All structures posure to has pressures around i0as being due to adsorbed discussed been identified hydrogen by observing the ~ isotope energy shift -
Koninklijke Shell Laboratorium Amsterdam B.V.), Amsterdam, The Netherlands.
(Shell Research
upon deuterium chemisorption. Typical results for vibration excitation of hydrogen chemisorbed on the three low-index faces of tungsten are shown in fig. 1. Each panel gives the normalized elastic peak on the left hand side, and data are shown for loss energies up to 280 meV energy. No losses have been found at higher vibration frequencies, e.g. near the H2 molecular vibration frequency at 545 meV, so dissociative chemisorption appears to occur at room temperature for all three faces. The spectra displayed for the (l10)*1 and (111) [4] face have been taken at saturation coverage. The lowest panel in fig. I shows the data reported by Froitzheim et al. [3] on the (100) face of tungsten for saturation (0 = 2) and at lower coverage (0 = 0.5). Here the single peaks at different frequencies for different coverages indicate a coverage. dependent adsorbate site change. A striking feature evident from fig. 1 is the fact that one loss peak near 160 meV is found on all three faces (vertical line), while a second peak occurs at various lower energies, dependent on the face, or is not observed at all in the case of the (Ill) face. For the (100) surface, Froitzheim et al. [3] deduce that the 155 meV loss corresponds to “on-top” site adsorption in the low coverage ~32 -phase which forms the observed c(2 X 2) superstructure [5]. A simple geometrical argument [2,3] indicates that the highest observed frequency should be due to a W—H vibration corresponding to adsorptionatom at a coordination. substrate site giving lowest adsorbate-substrate The The results on the (110) face have been obtained using electrons scattered 4°off the incidence plane and therefore show a strongly reduced background compared to the data taken in the specular beam for (111) and (100) planes.
145
Volume 60A, number 2
PHYSICS LETTERS
I
I
(110)
~--~
157
1JA~\A I
x20 I (111)
~
: ___________ z
x250
.
_____
•
J~i 155
I
~
x220 -
_________________________ 0 100 200 ENERGY LOSS (m.V)
Fig. 1. Energy loss spectra of H2 adsorbed on the three low index planes of tungsten. Top: (110) exposed to 10 L H2 impact energy is 4 eV and the detection angle is 4°out of the specular plane. Middle: (111) exposed to 20L H2 the impact energy is 5eV (ref. [4]). Bottom: the (100) spectra shown are taken at saturation coverage (0 = 2) and at 0 = 0.5. The impact energy is 5 eV (ref. [3]).
(111) surface of a bcc crystal is very open, such that all sites except the top site are equivalent to a large coordination number of tungsten atoms. This being the case, the 160 meV loss peak observed on the W(l 11) face is assigned to on-top site adsorption. For the (110) face, two spectral peaks are observed, and again the highest vibration frequency can be attributed to the on-top position. The conclusion to be drawn from this interpretation is that the vibration frequency, and therefore the 146
7 February 1977
.V~(
Fig. Models (100) proposed (110) for(111) the sites (111)at For saturation coverage for (110) and face tungsten. (100)2.face thethe sites occupied at 0adsorption = 0.5of (full circles) andthe 0= 2 (open circles) are given (ref. [31).
the on-top site on all three faces, independent of the metal relatedcrystallographic chemisorption bond environment. force, is This equivalent is supported for by the results of LCAO calculations for hydrogen on a variety in nickel faces [6], and on the W(l00) face in particular [7], allof which predict an energy of
approximately 170 meV for the on-top position irrefrom the present results that the top sites must be the energetically favourable and therefore represent those sites thesurface highest binding energy. This, in turn, would indicate thatones the order of equilispective of with crystal *2 relative Moreover, it follows brium binding energies for the various sites as obtained by LCAO calculations is indeed realistic Although adsorbate-adsorbate interaction at higher [6,7]. coverages does not appear to change the vibrational frequency, and hence the curvature of the associated potential energy well normal to the surface at equilibrium, it may well influence the relative magnitude of the minima as is suggested by the coverage dependent transition to different occupied sites on the (100) surface [3, 5].
The loss peak at 130 rneV on the (100) face has been assigned to hydrogen bonded in bridge sites [3] at full coverage, where 2 H atoms [8] have to be accommodated in equivalent sites within the surface unit cell. On the (110) face, only 1 H atom per W surface atom is adsorbed at saturation [8]. The observation of two peaks in the loss spectrum (top, fig. 1) indicates however that two separate sites are occupied, in agreement with conclusions reached from photoemission data [9], but in contrast to the currently accepted model *2
The vibration frequencies have been derived from the potential energy curves of refs. [6,7] by making a parabolic fit to their curves at the equilibrium distance.
Volume 60A, number 2
PHYSICS LETTERS
[10, 11] that proposes the H atoms to be adsorbed in the 5-fold coordination site where the W atom in the next plane would be located, forming a p(2 X 1) superstructure at half coverage [12]. The present results indicate that hydrogen is adsorbed in the top site, giving rise to 157 meV loss, while simultaneously a second different site is occupied. The associated low energy loss of 95 mey*3 is indicative of a higher coordination than twofold, based on the observation that the bridge site on W(100) gives rise to a 130 meV loss. The tungsten atoms forming a bridge site on W(l 10) are closer together than on the (100) face, which would produce a narrower W—H—W bond angle and therefore a higher vibrational frequency than 130 meV. The 5-fold coordination site, therefore, is a more likely position for the hydrogen atoms since this would account for the much lower 95 meV vibration frequency observed on W(l 10). A summary of these conclusions in terms of particular site models derived for hydrogen chemisorption on tungsten by energy loss spectroscopy is presented in fig. 2. The model for the (100) face is that described by Froitzheim et al. [3] including a shift from top sites at low coverage to bridge sites (open circles) at saturation. The structure proposed for the (110) face is consistent with the present coverage data, but not with RHEED results indicative of p(2 X 1) superstructure [12]. The room-temperature coverage results for the (Ill) face [8, 11] suggest that the presence of a second hydrogen atom on the surface is not observed in electron-energy4oss vibration spectroscopy. ThIS 15 due to the very open lattice structure of this surface, any “extra” chemisorbed atoms being “buried” within the surface with a high coordinate number of surroundbig W atoms such that the corresponding vibration fre*3
7 February 1977
quency normal to the (111) plane is very soft, too low in frequency in fact to be observed with the resolution achieved to data. In summary, two co-existing sites are found on the (1 10)W surface at all coverages in contrast to a coverage dependent site transition observed on (lOO)W. No coverage dependence of occupied sites has been observed on the (lll)W surface to date. The “on-top” site appears to be energetically the most favourable site on all three tungsten planes. For this site, the same vibrational frequency hw~b 157 ±3 meV is observed for chemisorbed hydrogen on the three different surfaces, independent of the local geometric spatial location of the crystal-field split d-band orbitals. This would indicate that strict applicability of an atomic bond orbital model in the prediction of likely adsorption sites [11] to be of very limited value.
References [1] F.M. Propst and T.C. Piper, J. Vac. Sci. Technol. 4 (1967) 53. [2] H. Froitzheim, H. Ibach and S. Lehwald, Phys. Rev. B14 [3] H. Froitzheim, H. Ibach and S. Lehwald, Phys. Rev. Lett. 36 (1976) 1549. [4] C. Backx, B. Feuerbacher, B. Fitton and R.F. Willis, Surface Sci. (in press). [5] For a review of the experimental situation see L.D. Schmidt, in: Interactions on metal surfaces, ed. by R. Gomer (Springer, Heidelberg, 1975) p. 64. [6] D.J.M. Fassaert and A. van der Avoird, Surface Sci. 55 (1976) 291; ibid., 55 (1976) 313. [7] L.W. Anders, R.S. Hansen and L.S. Bartell, J. Chem. Phys. 59 (1973) 5277. [8] T.E.Madey, Surface Sci. 36 (1973) 281; ibid., 29(1972)
This peak appears to be broader than the 157 meV struc-
[9] B. Feuerbacher and B. Fitton, Phys. Rev. B8 (1973) 4890. [10] E.W. Plummer, et al., Progr. Surface Sci. 7 (1976) 149.
ture. The larger width is due to a carbon peak at 73 meV arising from the carbon monoxide background pressure due to the high excitation cross section of this vibration.
[11] P.W. Tamm and L.D. Schmidt, J. Chem. Phys. 54 (1971) 4775. [12] K.J. Matysik, Surface Sci. 29(1972) 324.
147