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Interaction of oxygen with a polycrystalline palladium surface over a wide temperature range
A406 346
Surface Science 218 (1989) 346-362 North-Holland, Amsterdam
ADSORPTION
OF WATER
ON Ru(100)
P.K. LEAVITT,
J.L. D A V I S
*, J.S. D Y E R
and P.A. THIEL
**
Department of Chemistry and Ames Laboratory, Iowa State University, Ames, 1,4 50011, USA Received 20 January 1989; accepted for publication 30 March 1989 We have investigated adsorption of water on Ru(100) using electron energy loss spectroscopy and thermal desorption spectroscopy. On this row-and-trough substrate, there is a single desorption state associated with the chemisorbed layer, which shifts down from 240 to 180 K as coverage increases. There is no isotope effect, i.e., desorption spectra of H 2 0 are the same as those of D20, within experimental error. Less than 0.1 monolayers of H 2 0 dissociate and exchange between chemisorbed hydrogen and D20 is not measurable. Pre-adsorbed oxygen and hydrogen cause distinctive changes in the shapes of the water desorption peaks on Ru(100). Electron energy loss spectroscopy indicates that the extent of hydrogen bonding increases as water coverage increases at 85 K. Upon annealing and desorption, most changes in the vibrational spectra can be attributed to decreasing coverage, with the exception of some changes in the low-frequency modes. Comparison with previous studies of the hexagonally close-packed Ru(001) surface reveals that surface morphology plays a major role in determining the properties of water adlayers on ruthenium surfaces.
Surface Science 218 (1989) 363-388 North-HoUand, Amsterdam
363
INTERACTION OF OXYGEN WITH A POLYCRYSTALLINE PALLADIUM SURFACE OVER A WIDE TEMPERATURE RANGE M . M I L U N , P. P E R V A N
Institute of Physics of the University of Zagreb, P.O. Box 304, 41001 Zagreb, Yugoslavia and K. WANDELT
*
Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-IO00 Berlin 33, Germany Received 17 August 1988; accepted for publication 31 March 1989 Using thermal desorption spectroscopy (TDS) the interaction of gaseous oxygen with a polycrystalline palladium foil was studied for the first time over a very wide temperature (110 < T < 1400 K) and exposure (10000 L) range. Since studies with polycrystalline and defect surfaces are generally hampered by a serious lack of structural information, the Pd(fofl) surface was first characterized by means of the PAX technique (photoemission of adsorbed xenon). Hereafter the surface was found to consist of -605~ (Pd(ll0)-Iike) defect sites and - 4 0 % Pd(100)-like sites. This structural information proved very helpful for the interpretation of the complex high-coverage spectra of oxygen from this surface. Despite the general belief we find desorption of some amount of molecularly adsorbed oxygen from this highly defective surface before the detection of an atomically adsorbed oxygen species. This molecular oxygen desorbs at 160 K. Exposures greater than 0.7 L are required to first detect atomically adsorbed oxygen.
A407 Oxygen adsorption at Tad > 150 K results in atomic oxygen chemisorption which manifests itself as a single peak of second-order desorption kinetics near 900 K desorption temperature and which grows with increasing exposure. Its width, however, is not significantly affected by the presence of surface defects as revealed most clearly by a so-called Y-parameter (FWHM/Tp) analysis. Beyond an initial coverage of 00 = 0.25 this peak develops several shoulders at lower temperatures ( - 700 K) which did not saturate even after 10000 L and which are assigned to oxygen chemisorbed on the Pd(100) surface patches and the Pd(ll0)-like surface defects, respectively. This assignment was possible on the basis of the PAX results and is consistent with literature data for the oxygen desorption from Pd(100) and Pd(ll0) single-crystal surfaces. At an adsorption temperature of Tad = 510 K the highest exposures lead to the formation of a surface oxide species which decomposes above 700 K with zeroth-order kinetics. A small fraction of each new exposure penetrates deeper into the bulk and is given off only at temperatures _>1300 K.
Surface Science 218 (1989) 389-405 North-Holland, Amsterdam A SEMICLASSICAL TREATMENT STIMULATED DESORP'rION Z.L. MISKOVIC
389 OF ION DYNAMICS
IN ELECTRON
a n d J.V. V U K A N I C
"'Boris Kidri?'" Institute for Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Yugoslavia Received 10 January 1989; accepted for publication 3 April 1989 The kinetic energy distribution of desorbed ions is analysed on the basis of the factorized electron stimulated desorption cross section which contains the Franck-Condon nuclear overlap integral and the ion survival probability. Approximate expressions for both factors are derived by solving the one-dimensional time-independent nonhomogeneous Schr~Sdinger equation which contains an optical potential for the final state adparticle motion. The method used is an extension of the uniform semiclassical approximation into the complex plane. The effect of the decay of the final state as well as of the curvature of the final state potential on the Franck-Condon integral are investigated in some detail. The limit of small decay rate as well as the linearized potential approximation are discussed and the connection with the reflection approximation for the Franck-Condon integral is made. Also, the semiclassical expression for the ion survival probability is analysed and related to the classical one. The results derived are tested by simple model calculations appropriate for the system oxygen on tungsten.
406 ELECTRON ADSORBED
Surface Science 218 (1989) 406-430 North-Holland, Amsterdam STIMULATED DESORPTION FROM CO AND O ON CLEAN AND Cu COVERED W(ll0) SURFACES
J.C. L I N a n d R . G O M E R Department of Chemistry and The James Franck Institute, University of Chicago, Chicago, I L 60637, USA Received 30 December 1988; accepted for publication 28 March 1989
Surface Science 218 (1989) 346-362 North-Holland, Amsterdam
ADSORPTION
OF WATER
ON Ru(100)
P.K. LEAVITT,
J.L. D A V I S
*, J.S. D Y E R
and P.A. THIEL
**
Department of Chemistry and Ames Laboratory, Iowa State University, Ames, 1,4 50011, USA Received 20 January 1989; accepted for publication 30 March 1989 We have investigated adsorption of water on Ru(100) using electron energy loss spectroscopy and thermal desorption spectroscopy. On this row-and-trough substrate, there is a single desorption state associated with the chemisorbed layer, which shifts down from 240 to 180 K as coverage increases. There is no isotope effect, i.e., desorption spectra of H 2 0 are the same as those of D20, within experimental error. Less than 0.1 monolayers of H 2 0 dissociate and exchange between chemisorbed hydrogen and D20 is not measurable. Pre-adsorbed oxygen and hydrogen cause distinctive changes in the shapes of the water desorption peaks on Ru(100). Electron energy loss spectroscopy indicates that the extent of hydrogen bonding increases as water coverage increases at 85 K. Upon annealing and desorption, most changes in the vibrational spectra can be attributed to decreasing coverage, with the exception of some changes in the low-frequency modes. Comparison with previous studies of the hexagonally close-packed Ru(001) surface reveals that surface morphology plays a major role in determining the properties of water adlayers on ruthenium surfaces.
Surface Science 218 (1989) 363-388 North-HoUand, Amsterdam
363
INTERACTION OF OXYGEN WITH A POLYCRYSTALLINE PALLADIUM SURFACE OVER A WIDE TEMPERATURE RANGE M . M I L U N , P. P E R V A N
Institute of Physics of the University of Zagreb, P.O. Box 304, 41001 Zagreb, Yugoslavia and K. WANDELT
*
Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-IO00 Berlin 33, Germany Received 17 August 1988; accepted for publication 31 March 1989 Using thermal desorption spectroscopy (TDS) the interaction of gaseous oxygen with a polycrystalline palladium foil was studied for the first time over a very wide temperature (110 < T < 1400 K) and exposure (10000 L) range. Since studies with polycrystalline and defect surfaces are generally hampered by a serious lack of structural information, the Pd(fofl) surface was first characterized by means of the PAX technique (photoemission of adsorbed xenon). Hereafter the surface was found to consist of -605~ (Pd(ll0)-Iike) defect sites and - 4 0 % Pd(100)-like sites. This structural information proved very helpful for the interpretation of the complex high-coverage spectra of oxygen from this surface. Despite the general belief we find desorption of some amount of molecularly adsorbed oxygen from this highly defective surface before the detection of an atomically adsorbed oxygen species. This molecular oxygen desorbs at 160 K. Exposures greater than 0.7 L are required to first detect atomically adsorbed oxygen.
A407 Oxygen adsorption at Tad > 150 K results in atomic oxygen chemisorption which manifests itself as a single peak of second-order desorption kinetics near 900 K desorption temperature and which grows with increasing exposure. Its width, however, is not significantly affected by the presence of surface defects as revealed most clearly by a so-called Y-parameter (FWHM/Tp) analysis. Beyond an initial coverage of 00 = 0.25 this peak develops several shoulders at lower temperatures ( - 700 K) which did not saturate even after 10000 L and which are assigned to oxygen chemisorbed on the Pd(100) surface patches and the Pd(ll0)-like surface defects, respectively. This assignment was possible on the basis of the PAX results and is consistent with literature data for the oxygen desorption from Pd(100) and Pd(ll0) single-crystal surfaces. At an adsorption temperature of Tad = 510 K the highest exposures lead to the formation of a surface oxide species which decomposes above 700 K with zeroth-order kinetics. A small fraction of each new exposure penetrates deeper into the bulk and is given off only at temperatures _>1300 K.
Surface Science 218 (1989) 389-405 North-Holland, Amsterdam A SEMICLASSICAL TREATMENT STIMULATED DESORP'rION Z.L. MISKOVIC
389 OF ION DYNAMICS
IN ELECTRON
a n d J.V. V U K A N I C
"'Boris Kidri?'" Institute for Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Yugoslavia Received 10 January 1989; accepted for publication 3 April 1989 The kinetic energy distribution of desorbed ions is analysed on the basis of the factorized electron stimulated desorption cross section which contains the Franck-Condon nuclear overlap integral and the ion survival probability. Approximate expressions for both factors are derived by solving the one-dimensional time-independent nonhomogeneous Schr~Sdinger equation which contains an optical potential for the final state adparticle motion. The method used is an extension of the uniform semiclassical approximation into the complex plane. The effect of the decay of the final state as well as of the curvature of the final state potential on the Franck-Condon integral are investigated in some detail. The limit of small decay rate as well as the linearized potential approximation are discussed and the connection with the reflection approximation for the Franck-Condon integral is made. Also, the semiclassical expression for the ion survival probability is analysed and related to the classical one. The results derived are tested by simple model calculations appropriate for the system oxygen on tungsten.
406 ELECTRON ADSORBED
Surface Science 218 (1989) 406-430 North-Holland, Amsterdam STIMULATED DESORPTION FROM CO AND O ON CLEAN AND Cu COVERED W(ll0) SURFACES
J.C. L I N a n d R . G O M E R Department of Chemistry and The James Franck Institute, University of Chicago, Chicago, I L 60637, USA Received 30 December 1988; accepted for publication 28 March 1989