A 2 4 9

dissipation due to substrate excitations is shown to drastically influence the reaction probability. Within the context of the trajectory theory, dissipation places upper limits on the number of trajectories.leading to non-reactive events and hence on the reaction probability. This is illustrated by example, using sticking as a prototypical event. A criterion for large sticking coefficients is given in terms of vibrational frequencies and linewidths and of desorption energies, all quantities being experimentally accessible with current state-of-the-art measurement techniques. An experi- ment, based on the present theory, is proposed which could differentiate between electronic and phonon dissipation.

Surface Science 118 (1982) 193-207 193 North-Holland Publishing Company

E N E R G Y - R E S O L V E D A N G U L A R D I S T R I B U T I O N S O F S E C O N D A R Y

E L E C T R O N S E M I T r E D F R O M (111) AI T A R G E T B O M B A R D E D

B Y D I F F E R E N T I N E R T G A S I O N S

J o s e t t e M I S C H L E R , M i c h e l N I ~ G R E a n d N i c o l e B E N A Z E T H

Laboratoire de Physique des Solides, Associb au CNRS, Universitb Paul Sabatier, 118 Route de Narbonne, 1;'-31062 Toulouse Cbdex, France

Received 2 November 1981 ; accepted for publication 12 February 1982

Energy-resolved angular distributions of secondary electrons emitted from (11 I) A1 targets bombarded by noble gas ions (Ne + , Ar + , Kr +, Xe + ) are studied. These experimental results show an anisotropy less than 10%; they are compared with two theoretical models which apply to continuum background electrons and Auger electrons, respectively. These models take into account the multiple scattering effects of the electrons inside the crystal and their passage through the surface and furthermore, for the Auger electrons, the matrix elements of the corresponding transition. For Kr + and Xe + ions the experimental angular distributions are in reasonable agreement with the calculated curves. For Ne + and Ar + ions experimental results disagree with calculated profiles both for Auger electrons and for continuum background electrons. The analysis of these results lead to presumptive evidence of a preferential ejection of electrons along the [I 10] directions.

208 Surface Science 118 (1982) 208-222 North-Holland Publishing Company

A D S O R P T I O N O F A L K A N E S A N D T H E I R C O A D S O R P T I O N W I T H

H Y D R O G E N O N P L A T I N U M T H I N F I L M S

W. L I S O W S K I a n d R. D U S

Institute of Physical Chemistry, Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224 Warsaw, Poland

Received 6 March 1981; accepted for publication 3 February 1982

Methane, ethane and propane adsorption on platinum thin films at 195 and 298 K was studied by means of surface potential measurements with the application of the rapidly recording static capacitor method, Observation of the gas phase composition over the adsorbate, and volumetric measurements. The interaction of hydrogen with the hydrocarbon preadsorbed, and also the

A 2 5 0

interaction of C I -C 3 paraffines with hydrogen deposited on platinum thin films was investigated. Two different molecular forms of methane adspecies were registered, while they were dissociatively adsorbed, and molecular forms of ethane and propane deposit were observed. Hydrogen pread- sorbed on plat inum thin films precluded the dissociative adsorption of ethane and propane, and diminished the population of the molecular form of the deposit.

Surface Science 118 (1982) 223-247 North-Holland Publishing Compan 5

S U P E R L A T F I C E S F O R M E D B Y I N T E R A C T I O N O F P O L A R S O L V E N T S W I T H P t ( l l l ) S U R F A C E S S T U D I E D B Y L E E D ,

A U G E R S P E C T R O S C O P Y A N D T H E R M A L D E S O R P T I O N M A S S S P E C T R O M E T R Y

G e r a l d A . G A R W O O D , Jr . a n d A r t h u r T . H U B B A R D

Department of ChemisttT, Universi O' of ('ahfornia, Santa Barbara, California 93106, USA

Received 8 September 1981; accepted for publication 26 January 1982

223

Eleven compounds commonly used as polar solvents in electrochemistry and other liquid-solid catalytic processes have been studied at P t ( l l l ) by LEED, Auger spectroscopy and thermal desorption mass spectrometry. Eight of the compounds chemisorb irreversibly on Pt( l l 1): sulfo- lane, dimethylsulfoxide, pyridine, dimethylformamide, acetonitrile, acetic acid, propylene carbonate and p-dioxane. Limiting chemisorbed coverages are reached at pressures (less than 3 × 10 6 Tort) many orders of magnitude below the vapor pressures of these materials. The remaining three compounds (water, dichloromethane and ammonia) do not chemisorb to an appreciable extent on Pt(111) at pressures as high as 0.3 Torr. The chemisorbed compounds form highly ordered layers at certain coverages. Adsorption is accompanied by varying degrees of dissociation. Heating the surface to 1075 K following chemisorption at room temperaure removes only a small fraction of the total chemisorbed matter; the rest forms a very heat-resistant overlayer composed of most elements of the original adsorbed layer (C, N, O, S).

248 Surface Science 118 (1982) 248 256 North-Holland Publishing ( 'ompan).

P H O T O E M I S S I O N S T U D I E S O F R O O M - T E M P E R A T U R E O X I D I Z E D Ga S U R F A C E S *

C . Y . S U , P . R . S K E A T H , I. L 1 N D A U a n d W . E . S P I C E R **

Electrical Engineering Department, Stanford Unicersity, Stanford, California 94305, USA Received 24 July 1981; accepted for publication 2 March 1982

The oxidation of in situ prepared Ga films at room temperature has been studied with photoemission. Direct formation of Ga203 has been observed. An interpretation of the valence band spectrum of Ga203 in terms of the electronic structure of Ga203 is offered. Correspendence between a donor-like bonding band, due to the interaction between a doubly occupied oxygen lone pair and an empty Ga orbital, an oxygen non-bonding band. and a Ga O - G a bonding band and three features in the valence band spectrum of oxidized Ga has been found. Interpretation of the photoemission spectrum of the recently reported chemisorbed-oxygen on Ga metal at 70 K is suggested based on the understanding of the spectrum of Ga203. 7'he possible existence of an intermediate oxidation state formed by the room temperature oxidation of Ga films is also discussed based on the presence of intermediate chemical shifts (A E <~ 2.0 eV) m the Ga-3d level.