A39

Surface Science 166 (1986) 129-140 North-Holland, Amsterdam

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

P A R T I C L E S D E S O R B E D F R O M L A S E R I R R A D I A T E D C d S

A. N A M I K I , T. K A W A I a n d K. I C H I G E

Department of Electrical Engineering and Electronics, Tovohashi University of Technology, Tempaku, Tovohashi 440, Japan

Received 8 July 1985; accepted for publication 5 September 1985

129

Angle-resolved time-of-flight spectra of neutral particles desorbed from laser irradiated CdS have been investigated with a pulse mass counting method. Three quantities to characterize the desorption dynamics - the desorption flux, the mean kinetic energy and the speed ratio - depend remarkably on the incident laser power and the ejection polar angle 0. The desorption flux is strongly peaked forward deviating from the cosine distribution. For low laser power, the desorbing S: follows the same Maxwellian velocity distribution over the polar angles below 50 °. For high laser power, the Maxwell distribution is limited to a narrow cone around the polar axis and with increasing polar angle the spectra of the velocity distribution become broader than the simulated Maxwellian one. With increasing laser power the mean kinetic energy increases for 0 < 40 °, while for 0 > 40 ° it decreases abnormally. For these apparent breakdowns of the Maxwell law and Knudsen law, the dynamic behaviour of desorbing particles obeys a non-equilibrium thermal mechanism, which may involve a solid-gas phase transformation.

Surface Science 166 (1986) 141-158 141 North-Holland, Amsterdam

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

R A D I A T I O N (132.3 eV)

K. D O C K E R S , H.P . B O N Z E L a n d D .A. W E S N E R

lnstitut fur Grenzfli~chenforschung und Vakuumphysik, Kernforschungsanlage J~lich GmbH, Postfach 1913, D-5170 Ji21ich, Fed. Rep. of Germany Received 16 July 1985; accepted for publication 16 September 1985

An Y anode was prepared in situ under UHV conditions and used for core level spectroscopy of Pt 4f levels of a clean and CO covered Pt(111) crystal. The Y M~" photons of 132.3 eV energy and 0.45 eV half width were sufficiently surface sensitive in order to produce a large fraction of surface to bulk emission. The background of Auger and secondary electrons in thephotoemission spectra was subtracted by use of an analogous spectrum excited by Mo M£ radiation (193.3 eV). The Pt 4f spectra were fitted to Doniach-Sunji6 line shapes and decomposed into surface and bulk contributions. The surface core level shift was found to be -0.37 and 0.63 eV for clean Pt( l l l ) and CO covered Pt(lll) , respectively. The intensity of the clean Pt( l l l ) surface peaks was zero after CO adsorption, hence all surface Pt atoms are affected by CO bonding. The ratio of bulk to surface peak intensity was 2.3. The half width of the CO related surface peak was twice that of the clean Pt surface peak consistent with the presence of two different adsorbed CO species.