PENNING IONIZATION OF THE 'X-COVERED Pd(ll1) SURFACE BY METASTABLE HELIUM (2+, 23S)-BEAMS*

Surface Penning ionization (SPI) in the process He*(2lS, 23S) + CO + He(llS) + CO+ + eis investigated theoretically for CO molecules chemisorbed on a Pd(ll1) surface at low collision energy of metastable He*-beams. The actual process can well be explained in terms of Auger deexcitation. The entire SPI process occurs at the topmost layer, thereby increasing the surface sensitivity. The potential energy surfaces between the collision partners in the initial and final states are calculated using the local spin density functional formalism. The angle distribution and ionization rate of emitted electrons are calculated based on the Golden Rule, assuming CO standing straight on the substrate for various impact geometries. Pronounced "shadow" effect due to neighboring CO molecules at grazing He* incident angles and in the emitted electron angle distributions is obtained. Comparisons with experimental data are made. Our results exhibit the usefulness of SPI in studying the electronic and geometrical structure of adsorbates on surfaces. In a previous paper 11) (referred to as I) energy distributions of electrons ejgcted from clean and CO-covered Pd(ll1) surfaces by impact with metastable He 2lS (excitation energy EX=20.6eV) and 23S(E*=19.8eV) have been reported. These results have been verified both experimentally and theoretically as due to the operation of the Penning ionization (=Auger deexcitation)mechanism, via He* + CO/Pd(lll) -+ He + CO+/Pd(lll) f e. (1) Recently a first theoretical analysis (2) (referred to as II) of surface Penning ionization (SPI) appeared in which the potential energy curves, ionization rates and angle distributions of the emitted electrons for the same system were obtained. The results of this analysis can be summarized as follows: (1) The well depths of the interaction potential energy curves between the collision partners in the initial and final states are both very small+and indicate a "hard core" type collision process.. The ionization rate I'(R) decays exponentially as a function of 0 **-He* internuclear separation. This means $hat most of the ionization events take place at the classicai turning point Rm between the collision partners and it is a reasonable approximation to assume the ionization transition to occur only at Xm. (2) The angle distributign foq electrons emitted from an isolated CO molecule with a single He at Rm is written in partial -wave expansion as