Surface Penning Ionization: Theory and Application of a New .'technique for the Analysis of Chemisorption Systems

Metastable He*(21S,23S) atomic beams at thermal energies have been used more frequently as soft surface probes recently. Due to the He*J< surface (M) interactions there is a certain probability I' of He being deexcited in front of the surface. The deexcitation energies (E* = 20.6 and 19.8 eV respectively for He * 1 -2 S and -23S) that are released are sufficient to ionize electrons of surfaces. This process, i.e., He* +M+He+M++ e-, is the so-called surface Penning ionization (SPI). In this work, we derive general theoretical formulae for the energy and angular distributions of electrons ejected in the SPI process for surfaces covered with adsorbates. Our derivations include: (1) obtaining the complete potential energy surfaces (PES) of the * initial collision partners He .*. M and final collision partners He l ** M+ by overlapping the individual electron densities of these partners and by calculating their interaction energies using the local spin-density functional formalism; (2) calculating the ionization probability P, which decays roughly exponentially as a function of * Work supported by Department of Energy, contract DE-AC03-76SF00515.