A simple theoretical model is described for Penning ionization (M+A*→M++A+e−) in which a metastable-state rare gas atom (A*) extracts an electron from a target molecule (M) and ejects another electron into a continuum state. One of the most important factors governing Penning ionization is electron distributions of molecular orbitals which are directly connected with the electronic transition probabilities. Another key factor is the repulsive molecular surface which divides the unreactive electron densities in the interior region from the reactive electron densities in the exterior region, because the metastable atom A* cannot penetrate into the interior region. In the present model, exterior electron densities (EED) for individual molecular orbitals are considered to be proportional to Penning ionization branching ratios. Good agreements between ab initio MO calculations of EED’s and experimental branching ratios of Penning ionization were obtained for various unsaturated hydrocarbon molecules.

tribution would be sharper on single crystal surfaces.Thermal desorption spectra of CO are shown in Figure 7. Only a single peak was observed around 620 K with small exposures.This peak was shifted to lower temperatures with an increase in CO coverage. Above 1 .O langmuir of CO, the desorption spectra show a shoulder at lower temperatures converging to 510 K. These spectra are quite similar to those on Ir(ll0) reported by Taylor et al.*' They have estimated an activation energy for the desorption of 37 kcal/mol at low coverages. Figure 8 shows the angular distribution of the desorption with 2 langmuirs of CO. The relative peak height of the CO desorption at 600 K is plotted against the desorption angle. It varies as cos 6. The sticking probability is close to unity.23 The adsorption process of CO is not activated. Therefore, the desorption may be expected to obey the cosine law. Acknowledgment.He*(2'S) Penning ionization electron spectra (PIES) and He I photoelectron spectra of some alkanenitriles, acrylonitrile, and 3-butenenitrile were measured. Band assignments were made based on the characteristics of the PIES; the nonbonding orbital of the CN group showed a remarkably enhanced band and the relative activities of the a orbitals were found to depend on the size of the substituent next to the a-bond. It is confirmed that relative PIES intensities depend on the following two factors: (i) the electronic factor, which is mainly related to electron distributions of individual molecular orbitals, and (ii) the steric factor involving the size of bulky groups which protect some orbitals from the impact of metastable atoms.

The exterior electron model was found to provide a means to test wave function tails of individual molecular orbitals and also to study reactivity of molecules. Demonstrative ab initio calculations were made for NH3, CO, H2O, and H2S at various levels of basis sets.