Penning ionization of (CH3)4C and (CH3)3CCl by collision with He*(2 3S) metastable atoms

Abstract

Penning ionization of (CH3)4C and (CH3)3CCl upon collision with He*(2 3S) metastable atoms were studied by angular and collision-energy resolved Penning ionization electron spectroscopy. The observed Penning ionization electron spectra (PIES) revealed peculiar features for the totally symmetric ionized state of the target molecules which can be ascribed to ionization from a molecular orbital having carbon 2s atomic orbital characters; (1) the relative PIES intensity is extremely enhanced, (2) the intensity distribution of the vibrational fine structure in PIES is clearly different from that observed for ultraviolet photoelectron spectra, (3) the angular distribution of Penning electrons is highly isotropic, and (4) the collision energy dependence of the partial Penning ionization cross section is slightly increasing and rather flattened with respect to the increase of the collision energy. These features were found to be due to the formation of an excimer-like state involving C2s hole characters in the target molecule which induces Auger-like transitions selectively from orbitals having the C2s characters. Effects of attractive interaction potentials were observed for (CH3)3CCl on the ionized state ascribed to the Cl nonbonding orbital.