A vibrationally adiabatic theory of molecular Penning ionization

Abstract

Based on available theoretical and experimental information on the Penning ionization of molecules by metastable helium and neon atoms at thermal energies, an approximate theoretical approach for incorporating the vibrational degrees of freedom is developed. The electronically excited, metastable atoms have diffuse outer orbitals, giving rise to relatively soft intermolecular repulsion in nonbonded excited state potentials. A low-energy ionizing collision is then near-adiabatic in respect to its effect on the vibrations of the molecule under attack. In addition for the great majority of experimentally studied molecules, nearly vertical vibrational populations in the Penning molecular ion are observed in Penning ionization electron spectroscopy (PIES). In the simplest limit of vibrational adiabaticity, the bond oscillator remains unperturbed by the collision, and small deviations from verticality may then be interpreted as a reflection of the bond-length dependence of the discrete-continuum coupling that gives rise to ionization. The theory presented may be cast into an approximate but simple form that allows the ready extraction of such information from a complete set of vibrational populations. Recent experimental studies of the He*+H2 system provide both justification and an example of the application of the theory to reduction of population data.