An Isotope Effect in the Probability of ionizing a Molecule by Energy Transfer from a Metastable Noble-Gas Atom

Abstract

EXCITATION of a molecule to an electronic state lying above the ionization potential need not result in ionization. The excitation energy can instead be degraded in a re-organization of the positions of the constituent atoms—by dissociation, predissoeiation, or internal conversion. This conclusion stems from the fact that the fraction of excitation acts leading to ionization; the ‘efficiency of ionization’, η(E), is almost always substantially smaller than unity, at least when the excitation energy, E, is not more than 15–20 eV greater than the lowest ionization potential. Experimental values of η(E) for many diverse molecules have been obtained, both for excitation by absorption of a photon¹ (η_p) and for excitation by collision of the second kind with an excited atom² (η_C). No information is available for the equally interesting case of excitation by electron impact.