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

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 photon 1 (η p ) and for excitation by collision of the second kind with an excited atom 2 (η C ). No information is available for the equally interesting case of excitation by electron impact.