Isotope Effect in the Ionization of Polyatomic Gases by Metastable Atoms

Abstract

An isotope effect in the probability of ionizing a polyatomic molecule by energy transfer from a metastable noble-gas atom has been investigated for a large number of gases, especially in an attempt to relate the magnitude of the observed effect to the quantity of energy transferred from the metastable atom. In the experiments the current in an ionization chamber was measured for an initially highly purified noble gas under bombardment by beta particles from a 63Ni or 3H source, as successive measured concentrations (up to 1% by volume) of the gas investigated were introduced. The increase in ionization, relative to that found initially in the pure noble gas, gives a measure of the number of ion pairs resulting from collisions of the second kind between metastable atoms and molecules of the contaminant gas. This relative increase varies through wide limits with the nature of the polyatomic contaminant gas introduced. Moreover, the increase is found consistently larger for a fully deuterated hydrocarbon sample than for an undeuterated one. The isotope ratio, i.e., the ratio of the current increase for a deuterated to that for an undeuterated sample, has been found in ethylene, n-butane, benzene, and toluene to decrease as the metastable state energy increases (in benzene from a ratio 1.16 for an energy of 9.4 eV to 1.04 at 20.2 eV). Over a smaller range, ethane and methane show increases in the ratio with increasing energy. In general, the present values for this isotope ratio are in good accord with the theory of Platzman, which predicted such an isotope effect.