Pulse Technique for Measurement of the Probability of Formation and Mobility of Negative Ions

Abstract

A new technique for measurement of the attachment coefficient of electrons to molecules has been devised. The technique involves production of a beam of electrons in an attaching gas. The transient current flowing after the initiation and termination of the electron beam is interpreted as a flow of negative ions produced by electron‐molecule collisions. Analysis of the transient current which may last several milliseconds yields a value for the mobility of the negative ions thus formed. The ratio of the magnitude of the electron beam current to the negative ion current yields a direct measure of the attachment coefficient. Preliminary results obtained with SF₆ show a very large attachment coefficient. Values of α/p (the ratio of attachment coefficient to pressure) are considerably larger in SF₆ than those obtained for O₂ in studies elsewhere. As might be expected the mobility of negative ions formed in SF₆ is much lower than that observed for other gases. This is attributed to the very large collision cross section resulting from (1) the interaction between the negative ion and neutral gas molecules comprising polarization attraction and hard sphere repulsion and (2) resonance or exchange interaction.