a New Technique for Measurement of Electron Attachment to Molecules

The electron attaching properties of molecules in gases have been investigated for many years but there remain many gases and mixtures of gases that have not been thoroughly investigated. The fact that the environment of the attaching molecule affects its attachment properties, greatly increases the range of experimental conditions that have to be considered. One of the goals of this dissertation has been to develop a faster method of measuring the attachment properties of molecules. An apparatus has been successfully developed that employs a pair of coaxial cylindrical electrodes with the inner one serving also as a pulsed photoelectron source. An electron swarm is driven radially outward through a mixture of an attaching gas and a buffer gas. Both the electrons and resulting negative ions are detected as time -resolved currents by a cylindrical detector contained within the outer electrode as a Faraday cage. Data collection and analysis are handled by a minicomputer based data acquisition system with two independent digitizers. Data have been obtained for oxygen in helium or nitrogen as a buffer gas and for sulphur dioxide in helium. Attaching gas percentage were generally below 1%. The electric field to number density ratio has been in the range of 1.7 x 10('-19) V cm('2) to 3.8 x 10('-18) V cm('2). Attachment coefficients are obtained firstly by treating the negative ion currents as a measure of electron attenuation through a gas mixture and secondly by reconstructing the spatial distribution of negative ions at the time of electron passage from the time-resolved currents. Because the ions are formed by collisionally stabilized direct attachment, a three body rate coefficient was caclulated by the first method for He-O(,2), N(,2)-O(,2), and He-SO(,2) mixtures as a function of the electric field to number density ratio. These results are compared with existing data. The second method produced results with certain internal consistencies. Further investigations uncovered an electron detachment mechanism that is most pronounced at low electric fields. The effects of outgassing from vacuum chamber surfaces and loss of attaching gas molecules by adsorption have been studied extensively and the complete procedure for correcting for them is described in detail. Suggestions for further investigations are given, together with new applications.