The behaviour of electron swarms in polyatomic gases.

Abstract

This work is concerned with collision processes occurring between simple polyatomic gas molecules and electrons of low incident kinetic energy (0-5 eV). The principal methods of experimental investigation and previous work in the field are reviewed. An outline of fundamental concepts of wave-mechanical scattering theory is presented and applications of theory to low energy electron-molecule collisions are reviewed and discussed. After consideration of the shortcomings of existing theories in relation to triatomic and larger molecules it is concluded that appreciable direct excitation of in frared-active vibrational modes is to be expected. A description is given of the design and construction of a Townsend-Huxley type diffusion apparatus to measure the ratio of diffusion coefficient to mobility {D/µ) for electrons in gases. The principal feature of this apparatus is its suitability for accurate measurement in low-energy swarms achieved by choice of geometry, mechanical accuracy, uniformity of electric field and use of ultra-high vacuum techniques. The results obtained using this apparatus are presented as D/µ values in methane, ethylene, acetylene, cyclopropane and hydrogen sulphide. In each case the measurements extend to considerably lower values of field strength/pressure ratio than hitherto published results. An account is given of the method of swarm transport coefficient analysis by solution of the Boltzmann equation for trial cross-section values. A computer program is described which automatically adjusts the cross-sections until they are consistent with experimental data. For each gas studied, the results of the analysis are given. For methane and ethylene, the momentum-transfer cross-section is derived along with two inelastic cross-sections corresponding to excitation of infrared-active vibrational modes. For acetylene and cyclopropane only one vibrational cross-section is used. The likely contributions from other inelastic processes are discussed. The results suggest that the large inelastic energy losses in these molecules can be explained by vibrational excitation cross-sections peaking just above threshold energy with magnitudes of the order of 10-16 cm2. No evidence is found to support the idea that this excitation may occur via an intermediate negative ion "resonance”.