Low-energy electron impact spectroscopy

Abstract

An electron scattering spectrometer has been constructed to explore electron impact excitation cross-sections close to threshold. A trochoidal electron monochromator produces a low energy dispersion beam which interacts with target gas in a collision chamber. Inelastically scattered electrons are detected by the trapped electron technique. The design and performance of this instrument are discussed in detail. A technique to measure total (angle integrated) inelastic cross-sections at fixed incident energy is demonstrated. We know of no other experiment in which this is achieved directly. Spectra from this technique yield (1) the behaviour of the total electron impact ionisation cross-section for helium up to about 6 eV above threshold, and (2) the form of PE(ER), the energy distribution function of the ionisation electrons. Our helium excitation functions are placed on an absolute basis by comparison with known ionisation cross-sections. The experimental results obtained for the excitation of He, N2, CO2, COS and CS2 are presented and discussed.