Measurement of excited state lifetimes of gases

Abstract

An apparatus is described for the direct and general measurement of excited state lifetimes in atomic or molecular gases down to 3 ns. States with energies up to 50 eV are excited by electron impact using a nearly monoenergetic pulsed electron beam with a 2 ns pulse time cut-off and energy close to the excitation threshold. Weak light signals are recorded using single photon counting, time-to-amplitude conversion, and integration by a multichannel analyser (method of delayed coincidence). A description of the technique with a discussion of single photon counting statistics applicable to the experiment, assessment of apparatus performance and treatment of data are given with sample results for some levels of HeI, under gas density variation.