On electron scatter and the sensitivity of photoelectron spectrometers

Abstract

A quantitative description of the pressure-dependent phenomena associated with the photoelectron spectra of gases is derived from the semiempirical treatment of electron—molecule collisions. This serves to emphasise that spectra of samples ionised by UV radiation are more sensitive to pressure effects than those obtained by X-ray ionisation and leads to the following conclusions: the initial increase of signal intensity with sample pressure is not linear; the highest intensity obtainable is heavily dependent on the geometrical parameters of the instrument; when the signal is maximised, at least 63.2% of the electrons emitted will be scattered regardless of the nature of the sample. High sensitivity is therefore best achieved at lower pressures by having a long interaction volume. In the context of spectrometer design we show that the predictions of electron optical theory may not be fulfilled in practice because of electron scatter. Applied to the capillary jet technqiue for the study of liquids the theory gives an upper limit of 73 mtorr for the vapour pressure of a typical sample ionised by UV radiation.