Electron thermalisation in a pulsed e-beam high pressure mass spectrometer

Abstract

A pulsed e-beam high pressure mass spectrometer (HPMS) is shown to be well-suited to the measurement of electron thermalisation and thermal electron capture (EC) processes. By measurement of the intensity of the negative ions observed as a function of time after the e-beam pulse, it is possible to verify that EC reactions, alone, are responsible for the formation of the negative ions. It is also demonstrated that most of the electrons within the ion source can be brought into thermal equilibrium with the buffer gas prior to their attachment to an EC-active compound. Of five different buffer gases studied, methane and hydrogen are shown to be particularly effective in thermalising electrons. Additional experiments indicate that less control of electron energy and/or EC processes is achieved with use of the continuous e-beam mode of the HPMS. However, good agreement was obtained between the continuous and pulsed modes of the HPMS when hydrogen was used as the buffer gas.