High transmission and dual electron beam ion sources for mass spectrometry

Abstract

Two high transmission electron-bombardment ion sources, based on a principle of electrical rather than mechanical ion collimation, have been developed and tested with a 6 in. radius-sector type of mass spectrometer. The new ion sources had a measured transmission efficiency of 88 and 85% as measured by two different techniques. One ion source was designed for general use for experiments in which high sensitivity is required, while the second was designed for the study of unstable species such as excited neutral molecules, products of ion-molecule reaction and free radicals. These sources are capable of measuring partial pressures of both unstable and stable species as low as 10 ftorr. Collimating slits of 0.250 in. × 0.500 in. are used throughout the general purpose ion source to give electrical rather than mechanical collimation of the ion beam. Electrical collimation increased the transmission by over two orders of magnitude with respect to mechanical collimation, whereas the resolution was reduced only by about a factor of six. The `unstable species' ion source utilizes a principle of two parallel electron beams. One electron beam is used to produce a desired species and the second electron beam ionizes the species for mass identification. An experiment in which helium is excited to the 1s2s metastable level by 23 ev electrons in the first electron beam and subsequently ionized in the second electron beam by 6 ev electrons was performed to demonstrate the operation of the source.