Electrostatic optical device for high-resolution differential cross section measurements in scattering experiments

Abstract

A new electrostatic optical system designed for high angular and energy resolution in ion–molecule-scattering experiments is described. This device is based upon a theoretical lens model (’’zero gap electrostatic double-aperture lens’’) proposed by Read. A first pair of these lenses accelerates a low-energy ion beam, with a relatively wide angular spread, up to a chosen collision energy and makes the beam parallel in the scattering region, without any system of slits. A second pair of lenses, symmetrical to the first, decelerates the ions scattered at a given angle and focuses them on the entrance slit of a 127° electrostatic analyzer. This analyzer and the final lens are scanned in order to obtain energy loss spectra. A simple theoretical study of the system which allows the angular spread of the ion beam in the collision region to be predicted as a function of the initial energy and the chosen scattering energy (between about 100 and 3000 eV) is given, the respective positions of the lenses being adjustable parameters. These positions are chosen according to the theoretical results in order to achieve the best angular resolution. The first experimental data obtained are in very good agreement with the theoretical predictions. An example of application to elastic He+–He collisions is presented.