Crossed-field (trochoidal) electron monochromators and their optimization

Abstract

In electron spectroscopy a wide variety of methods using the dispersive properties of both magnetic and electric fields are applied to analyse the energies of charged particles moving in a longitudinal magnetic field. Retarding potential analysers where a uniform magnetic field is used, however, possess a disadvantage: an integral characteristic (retarding curve) must be differentiated in order to determine the energy spectrum. In analysers with a non-uniform magnetic field (which is increased or decreased in the retarding region), electrons are analysed by both the longitudinal and total components of electron velocity. The method of using a Wien filter with non-uniform transverse fields, immersed in a weak longitudinal magnetic field, has also been reported. A complicated system of magnetic field formation of these analysers is coupled with restrictions on the magnetic field alteration rate. Trochoidal electron monochromators (TEMs) have a favourable advantage-principle simplicity-enabling low-energy electron beams to be obtained. Since being created, the TEM has been used in a series of studies of electron scattering by atoms and molecules and solid surfaces. The monochromator theory has been considered in detail in the literature where the principal parameters of the TEM were calculated. Here we consider the influence of transverse potential drop at the entrance slit and suggest the minimization of beam distortion at the exit by means of the proper choice of non-uniform transverse electric field parameters.