Coherence in inelastic low-energy electron scattering

Abstract

The results of electron-photon coincidence experiments have given a new insight into the excitation of atoms by electron impact. This article reviews the way in which this new experimental technique has provided an exacting and fundamental test for the theory of the excitation process. The basic experimental method is first described, and a comprehensive treatment of excitation of the nlP states of helium is given. The extension of the technique to the study of excitation of the n = 2 states of atomic hydrogen is then presented. Coincidence measurements also provide information on the role of spin in the collision, and this subject is treated in detail for excitation of heavy atoms by both unpolarised and spin-polarised beams of electrons. Finally, the electron-photon coincidence method permits observation of the time evolution of excited atomic states, and this review describes the attempts made to observe quantum beats in the radiative decay.

Differential cross sections for excitation to the 3s, 3p and 3d states of atomic hydrogen by electron impact at energies from 16.5 to 54 eV

“…These developments led to the measurement of the ratios of differential cross sections for excitation to the n = 2, 3 and 4 levels at energies from 14.6 to 40 eV ; their values support earlier n = 2 measurements (Williams and Willis 1975, Williams 1981, Slevin 1984 as well as providing new data for the n = 3 and 4 levels. Their approach and cross section calibrations were established by two supporting measurements of (i) new double angular differential ionization cross sections from 14.6 to 40 eV (Childers et al 2003 and (ii) differential elastic scattering cross section measurements at 20 and 40 eV .…”

Polarisation and Angular Correlations of Excited P States of Helium and Hydrogen

“…A full discussion has been given by Blum and Kleinpoppen (1979) and Slevin (1984). The aim of the present measurements was to completely characterise the excited 2P state, apart from the undetected electron spin.…”

“…A full description of the problem has been given by Blum and Kleinpoppen (1979) and Slevin (1984). Considerable interest exists in the measurement which requires the observation of the initial coherence between states of opposite parity in the presence of an external field to mix the states.…”

“…Subsequently many people, for example, Blum and Kleinpoppen (1979), Herman and Hertel (1982) and Andersen et al (1985), have discussed the coherence of the excited states, the symmetry properties of the scattering process, the angular momentum transfer in the collision and the interpretation of the angular, and polarisation, correlations in terms of the shape and orientation of the electron charge cloud of the excited state. The topic has been reviewed most recently by Hanne (1983) and Slevin (1984). The most fundamental advances in the subject have been the measurement of scattering amplitudes and their relative phases as well as the measurement of Stokes parameters,…”

Measurements of circular polarization correlations following excitation of Ar and Kr by electron impact

“…Angular correlation experiments detect the inelastically scattered electrons which have excited a particular target state in coincidence with the subsequently emitted decay photons without regard for the light polarization, while polarization correlation experiments also require a linear/circular polarization analysis of the emitted photons. Several reviews have summarized the experimental and theoretical progress in this field at regular intervals [1][2][3][4][5]. The review of Andersen et al [3] deserves particular attention, since these authors also introduced a by now widely used set of frame-independent param-L + + eters, the natural parameters (y, _~, P~m, Poo).…”