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Physical Review A

‘‘Perfect’’ electron-scattering experiment in sodium

J L Riley, P. J. O. Teubner, M. J. Brunger

Published Mar 1, 1985• 16 citations
Publisher

Abstract

An electron-photon coincidence technique was used to probe the coherence of the excitation of the 3 ${\mathrm{}}^{2}$P state in sodium by 22.1-eV electrons. The observed coherence implied that spin-dependent interactions could be neglected for forward-angle scattering at this energy. The magnitude and phase of the scattering amplitudes are compared with those derived from experiments which are the time and parity inverse of the present experiments.

DOI
10.1103/physreva.31.1959

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Physical Review A•Jan 1, 1982•5 citations
An electron-photon coincidence technique was used to probe the coherence of the excitation of the 3 2{\mathrm{}}^{2}2P state in sodium by 22.1-eV electrons. The observed coherence implied that spin-dependent interactions could be neglected for forward-angle scattering at this energy. The magnitude and phase of the scattering amplitudes are compared with those derived from experiments which are the time and parity inverse of the present experiments.
Components of the Stokes vector have been measured using an electron-photon coincidence technique for radiation emitted normal to the scattering plane in the electron excitation of the 3 2 P state in sodium at several energies from 5.1 to 30 eV. The coherence of the excitation process is probed through the concept of the reduced polarisation. The data are compared with those derived from optical pumping experiments, which are the time-reversed equivalent of the present experiments. In general, very good agreement is found with the data from the two techniques. Scattering amplitudes derived from the components of the Stokes vector are compared with theoretical predictions. It is observed that there is only moderate agreement between theory and experiment.
An electron-photon coincidence technique has been used to study the coherence of the excitation of the 3 2 P state in sodium. The total polarisation of the decay photons has been determined by measuring the linear and circular components of the Stokes's vector. Analysis of the reduced degree of polarisation mod P mod shows that the excitation is coherent for electrons which have been scattered through the angular range from 3 to 10 degrees . The concept of reduced polarisation assumes that the Percival-Seaton hypothesis applies to the collision. Thus the authors deduce that the nuclear spin plays no role in the excitation process at this energy and over these scattering angles. Scattering parameters have been deduced from the individual components of the Stokes's vector. These have been compared with those predicted by calculations based on the first Born and distorted-wave polarised orbital approximations. It is found that neither theory adequately describes the interaction.
A series of superelastic scattering experiments have been carried out on optically pumped sodium atoms in the 3 2 P 3/2 state. The total polarization P + has been observed at electron energies of 12.1 eV and 22.1 eV referred to the ground state, over a wide range of electron scattering angles. The connections between this parameter, the coherence of the excitation process, and exchange are discussed. Significant departures from P + =1 are observed. A qualitative explanation of this phenomenon is described which provides some insight into the collision process.
We generalize the existing description of collision-induced atomic S\ensuremath{\leftrightarrows}P transitions by spin-polarized electrons on spin-polarized target atoms. Analysis of existing experimental and theoretical data on the 3s\ensuremath{\leftrightarrows}3p transitions in Na determines how closely the ``perfect scattering experiment'' has been achieved to date. Using a newly developed inversion technique, we demonstrate to what extent the various scattering amplitudes can be extracted from present experiments, and how state-of-the-art theory can be used to remove the remaining ambiguities.
A superelastic electron-scattering technique is described in which electrons are scattered from optically pumped sodium atoms. The scattering parameters derived from these experiments are shown to provide a sensitive and dynamic description of the excited 32P state of the sodium atom. These parameters are compared with those derived from electron-photon coincidence experiments which are the time reversed equivalent of the present experiments. Although the present results agree with the previous data, the much larger kinematic range of the present work indicates that optical pumping will be the technique of choice for future measurements. The parameters are also compared with those predicted by a coupled-channels theory where fair agreement is found.
Electron-photon angular-correlation parameters are given for the 2P^{2}P2P states of sodium which include both fine and hyperfine effects. These expressions contain interference terms between scattering amplitudes which are only observable in angular-correlation experiments and give rise to zero-field quantum beats in the excitation of the 2P32^{2}P_{\frac{3}{2}}2P23​​ state. By neglecting fine structure, partial cross sections for the $^{2}P_{J}(F,{M}_{F})\ensuremath{\leftarrow}^{2}S_{\frac{1}{2}}(F=2,{M}_{F}=2)$ excitations are given in terms of direct and exchange-scattering amplitudes.