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Physical Review A

Sodium excitation by spin-polarized electrons: A reanalysis of existing experiments

N. Andersen, Klaus Bartschat

Published May 1, 1994• 2 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.49.4232

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Australian Journal of Physics•Apr 1, 1996•1 citation
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.
It is shown how the results of a recent experiment by Jiang, Zuo, Vu\ifmmode \check{s}\else \v{s}\fi{}kovi\ifmmode \acute{c}\else \'{c}\fi{}, and Bederson [Phys. Rev. Lett. 68, 915 (1992)], who investigated low-energy electron scattering from laser-excited polarized sodium atoms in the initial (3p) 2^{2}2P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{3/2}$ (F=3, MF{\mathit{M}}_{\mathit{F}}MF​=3) state, can be related to the inelastic 3S\ensuremath{\rightarrow}3P transition involving initially unpolarized electron and atom beams. Hence, this method can provide an independent check of the traditional electron-scattering experiment with unpolarized beams.
An overview is given of the present understanding of excitation in electron–atom collisions, with particular emphasis on the extent to which a "perfect scattering experiment" in the Bederson sense has been achieved. Recent experimental and theoretical results are put into a common framework, generalizing the ideas and systematics, presented in a review paper, of the case of excitation by spin-polarized electron beams. For various levels of complexity, complete sets of coherence parameters are suggested, and their relationships to generalized Stokes parameters and generalized STU parameters are pointed out.
An experimental investigation of electron - impact induced transitions from the laser - excited 3P state of sodium atom to the 4S and 3D excited states is presented, the main objective being measurement of the atomic collision parameters ( ACPs ) that are related to individual scattering amplitudes. These parameters are complementary to the differential excitation cross - sections and provide finer details on the collisional excitation process. The experiment involved the preparation of the 3P - state with coherent laser radiation of a particular polarisation and measuring the rate of electrons scattered from the laser - excited state as function of laser polarisations. The ACPs were then deduced from these measurements and optical pumping parameters, which account for the optical preparation of the 3P - state. To formulate different measurement schemes, a measurement theory based on the general description of transitions between states of different angular momentum using density matrix formalism is derived. The investigation of the 3P - 4S transition is very similar to the studies of the 3S - 3P excitation performed by superelastic method involving the 3P - 3S scattering in the 'time - inverse' geometry. 'Standard' ACPs are used and, similarly to the superelastic method, the measurements for the 3P - 4S excitation performed in the 'time - inverse' geometry also reveal ACPs for the 4S - 3P de -excitation process. For the more complex 3P - 3D transition, however, the 'standard' ACPs may be less informative. It is shown that certain sums of individual magnetic sublevel excitation probabilities can be measured which may be considered as atomic collision parameters for transitions between P and D - states.
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.
A spin-exchange optical pumping experiment to study collisions between sodium atoms and free electrons is reported. In this experiment, electrons in a weak magnetic field were polarized by spin-exchange collisions with optically pumped sodium atoms. The sodium-electron collisions were the principal source of the electron resonance linewidth, and they also produced a shift in the electron resonance frequency. The magnitudes of the linewidth and the frequency shift depend upon the scattering amplitude for sodium-electron collisions, the sodium polarization, and the sodium atom density. The ratio of the frequency shift to the linewidth was 0.03. The sodium polarization was approximately 30%. The electron resonance frequency was lower when the sodium polarization was positive. The measurements of the electron linewidth and the frequency shift agree with theoretical values of the linewidth and frequency shift obtained from the sodium-electron scattering phase shifts which have been recently calculated by Garrett.
The density matrix parametrisation of collisionally excited atomic ensembles is generalised to account for projectile and target spin polarisations. The density matrix elements, containing spin-resolved alignment and orientation parameters, can be determined by measurements of the 'generalised Stokes parameters' introduced by Andersen and Bartschat (1994) to describe scattered-projectile-polarised-photon coincidence experiments. Focusing on electron impact excitation of light alkali-type targets and mercury, the present experimental status of such experiments is reviewed, in particular with regard to the 'perfect scattering experiment' whereby all independent scattering amplitudes are determined.