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Physical Review Letters

Electron-Photon Coincidence Studies in Collisions of Polarized Electrons with Mercury Atoms

A. Wolcke, Johannes Goeke, G F Hanne, Joachim Keßler, W. Vollmer, Klaus Bartschat, Karl Blum

Published Mar 26, 1984• 29 citations
Publisher

Abstract

Electron-photon coincidences with initially polarized electrons have been measured for the first time. Results for linear and circular polarization components of the 254-nm line (Hg $6^{3}P_{1}\ensuremath{-}6^{1}S_{0}$ transition) are presented as a function of electron energy for electrons which are scattered inelastically ($6^{3}P_{1}$ excitation) in the forward direction. The analysis of these polarization components allows one to determine the role of spin-dependent interactions during collision as well as to determine the relative phase of the two scattering amplitudes involved.

DOI
10.1103/physrevlett.52.1108

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Journal of Physics B Atomic Molecular and Optical Physics•Apr 28, 1992•19 citations
Electron-photon coincidences with initially polarized electrons have been measured for the first time. Results for linear and circular polarization components of the 254-nm line (Hg 63P1\ensuremath−61S06^{3}P_{1}\ensuremath{-}6^{1}S_{0}63P1​\ensuremath−61S0​ transition) are presented as a function of electron energy for electrons which are scattered inelastically (63P16^{3}P_{1}63P1​ excitation) in the forward direction. The analysis of these polarization components allows one to determine the role of spin-dependent interactions during collision as well as to determine the relative phase of the two scattering amplitudes involved.
The polarisation of 2154 nm photons emitted from Hg atoms which are excited by electrons scattered in the forward direction has been measured for collision energies between 5.5 and 15 eV. From the results the relative contribution of exchange excitation could be determined. Exchange is found to be the dominant excitation mechanism from threshold up to 8 eV collision energy. The results are also expressed by the alignment parameter lambda and good agreement is found with a previous measurement using incident polarised electrons. The results are explained in terms of intermediate coupling.
A new type of electron/photon coincidence experiment is presented which utilises a laser -electron stepwise excitation technique to prepare atoms in an excited state. An electron collision initially excites an atom from the ground state to an intermediate state. Following this electron excitation process, coherent single mode laser radiation further excites the atom to an upper state, from which the fluorescence is observed. The nature of the laser excitation transfers both amplitude and phase information about the electron excited state to the upper excited state in a coherent way. This information is extracted from the upper state fluorescence by analysis of the fluorescence polarization in coincidence with the electron scattered from the atom for particular polarizations of the laser radiation. The coincidence technique allows an identically prepared sub -ensemble of excited atoms to be selected for measurement. The electron excited state studied was the 61131 state of mercury, excited by a beam of unpolarized electrons at an impact energy of 16 eV. The mercury atomic beam was isotopically selected following electron excitation using the narrow bandwidth of the laser radiation. Excitation from the electron excited 61P1 state was via 579 nm laser radiation to the 61D2 state and fluorescence was monitored from this state to the 63P1 state via a 313 nm transition. Stokes parameters were measured for two different laser polarization angles, since this allowed a complete set of collision parameters to be deduced for a single experimental geometry. Results for three different scattering angles at this energy are presented. The laser excitation process was modelled in both the weak and the strong laser excitation regimes. A perturbation approach developed to model single photon laser excitation allowed the collision parameters to be extracted from the measured coincidence Stokes parameters analytically, whereas a Quantum Electrodynamic model allowed high laser intensity effects on the coincidence signal to be explored theoretically. The Quantum Electrodynamic model indicated that the weak excitation model could be used to extract the collision parameters from the Stokes parameters at the laser intensities used in the experiment. Effects due to variation of the laser intensity were predicted from the Quantum Electrodynamic model and experiments conducted to test these predictions are presented. Collision parameters deduced from the measured stepwise coincidence Stokes parameters following optimization of the experimental conditions are presented. These parameters are compared qualitatively with existing data from heavy atomic systems and with a theoretical model based upon the Distorted Wave Born Approximation. Similarities between the experimental data are found, however significant discrepancies are noted between the parameters obtained here and with the predictions of the theoretical model.
We present linear and circular polarization measurements of $3^{1}P\ensuremath{-}2^{1}S$ (501.6 nm) photons detected in delayed coincidence with electrons inelastically scattered from the 31P3^{1}P31P level of helium. Vector polarization and coherence parameters of the coincident radiation provide direct experimental evidence of the coherent nature of the excitation process. Atomic orientation is obtained directly from the circular polarization measurements. Analysis of the data also yields $\ensuremath{\lambda}$ and $\ensuremath{\chi}$ parameters for 31P3^{1}P31P excitations, which are compared with previous data and theory.
The cross section for the transition $6^{3}P_{0}\ensuremath{\rightarrow}6^{3}P_{1}$ in mercury, induced in collisions with N2{\mathrm{N}}_{2}N2​ molecules, has been determined by the method of delayed coincidences. The Hg 3P1^{3}P_{1}3P1​ state was excited by pulses of 2537-\AA{} Hg resonance radiation and the decay of the long-lived 2537-\AA{} afterglow was studied in relation to N2{\mathrm{N}}_{2}N2​ pressure. The resulting cross section ${Q}_{12}(^{3}P_{0}\ensuremath{\rightarrow}^{3}P_{1})=4.18\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ A˚2{\mathrm{\AA{}}}^{2}A˚2 was found to be good agreement with the prediction of the principle of detailed balancing when compared with the cross section ${Q}_{21}(^{3}P_{1}\ensuremath{\rightarrow}^{3}P_{0})$. It was also found that, within the sensitivity of the method, collisions with N2{\mathrm{N}}_{2}N2​ molecules do not quench 3P1^{3}P_{1}3P1​ or 3P0^{3}P_{0}3P0​ Hg atoms to the ground state.