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Journal of Physics B Atomic and Molecular Physics

Electron-photon coincidence experiment for measuring the polarisation of the 63P1-61S0resonance line of Hg after electron excitation with ΔML=0

G F Hanne, K Wemhoff, A. Wolcke, Joachim Keßler

Published Aug 14, 1981• 20 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3700/14/15/007

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•Jan 1, 1989•0 citations
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.
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 (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 authors present circular-polarisation measurements of 3 1 P to 2 1 S photons (501.6 nm) detected in delayed coincidence with electrons inelastically scattered upon excitation of the 3 1 P state of helium. They performed these experiments as a function of the electron scattering angle at incident electron energies of 80, 60 and 50 eV in order to study the orbital angular momentum transfer during the collision. The experimental results are compared with previous angular correlation data and with theory. For each incident electron energy they found a sign reversal of the angular momentum transfer as a function of the electron scattering angle.
The excitation by electron impact at 50 eV, of the 104.8 and 106.7 nm lines of Ar, have been studied separately using the electron-polarised-photon coincidence technique. Polarisation correlations have been measured for electron scattering angles of 0 and 5 degrees . From the measurements at 5 degrees the authors derived the parameters lambda (0.72+or-0.06, 0.60+or-0.05) and mod chi mod (0.99 rad+or-0.19, 0.0) for the 104.8 and 106.7 nm lines, respectively. Thus, contrary to the assumptions of Arriola et al. (see ibid., vol.8, p.1275 (1975)) significant differences may occur between the two lines. The value of chi obtained for the 106.7 nm line implies total coherence at the energy and angle studied. The measurements at 0 degrees yielded 'threshold' polarisations of 0.41+or-0.13 (104.8 nm) and 0.94+or-0.14 (106.7 nm).
The coherence analysis technique has been used to determine the state of polarizatitn of 228.8 nm radiation emitted by cadmium atoms excited to 51P1 state by electron impact. Data have been obtained for incident electron energy of 100 eV for scattering angles in the range from 10° to 40°. Reduced Stokes parameters and electron impact coherence parameters (EICP), characterizing the state of the excited atoms immediately after the collision, have been extracted from the measurements and compared with results of relativistic distorted-wave approximation calculations. Theoretical predictions are in good qualitative agreement with experimental values.
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.