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Journal of Physics B Atomic Molecular and Optical Physics

Application of stepwise electron and laser excitation techniques to electron-photon correlation studies: II. Experiment

Andrew James Murray, R Pascual, W. R. MacGillivray, M. C. Standage

Published Apr 28, 1992• 19 citations
Publisher

Abstract

For pt.I see ibid., vol.23, no.19, p.3373-92 (1990). A stepwise excitation electron-photon correlation experiment is presented in which a combination of electron-photon coincidence and laser excitation techniques is used. Atomic collision parameters were measured for the electron impact excitation of the Hg 6 1 P 0 -6 1 P. (185 nm) transition at 16 eV incident electron energy and for electron scattering angles in the range 5'-30'.

DOI
10.1088/0953-4075/25/8/023

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For pt.I see ibid., vol.23, no.19, p.3373-92 (1990). A stepwise excitation electron-photon correlation experiment is presented in which a combination of electron-photon coincidence and laser excitation techniques is used. Atomic collision parameters were measured for the electron impact excitation of the Hg 6 1 P 0 -6 1 P. (185 nm) transition at 16 eV incident electron energy and for electron scattering angles in the range 5'-30'.
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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.
A theoretical description is presented for the process in which photons emitted from atoms which have been stepwise excited firstly by electron impact and then by resonant interaction with coherent radiation are detected in coincidence with the inelastically scattered electrons. The optical excitation is modelled using a full quantum electrodynamic theory. Calculations of the Stokes parameters are performed for the Hg (I=0) scheme; electron excitation from the 6 1 S 0 ground state to the 6 1 P 1 state followed by resonant laser excitation to the 6 1 D 2 state, with analysis being performed on the fluorescence from the 6 1 D 2 -6 3 P 1 relaxation channel. It is found that for this case, even though manifestations of optical pumping are evident, the Stokes parameters do not vary by more than +or-4% from the weak intensity limit.
The observation of radiation trapping effects in time-resolved, stepwise-excited, electron-photon coincidence signals is reported. Hg atoms were excited by electron impact to the 6 1^{1}1P1{\mathit{P}}_{1}P1​ state and then by laser excitation to the 6 1^{1}1D2{\mathit{D}}_{2}D2​ state. Coincidence signals were observed from the 6 1^{1}1D2{\mathit{D}}_{2}D2​--6 3^{3}3P1{\mathit{P}}_{1}P1​ (313-nm) transition. The decay time of these signals was found to depend upon radiation trapping of the electron-impact-excited 6 1^{1}1P1{\mathit{P}}_{1}P1​--6 1^{1}1S0{\mathit{S}}_{0}S0​ (185-nm) resonance line. An explanation of this effect is given in terms of the laser excitation coupling the 313- and 185-nm transitions.
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.