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Journal of Physics B Atomic Molecular and Optical Physics

Spin dependence of singlet and triplet excitation of mercury atoms

H Muller, Joachim Keßler

Published Dec 28, 1994• 9 citations
Publisher

Abstract

The change in the polarization vector perpendicular to the scattering plane is examined for collisional excitation of mercury atoms by electrons at a collision energy of 40 eV. The three measured polarization parameters for excitation of the states 6 1P1 and 6 3P1 give information about the influence of spin-orbit and exchange interaction on the excitation process. The results, which are quite different for the two excited states, are compared with the latest theoretical data.

DOI
10.1088/0953-4075/27/24/016

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Journal of Physics B Atomic Molecular and Optical Physics•Jan 29, 2002•9 citations
The change in the polarization vector perpendicular to the scattering plane is examined for collisional excitation of mercury atoms by electrons at a collision energy of 40 eV. The three measured polarization parameters for excitation of the states 6 1P1 and 6 3P1 give information about the influence of spin-orbit and exchange interaction on the excitation process. The results, which are quite different for the two excited states, are compared with the latest theoretical data.
The 3s-3p excitation of spin-polarized Na atoms by Ne+{\mathrm{Ne}}^{+}Ne+ ions has been studied for impact energies Elab=200{E}_{\mathrm{lab}=200}Elab=200​ eV to 6 keV, i.e., in the adiabatic regime. The total excitation cross section and the three Stokes polarization parameters of the fluorescence light have been measured. The linear polarization of the light shows a preferential excitation of the \ensuremath{\Vert}ml{m}_{l}ml​\ensuremath{\Vert}=1 magnetic substates. The circular polarization probes the spin orientation of the excited 3p state. At the highest impact energies investigated the experimental data are compatible with conservation of spin orientation during the collision. With decreasing impact energy, the spin polarization of the final 3p state becomes smaller than the spin polarization of the initial 3s state. This apparent spin depolarization is attributed to the exchange interaction between the Na valence electron and the unfilled Ne+{\mathrm{Ne}}^{+}Ne+ 2p5{p}^{5}p5 core in the quasimolecule formed during the collision.
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.
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 beam of unpolarized electrons is crossed at right angles with a beam of partially polarized sodium or potassium atoms (degree of polarization P A approximately=21%) and resonance lines from the excited 2 P 3/2, 1/2 states are observed in a direction perpendicular to both the atom and electron beams for electron energies from threshold to 200 eV. The polarization of the emitted photons is measured in the form of the Stokes parameters P 1 , P 2 and P 3 . The present measurements for sodium atoms are in better agreement with the theoretical values than the previously published experimental results. No other experimental or theoretical results are available for potassium atoms.
Abstract In a number of recent papers descriptions have been given of how the light from atoms excited by a directed stream of electrons is polarised, in the absence of any external field of force. In particular we may refer to the paper by one of the present writers (Paper I), where some of the characteristics of the effect in mercury were discussed. It was found that the different lines of the spectrum were differently polarised, both in magnitude and direction. The behaviour of the polarisation when a magnetic field is applied to the mercury atoms was found to be identical with the effect produced by a similar field on the polarisa­tion when the light is excited by the absorption of plane polarised light, the electric vector of which is parallel to the direction of the stream of electrons. In this way, the polarisation of the light excited by electron impact was shown to be closely related to the polarisation of resonance radiation, which had been investigated by Wood and Ellett and others. In the paper referred to, the measurement of the polarisation of the spectral lines was, except in an individual instance, only qualitative. Since its appearance two papers have appeared in which polarisation measurements of the mercury lines excited by electron impact have appeared. Eldridge and Olsen gave qualitative results, and these confirmed those which we had given. Quarder gives quantitative measure­ments which are also for the most part in general accord. The aim of the present paper is to describe quantitative results, and, as will be seen, these are not in all cases in detailed agreement with those given by Quarder. The second aim is to determine the variation of the polarisation with the velocity of the electron stream, a point which has not been previously attempted. In Paper I it was shown that the magnitude of the polarisation is intimately connected with the dynamics of the collision process. By the electron impact, the atom is raised from its normal state into an upper quantum state, and by its return into the normal state, or into another state, light is emitted. The magnitude of the polarisation depends on both of these processes, but the second process, the actual emission of the light, is well understood. Polarisation measurements therefore give a powerful means of investigating the collision of an electron with an atom in the case when excitation of the atom takes place, and it is in this that their main interest lies.
For the mercury 6s6p 1 P 1 and 6s6p 3 P 1 states excited by electron impact, the linear polarization of the light emitted in the transition to the ground state has been measured from threshold to intermediate energy for the decay photons of 254 and 185 nm wavelength, respectively. The results for both lines are in surprisingly good agreement with a calculation based on the Born-Ochkur approximation, neglecting spin-orbit terms for the continuum electron but allowing for electron exchange effects and intermediate coupling.