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Journal of Physics B Atomic and Molecular Physics

Hyperfine effect on the polarisation of the 63P1-61S0intercombination line of mercury excited by electron impact

Ali Zaidi, Ian A. McGregor, Hans Kleinpoppen

Published Mar 14, 1978• 9 citations
Publisher

Abstract

An isotope cell was used to eliminate the effect of hyperfine structure on the polarisation of radiation, near threshold, for the 6 3 P 1 -6 1 S 0 line of mercury (253.7 nm) excited by electron impact. These results are compared with the polarisation of the radiation when hyperfine structure is present.
DOI
10.1088/0022-3700/11/5/008

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Journal of Physics B Atomic and Molecular Physics•Aug 14, 1981•20 citations
The authors report measurements of the polarization of fluorescent radiation at 253.7 nm (6 1 S 0 -6 3 P 1 ) from even isotopes of mercury which arises from the alignment caused by electron beam excitation. The salient feature of the experimental technique adopted is that the use of an isotope cell allowed the performance of the electron impact excitation on an atomic beam of natural isotopic composition instead of using separated isotopes. The cell containing enriched samples of the odd isotopes of mercury filtered the fluorescent radiation and so reduced the contribution to the signal arising from these isotopes in the natural mixture atomic beam. In this way the hyperfine effect on the measurement of the polarization of this atomic line radiation has been eliminated. The experimental results are in very good agreement with theoretical values.
The theory of the polarisation of the 6 3 P 1 from 6 1 S 0 line of Hg after excitation by forward scattered electrons is reviewed. Certain problems encountered in previous studies are pointed out and it is shown that the Ochkur approximation fails to give adequate agreement with the experiment. It is also pointed out that in the case where hyperfine splitting is not resolved experimentally that a different means of averaging over the nuclear hyperfine contribution must be employed. A discussion is presented on why an Ochkur-type approximation might improve on the Born approximation near an excitation threshold.
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.
The incomplete polarization of $\ensuremath{\lambda}2537\mathrm{A}$ of mercury in resonance radiation is due to the peculiar behavior of the outer two hyperfine structure lines. The three remaining hyperfine structure lines behave in accordance with the predictions of the theory of the anomalous Zeeman effect for a 11{1}^{1}11S0{\mathrm{S}}_{0}S0​-23{2}^{3}23P1{\mathrm{P}}_{1}P1​ line. Von Keussler's observation of complete polarization in 7900 gausses is explained, in part at least, by the observed shift of the -25.6 hyperfine structure line, and complete polarization is obtained with a field only 2000 gausses using a resonance radiation source.
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.