On the excitation of polarised light by electron impact. II.—Mercury

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.