Electron impact excitation of polarised sodium and potassium

Abstract

A beam of unpolarised electrons is crossed at right angles with a beam of partially polarised sodium or potassium atoms (degree of polarisation PA = 21%). The resonance radiation from excited 2P3/2,1/2 states is observed in a direction perpendicular to both beams, and the Stokes parameters P1, P2 and P3, of the emitted photons are measured,

[equation omitted]

where 0 degrees, 45 degrees, 90 degrees and 135 degrees corresponds to the polariser angle measured from the electron beam direction and RHC, LHC to right and left hand circular polarisation in the spectroscopic definition. Because of reflection symmetries, only P1 is expected to be non-zero for unpolarised atoms. If polarised atoms are used with their spins parallel to the direction of the observed photons, P3 should also become non-zero (in addition to P1) while P2 remains zero. Such an experiment was originally proposed by Kleinpoppen, Phys. Rev. A3, 2015 (1971).

We report here an independent measurement on the neutral sodium NaI (32P3/2,1/2 – 32S1/2) transition (lambda = 589.0, 589.6 nm) and present new results for the neutral potassium KI (52P3/2,1/2 – 42S1/2) transition (lambda = 404.4, 404.7nm). The sodium results show close similarities to the work done by S. Osimitsch, Diplomarbeit Universitat Bielefeld (1983) and Jitschln at al J. Phys. B 17 1899 (1984) especially for P1 and P2. The potassium results for P2 agree well with the theoretical prediction of zero and the results for P1 show close similarities to those of sodium results at high and intermediate energies, while at low energy especially close to threshold they differ. The threshold polarisation (P1) in the case of potassium is about 10%. Our values for sodium are somewhat higher than the results of Osimitsch (1983) and Jitschin et al (1984) and in very close agreement with the theoretical work done by Moores and Norcross, J. Phys. B5, 1482 (1972) and Kennedy et al, J. Phys. B10 3759 (1977) .The trend of P3 for potassium is similar to that of sodium in such a way that it is low at low energy and Increases in value as the energy increases, until it reaches a limit of 17% at high energy. The circular polarisation, P3, depends on the particular interaction processes which took place. At low energy, the three interaction channels, the exchange, the direct and the interference interaction are comparable and this will explain the low value of P3, at low energy due to spin flips of the electron of the atom in the exchange processes. At high energy the circular polarisation P3, is high which indicates that the exchange processes are minimal.