Disalignment of excited atoms by radiation re-absorption: neon 2p2atoms in a discharge plasma

Abstract

In a positive column of a glow discharge, a linearly polarizedlaser pulse produces aligned neon atoms in the 2p2(Paschen notation) level, and the subsequent fluorescence is observed with its polarizedcomponents resolved. The relaxation rate of the alignment, or the disalignmentrate, is thus determined. The populations of the 1s2 – 1s5levels are found to be strongly dependent on the discharge current, and so are thedegrees of radiation re-absorption of the transition lines from 2p2to these levels. A Monte Carlo simulation is performed by which the disalignmentrate by radiation re-absorption for these transitions is determined, wherethe isotope splitting of the transition lines is taken into account. Thisrate is subtracted from the observed disalignment rate, resulting in therate being independent of the degree of radiation re-absorption. Thus,disalignment due to atom collisions and that due to radiation re-absorption areseparated. For the latter disalignment the disalignment rate deduced bythe simulation, treating the four transitions simultaneously, is found tobe virtually equal to the sum of the disalignment rates for each of thetransitions. The approximate expression for the disalignment rate given byD’Yakonov and Perel’ (1965 Sov. Phys.–JETP 20 997) is quantified, sothat it can be applied to estimating the disalignment rate of an upperlevel having several transition lines with finite optical thickness.