Monte Carlo study of population and alignment relaxation by trapped line radiation.

Abstract

Population and alignment relaxation for the Doppler broadening case in the infinite plane parallel geometry has been studied in the range of the optical thickness at the line center between ${10}^{\mathrm{\ensuremath{-}}3}$ and ${10}^{2}$. The obtained population and alignment relaxation rates are compared with analytical estimates. The validity of two approximations used in the estimates, i.e., the complete frequency redistribution of absorbed and successively emitted photons and the pure-parabolic distribution of excited atoms, is discussed. Also studied is the mixing of the population and the alignment, which is due to the spatial anisotropy of the geometry. This mixing leads to the self-alignment effect, which develops at a sufficiently late time after excitation. A separate simulation with the alignment effect omitted has revealed that, at a large optical thickness, the population relaxation rate agrees with the result with the alignment effect included. It is therefore concluded that the previous estimate on the influence of the self-alignment to the population relaxation rate is too large. The velocity distribution of the excited atoms in the direction perpendicular to the planes shows an effectively lower (higher) temperature at the center (boundary) than the base temperature (\ensuremath{\approxeq}5%) under the self-alignment condition.