Emission and absorption profiles in a scattering atmosphere

Abstract

The equation of radiative transfer for a gas containing two-level atoms is formulated in terms of atomic parameters and velocity distribution functions. In this way one obtains explicit expressions for the normalized profiles of absorption, spontaneous emission, and induced emission occurring in the transfer equation. The respective profiles of spontaneous and induced emission are proved to be identical. On the other hand, the equality of the absorption profile and the emission profile is shown to depend essentially on the equality of the velocity distributions of the unexcited and the excited atoms, respectively. The steady-state balances for the occupation numbers and the velocity distribution functions corresponding to the two atomic levels are derived, and some special cases are considered in which the absorption profile is exactly equal to the emission profile.