MASER LINE WIDTHS

The frequency distribution of maser radiation must follow the shape of the thermal line profile when the interaction rate with the maser radiation exceeds all other rates because of rather general considerations. Goldreich and Kwan pointed out that the broadening to thermal shape from the narrow unsaturated profile need not coincide with the onset of saturation when molecules are transferred from line wings to center without affecting the overall population of each maser level. The specific mechanism they proposed involves frequency redistribution when photons are trapped in optically thick transitions between the maser system and other levels. The same effect is also produced by elastic collisions, which are always present. This Letter shows how to include the elastic collisions and derives the relevant frequency-dependent expression for the absorption coefficient. The method developed is then utilized to incorporate the photon scattering mechanism, which inherently involves other levels, into the phenomenological two-level maser model.