Astrophysical Masers.IV. Line Widths

The standard theory of maser line widths predicts line narrowing during unsaturated amplification and rebroadening to the full Doppler width during saturated amplification. These predictions conflict with the observations of narrow maser lines from sources which appear to be saturated. in an attempt to resolve this conflict, a new theory of maser line widths is developed which takes into account effects arising from infrared line radiation trapped between the molecular levels. it is shown that if a series of infrared lines connects the upper and lower maser levels, line rebroadening during saturated amplification does not occur provided that each infrared transition satisfies A![exp (hv/kT) - 1] R, where A and v are respectively the spontaneous emission rate and transition frequency of the infrared line, T is the kinetic temperature, and R is the stimulated emission rate across the maser levels. in fact, under these conditions, the maser line continues to narrow during saturated amplification since the gain profile is not affected by saturation. Subject headings: line profiles masers