Laser-induced rate processes in gases: Reduction of generalized to ordinary master equation

The generalized master equation (GME) describing the time evolution of energy level populations for a laser-pumped molecular gas, suffering phase-changing and thermalizing collisions, is analyzed in the weak radiation field limit for a two-level absorber. We show that the GME reduces to an ordinary master equation (OME) when the absorber response is perturbed by collisions, by phase interruptions in the light field, or by a combination of both. The GME to OME reduction is possible only in the low intensity and weak collision limits thereby placing a stringent limitation on the use of ordinary master equations to obtain accurate estimates of the evolution of energy level populations.