Pressure dependence of degenerate four-wave mixing in NO and NO2: effects of population and thermal gratings

A simple phenomenological modification to the two-level density matrix theory of DFWM is presented, which models the DFWM signal intensity produced by both population and thermal gratings as a function of buffer gas pressure. A comparison is made between this model and data from DFWM for NO and NO2, showing effects due to both population and thermal gratings. The model is able to describe the experimental observations when either population or thermal gratings are the dominant mechanism for signal generation.