Comparison of degenerate four-wave mixing line shapes from population- and thermal-grating scattering

We report detailed measurements of transition line shapes of degenerate four-wave mixing in the A2Σ+(υ′ = 0) ← X2Π(υ″ = 0) band of nitric oxide. We compared spectra obtained under conditions in which either population or thermal gratings were significant. Under conditions in which population gratings were significant, spectral lines were narrower and had intensities that varied more rapidly with the absorption B coefficient than in spectra dominated by thermal-grating scattering. In both cases we found excellent agreement with calculations that used perturbation theory to describe the population-grating contributions and the squared absorption spectra to describe the thermal-grating contributions. Our results show that quantitative prediction of degenerate four-wave mixing spectra requires relatively detailed knowledge of the ratio of contributions from the two mechanisms.