Knowledge Bank

The usual theory of non-linear four wave mixing is generalized to account for the homogeneous broadening of the initial (= final) state. Conditions for the coherent interaction and growth of the scattered radiation (with a spectral width given by the homogeneous width of the initial state) with incident monochromatic lasers are discussed for coherent anti-Stokes Raman scattering. These conditions are easily satisfied in most Raman active gases. The excellent signal level and stability observed in recent experiments favor strongly the use of such non-linear optical techniques for studying molecular homogeneous (i.e., Doppler-free) widths. By mixing lasers at frequencies $\omega 1$ and $\omega 2$ using an excited rotational level of $H2$ as an initial state, the homogeneous width of that particular state can be obtained by spectrally analyzing the generated radiation.