The degenerate four wave mixing (DFWM) spectrum of the A(1A2)← X(1A1) and
B(1B1)← X(1A1) transitions of SO2 in the 299.5–305 nm region is presented. It has been
found that the DFWM signal intensities are proportional to the cube of laser intensity
and the square of SO2 pressure. The DFWM signal increases dramatically with the
pressure of N2 as a buffer gas. The enhancement of the DFWM signal can be mainly
attributed to the thermal grating contribution.