SATURATION EFFECTS IN GAS-PHASE DEGENERATE 4-WAVE-MIXING SPECTROSCOPY- NONPERTURBATIVE CALCULATIONS

Saturation effects in gas-phase degenerate four-wave mixing (DFWM) are investigated for conditions of interest for diagnostic applications i n flames and plasmas. In particular, the case in which Doppler and col lisional broadening are comparable, as is often the case for flame spe cies such as NO and OH, is investigated. DFWM line shapes and signal i ntensities are calculated nonperturbatively and compared with high-res olution laser measurements. In the nonperturbative calculations the ti me-dependent density-matrix equations for a two-level system interacti ng with three laser fields are integrated directly on a grid of spatia l locations along the phase-matching axis. The electric-field amplitud e for the DFWM signal is determined by multiplying the time-varying, l aser-induced polarization at each grid point by the appropriate phase factor and then by summing over all grid points. The calculations are in excellent agreement with measurements of DFWM line shapes and signa l intensities for NO in a buffer gas of He over a wide range of He pre ssure. Under saturation conditions the pressure dependence of the DFWM signal is reduced greatly compared with the unsaturated case. The sig nal level is still dependent on the ratio of pure dephasing to quenchi ng collisions, even for saturation conditions. From the standpoint of minimal dependence on collisional processes it appears that operation with pump-laser intensities approximately equal to the saturation inte nsity is optimal. The dependence of the DFWM line shape and signal int ensity on the ratio of probe to pump-laser intensity is also investiga ted.