MEASUREMENT AND THEORETICAL MODELING OF QUANTUM BEATS IN PICOSECOND TIME-RESOLVED DEGENERATE 4-WAVE-MIXING AND POLARIZATION SPECTROSCOPY OFOH IN ATMOSPHERIC-PRESSURE FLAMES

Using tunable ultraviolet picosecond laser pulses pump-probe degenerat e four-wave mixing (DFWM) and polarization spectroscopy experiments we re conducted in atmospheric pressure flames to investigate the tempora l signal behavior in selected rotational transitions of the OH A (2)Si gma-(XII)-I-2 (0,0) electronic band. The relaxation behavior of simult aneously excited main and satellite transitions in the Q and P branche s was studied in premixed stoichiometric methane-air and hydrogen-oxyg en flames. Experimental signal traces are compared with expressions fr om a detailed theoretical treatment of the signal generation process u sing perturbation calculations. The theoretical approach consists in c alculating the energy density in the signal field mode taking into acc ount the frequency spread of the pump and probe beam radiation, collis ional relaxation effects, and the polarization configuration of the in cident beams. Relaxation times for population and orientation deduced from the fitting algorithm are in good agreement with DFWM line-shape studies [S. Williams et al., J. Chem. Phys. 104, 3947 (1996)]. It is s hown that quantitative agreement with experimental data obtained for d ifferent polarization configurations of pump, probe, and signal photon s can be achieved when appropriate time correlated interactions of pum p and probe photons are taken into account. In addition, it is shown t hat due to the frequency spread of the employed laser pulses the diffe rent frequency components in the signal beam contribute with different amplitude to the oscillating and nonoscillating parts in the temporal development of the signal intensity depending on the relative strengt h of the simultaneously excited transitions. [S1050-2947(98)00211-X].