Polarization and probe delay effect on degenerate four wave mixing of pyrazine

Abstract

Degenerate four wave mixing (DFWM) in supersonically cooled pyrazine was investigated using different polarization combinations of the three input beams. A common feature of these spectra was the strong Q branch, stronger than the corresponding spectra for a single photon process. The ratios for all the rotational branches demonstrated strong dependence on the polarization combination. The YYXX combination showed similar intensities for the P and R branches, while the YXYX combination had a much stronger R branch. The Q branch, relative to the P branch, was the strongest for the YYYY combination. All experiments probed for the same electronic transition with the same Hönl–London factor; therefore the variation in the rotational branching ratios was solely an effect of the polarization combinations. This polarization dependence of DFWM can be traced back to the selectivity in the magnetic quantum number for this multiphoton process. Based on the theoretical framework by Williams et al. [J. Chem. Phys. 101, 1072 (1994)], these spectra were simulated successfully. The calculations and analysis of the experimental conditions revealed that among the twelve gratings included in the original theory, only three of them had observable contributions to the experimental spectra, i.e., the gratings formed through the ground-state molecules via the two forward beams. The other nine gratings either were washed out or decayed in the collision free environment (supersonic expansion) under an excitation pulse of 7 ns. This argument was further confirmed by another experiment using a time delay between the backward probe beam and the two forward beams. The spectra recorded with a 15 ns delay in the probe beam (longer than the pulse duration of each input beam) were similar to those without delays. These results implied that (1) the backward beam did not participate in the grating formation process even when it arrived at the interaction region simultaneously with the two forward beams. (2) Relaxation from any of the three contributing gratings was not observable. (3) The fast intersystem crossing, known to exist in pyrazine, had no effect on the grating formation and relaxation processes. Although complex in nature, degenerate four wave mixing offers versatility in experimental arrangements, providing both dynamics and spectroscopy information.