DEGENERATE 4-WAVE-MIXING OF PYRIDAZINE FROM A SLIT NOZZLE

Using,a pulsed supersonic slit nozzle, the nonfluorescing pi<--n tran sition of pyridazine was investigated. The degenerate four wave mixing (DFWM) spectra showed numerous vibrational bands over a 1200 cm(-1) r egion. Most of these bands were parallel transitions with a strong Q b ranch and weaker but observable P and R branches. Based on our previou s model [H. Li and W. Kong, J. Chem. Phys. 107, 3774 (1997)] these tra nsitions were simulated with success. The polarization dependence of t he rotational branching ratios suggested that primary contributions to the DFWM signal were from large spaced gratings formed by ground stat e molecules. The lack of contributions from excited state gratings and small spaced gratings was attributed to the fast internal conversion process on the S-1 surface of pyridazine (0.3-3 ns), the wash-out time due to movements of the sample in a molecular beam, and the duration time of the excitation laser (7 ns). Two vibrational bands showed unex pected enhancement in the P or R branch, but for each band, one adjust ment factor was sufficient to reproduce the spectra recorded under all different polarization combinations. Perturbations were observable fr om the rotationally resolved spectra, however in most cases, rotationa l progressions did not seem to be affected by the perturbation in term s of both line positions and intensities. A more detailed analysis of the supersonically cooled spectra, together with data from a room temp erature gas cell and ab initio calculations, will be necessary to comp letely interpret the spectroscopy of pyridazine. This paper demonstrat es that with the increased sensitivity achievable through a slit nozzl e, DFWM is an effective technique for detailed spectroscopic studies, particularly for nonfluorescing species. (C) 1998 American Institute o f Physics.