SPECTROSCOPIC OBSERVATIONS OF IVR THRESHOLDS IN MOLECULES WITH INTERNAL ROTATION

Author Institution: Department of Chemistry, Indiana University; JILA, University of ColoradoTwo approaches are being used to learn about the onset of intermolecular vibrational redistribution (IVR) as one climbs the $S_{1}$ vibrational ladder of aromatic molecules with methyl internal rotation. The three molecules o-, m-and p-fluorotoluene provide a systematic variation in the interaction of the internal rotation with the ring. The molecule, p-difluorobenzene saves as a benchmark system with similar ring vibrational structure but no internal rotation. Cold jet $S_{1}\rightarrow S_{0}$ dispersed fluorescence spectra from a series levels about $400 cm^{-1}$ apart on the $S_{1}$ ladders of EACH molecule show that extensive Level mixing in cold $S_{1}$ p-fluorescence is displayed al a much lower energy ($1200 cm^{-1}$) than in p-difluorobenzene($200-2400 cm^{-1})$. Moving the rotor off-axis to the meta position reduces further the onset. Placing the rotor next to the fluorine atom causes little change from the meta behavior. Room temperature spectra in all fluorotoluenes show abundant state mixing at $400 cm^{-3}$. We have explored the effect of oxygen quenching (chemical timing) to look at the short-time (psec) spectrum, again as the initial excitation climbs the $S_{1}$ ladder. observations of IVR by the lime evolution of structure in the fluorescence spectrum are now in progress so chat muse remain state (of Indiana) secrets until the Conference