BREAKING SYMMETRY WITH HYDROGEN-BONDS - VIBRATIONAL PREDISSOCIATION AND ISOMERIZATION DYNAMICS IN HF-DF AND DF-HF ISOTOPOMERS

High-resolution near-IR spectra of jet-cooled HF-DF and DF-HF isotopom ers are presented and analyzed for fundamental excitation in the HF-st retching region (3870 cm(-1)-3960 cm(-1)) and DF-stretching region (28 40 cm(-1)-2880 cm(-1)), based on direct absorption of tunable IR diffe rence frequency radiation in a slit-jet supersonic expansion (10 K). S pectra are obtained for excitation of all four stretching modes, HF-D F, HF-DF, *DF-HF, and DF-*HF (* denotes the vibrationally excited sub unit), which probe both the hydrogen/deuterium bond donor and acceptor moieties in the complex. Vibrational redshifts and predissociation br oadening measurements are compared with full 6D quantum calculations o n theoretical HF dimer potential surfaces, which exhibit trends in qua litatively good agreement with experiment. Each of the three DF-stretc h-excited bands are fit to spectroscopic precision (Delta v less than or similar to 0.0001 cm(-1)) by conventional high-resolution rotationa l analyses, cm whereas each of the four corresponding HF-stretch-excit ed bands appear extensively perturbed (Delta v approximate to 0.01 cm( -1)). This H/D isotope effect is interpreted as vibrational state mixi ng of the HF-stretch-excited species with a dense manifold of combinat ion band states built on DF-stretch excitation, and therefore reflects intermolecular energy flow in the complex. Such vibrational stale mix ing is further corroborated by observation of ''dark'' state transitio ns that can be tentatively assigned to HF-DF isomer interacting with the nearly isoenergetic DF-HF isomer. This state mixing would corresp ond to a vibrationally promoted ''isomerization'' over the tunneling b arrier, and yield a spectroscopic measure of the difference in hydroge n bond dissociation energies [Delta D-0 = 74.7(5) cm(-1)] for the HF-D F and DF-HF isotopomers. (C) 1996 American Institute of Physics.