NEAR-THRESHOLD PHOTODISSOCIATION DYNAMICS OF HOBR - DETERMINATION OF PRODUCT STATE DISTRIBUTION, VECTOR CORRELATION, AND HEAT OF FORMATION

The photodissociation dynamics of HOBr to give OH + Br fragments is in vestigated at 490 and 510 nm, close to the threshold for dissociation. The available energy resulting from dissociation at these wavelengths corresponds respectively to similar to 3500 and similar to 2700 cm(-1 ). The nascent OH photofragments are characterized via polarization an d Doppler spectroscopy using laser-induced fluorescence. At both wavel engths the OH fragments are found to be in their vibrational ground st ate with approximately similar to 150 cm(-1) of rotational excitation. Almost the entire allotment of available energy is deposited into rel ative translation of the products (similar to 95%). An analysis of the OH Doppler line shapes reveals that the [<(mu)over right arrow>.(v) o ver right arrow] correlation parameter is strongly positive (beta(mu v ) = 0.72), indicating a definite preference for parallel alignment of the electronic transition moment and the recoil velocity vector of HOB r. This trend is independent of the photolysis wavelengths examined wi thin the visible absorption band. All other vector correlations ([<(mu )over right arrow>.(J) over right arrow], [(v) over right arrow .(J) o ver right arrow], and [<(mu)over right arrow>.(v) over right arrow .(J ) over right arrow]), although discernible, are not very pronounced. C omparison of the measured [<(mu)over right arrow>.(v) over right arrow ] correlation with predictions of recent ab-initio calculations leads us to propose that the visible absorption band of HOBr arises from exc itation to a low-lying tripler state of A '' symmetry which borrows in tensity from singlet electronic state(s) of A' character. Furthermore, the present measurements allow us to determine the heat of formation of HOBr to be Delta H degrees(f)(0 K) = -49.5 +/- 4 kJ/mol (Delta H de grees(f)(300 K) = -60.0 +/- 4 kJ/mol).