Near-Threshold Photodissociation Dynamics of HOBr:  Determination of Product State Distribution, Vector Correlation, and Heat of Formation

Abstract

The photodissociation dynamics of HOBr to give OH + Br fragments is investigated at 490 and 510 nm, close to the threshold for dissociation. The available energy resulting from dissociation at these wavelengths corresponds respectively to ∼3500 and ∼2700 cm -1 . The nascent OH photofragments are characterized via polarization and Doppler spectroscopy using laser-induced fluorescence. At both wavelengths the OH fragments are found to be in their vibrational ground state with approximately ∼150 cm -1 of rotational excitation. Almost the entire allotment of available energy is deposited into relative translation of the products (∼95%). An analysis of the OH Doppler line shapes reveals that the 〈μ·v⃗〉 correlation parameter is strongly positive (β μ v = 0.72), indicating a definite preference for parallel alignment of the electronic transition moment and the recoil velocity vector of HOBr. This trend is independent of the photolysis wavelengths examined within the visible absorption band. All other vector correlations (〈μ·J⃗〉, 〈v⃗·J⃗〉, and 〈μ·v⃗·J⃗〉), although discernible, are not very pronounced. Comparison of the measured 〈μ·v⃗〉 correlation with predictions of recent ab-initio calculations leads us to propose that the visible absorption band of HOBr arises from excitation to a low-lying triplet state of A‘ ‘ symmetry which borrows intensity from singlet electronic state(s) of A‘ character. Furthermore, the present measurements allow us to determine the heat of formation of HOBr to be Δ H ° f (0 K) = −49.5 ± 4 kJ/mol (Δ H ° f (300 K) = −60.0 ± 4 kJ/mol).