Vector correlation studies of HO2 photodissociation at 220 nm

Abstract

The 220 nm photolysis of the hydroperoxyl radical, HO2, is investigated by probing the ejected OH fragments using Doppler and polarization spectroscopy. Analysis of the measured line profiles reveals that the OH fragments are predominately (84%) formed with the partner oxygen atom in its electronically excited 1D state with a smaller component (16%) being associated with oxygen atoms in the 3P ground electronic state. Measurement of OH fragment internal state distribution indicates that the 23 200 cm−1 of available energy is primarily released as electronic excitation of the oxygen atom (fel=0.57) and to a lesser extent as relative translation of the products (ftr=0.41). The internal degrees-of-freedom of the OH fragment receive very little of the available energy and are found to be fairly cold (fvib<0.004 and frot=0.014). For the primary O(1D) dissociation channel the measured 〈μ⋅v〉 correlation is strongly positive (βμv=0.61) indicating a preference for parallel alignment of the electronic transition moment and the recoil velocity vector in HO2, consistent with the excited state being of A″ symmetry. All other bipolar moments are close to zero for this pathway (βμJ=−0.10, βvJ=−0.04, βμvJ=−0.06) independent of the probed rotational quantum state of the OH fragment. For the minor O(3P) pathway a comparable set of bipolar moments is obtained. An investigation into the source of OH fragment rotation reveals that the combined contributions from out-of-plane rotation, generated by initial parent thermal motion about A-inertial axis, and in-plane rotation, generated by the combination of bending mode zero-point energy and final state interaction on the excited potential energy surface, result in negligible 〈v⋅J〉 correlation in the photodissociation of a thermally distributed sample of HO2 at 300 K.