Product quantum-state-dependent anisotropies in photoinitiated unimolecular decomposition

Abstract

Angular distributions of state-selected NO and O products in the photoinitiated unimolecular decomposition of jet-cooled NO2 have been measured by using both the photofragment ion imaging technique with velocity map imaging and ion time-of-flight translational spectroscopy. The recoil anisotropy parameter of the photofragments, β, depends strongly on the rotational angular momentum of the photoproduct. O(3Pj=2,0) angular distributions are recorded at photolysis wavelengths 371.7, 354.7, and 338.9 nm. At these wavelengths, respectively, vibrational levels v=0, v=0,1 and v=0–2 of NO are generated. In addition, β values for NO(v=2) in specific high rotational levels are determined at ∼338 nm. The experimental observations are rationalized with a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. The model is general and applicable in cases where fragment angular momentum is large, i.e., a classical treatment is justified. It is applied here both to the experimental NO2 results, and results of quantum calculations of the vibrational predissociation of the Ne–ICl van der Waals complex. It is concluded that deviations from the limiting β values should be prominent in fast, barrierless unimolecular decomposition, and in certain dissociation processes where a large fraction of the available energy is deposited in rotational excitation of the diatom. The application of the model to NO2 dissociation suggests that the nuclear dynamics leading to dissociation involves a decrease in bending angle at short internuclear separations followed by a stretching motion. This interpretation is in accord with recent theoretical calculations.