Product quantum-state-dependent anisotropies in photoinitiated unimolecular decomposition

Angular distributions of state-selected NO and O products in the photoiniti ated unimolecular decomposition of jet-cooled NO2 have been measured by usi ng both the photofragment ion imaging technique with velocity map imaging a nd ion time-of-flight translational spectroscopy. The recoil anisotropy par ameter of the photofragments, beta, depends strongly on the rotational angu lar momentum of the photoproduct. O(P-3(j = 2,0)) angular distributions are recorded at photolysis wavelengths 371.7, 354.7, and 338.9 nm. At these wa velengths, respectively, vibrational levels upsilon = 0, upsilon = 0,1 and upsilon = 0-2 of NO are generated. In addition, beta values for NO(upsilon = 2) in specific high rotational levels are determined at similar to 338 nm . The experimental observations are rationalized with a classical model tha t takes into account the transverse recoil component mandated by angular mo mentum conservation. The model is general and applicable in cases where fra gment angular momentum is large, i.e., a classical treatment is justified. It is applied here both to the experimental NO2 results, and results of qua ntum calculations of the vibrational predissociation of the Ne-ICl van der Waals complex. It is concluded that deviations from the limiting beta value s should be prominent in fast, barrierless unimolecular decomposition, and in certain dissociation processes where a large fraction of the available e nergy 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 acc ord with recent theoretical calculations. (C) 1999 American Institute of Ph ysics. [S0021-9606(99)00239-1].