Fully state-selected VMI study of the near-threshold photodissociation of NO₂: variation of the angular anisotropy parameter.

Velocity-map ion imaging (VMI) has been used to study the angular distribution of the NO fragment generated in the photodissociation of NO₂ at a variety of photolysis wavelengths. Images were recorded for the channels NO ²Π_1/2 (v"_NO = 0, J"_NO= 3/2, 11/2 and 21/2) + O (³P_2,1), for excitation energies ranging from the onset (E_avl/hc = 0 cm^-1) to E_avl/hc ≅ 900 cm^-1. The angular anisotropy parameter β was obtained as a function of available energy. Photofragment multiphoton ionization (PHOMPI) spectra were also recorded in the energy range E_avl/hc = 0-300 cm^-1 for each of these channels. Large fluctuations of beta as a function of E_avl were found in all observed dissociation channels. These variations are discussed in terms of the lifetimes of the originally photoexcited overlapping resonances in the Ã ²B₂ state of NO₂, the dynamics of which are strongly influenced by nonadiabatic coupling with the X̃ ²A₁ state. The potential use of this photolysis process for production of cold oxygen atoms is discussed.