Rapidly fluctuating anisotropy parameter in the near-threshold photodissociation of NO2

A novel experiment for near-threshold photodissociation studies is presente d. State-selective excitation of the molecular photofragments to high-n Ryd berg states is used in a variation of the ion imaging technique, allowing f or undistorted detection of slow fragments produced close to the channel di ssociation threshold. As a first demonstration of this method, the angular anisotropy parameter beta for production of NO (J=17/2) and O P-3(2) in the photodissociation of NO2 has been obtained as a function of excess energy. A classical model for beta as a function of excess energy is presented, ac counting for the decrease of anisotropy in the angular photofragment distri bution upon approaching the channel threshold. The experimental values of b eta fluctuate substantially around the values predicted by the model, indic ating strong underlying fluctuations in the state-to-state rate constant. T his experiment offers in principle a unique route to measuring state-to-sta te reaction rate constants in situations where existing time- or frequency- resolved methods are inappropriate. (C) 2000 American Institute of Physics. [S0021-9606(00)01908-5].