The monoenergetic unimolecular reaction of expansion-cooled NO2: NO product state distributions at excess energies 0–3000 cm−1

Abstract

We report detailed vibrational, rotational, and electronic (V,R,E) distributions of nascent NO(X 2Π1/2,3/2) deriving from monoenergetic unimolecular reactions of expansion-cooled NO2. Near UV excitation above dissociation threshold (25 130.6 cm−1) prepares molecular eigenstates which are admixtures of the optically active 1 2B2 state and the ground X̃ 2A1 electronic state. The strong mixings among the vibronic states result in vibrational predissociation from states of predominantly ground state character, and the NO product state distributions (PSDs) are compared with the predictions of several statistical theories. The PSDs are combined with previously measured O(3PJ) distributions and unimolecular reaction rates, thereby providing a complete description of the decomposition of NO2 at these excess energies. All the rotational distributions show prominent fluctuations and structures, but tend on average to follow the statistical distributions predicted by phase space theory (PST). This behavior is observed in both NO(v=0) and NO(v=1) channels, although the relative population in NO(v=1) was always greater than expected by PST. The NO(v=1) fractional population is bounded by the predictions of the separate statistical ensembes (SSE) method, and recent variational Rice–Ramsperger–Kassel–Marcus (RRKM) calculations are in agreement with the experimental results. Prior distributions underestimate the degree of vibrational excitation even more than PST does, and also the relative populations of the lower NO rotational levels. The observed NO spin–orbit states are always colder than statistical. We conclude that a significant interplay between dynamical biases and statistical expectations is manifest from the onset of dissociation, and is particularly evident when the initial parent rotational state is well defined.