Unimolecular Reaction of NO2: Overlapping Resonances, Fluctuations, and the Transition State

Abstract

The unimolecular decomposition of expansion-cooled NO2 at excess energies 0−3000 cm-1 is described with emphasis on the manifestations of overlapping resonances. When NO2 is excited to energies above dissociation threshold, overlapping resonances interfere and give rise to final state-selected spectra which depend on the monitored final state of the NO product. The differences among the spectra diminish with the degree of incoherent superposition (e.g., thermal averaging). In this article, we describe the experimental manifestations of overlapping resonances in the case of barrierless unimolecular reactions and how they relate to transition state theories. We treat the unimolecular reaction of NO2 using resonance scattering theory combined with random matrix formalism and distinguish between the near-threshold region where the transition state is loose and dissociation at higher excess energies where the transition state has tightened significantly. The final state-selected spectra and their dependence on the degree of resonance overlap are simulated in a qualitative way. Experimentally, fluctuations are observed in the line shapes, positions, and intensities in the spectra; the rotational and vibrational NO state distributions; the spin−orbit states of the oxygen atom correlated with specific quantum states of NO; and the state-specific rates. We show how the patterns of fluctuations in the rotational distributions allow the distinction between the loose and tight transition state cases and discuss the evolution of the excited complex from transition state to final products. In spite of the fluctuations, the averaged results agree well with the predictions of statistical theories, and implications for the transition state and the adiabatic evolution of the NO degrees of freedom are discussed. Even when the product state distributions agree with the predictions of specific statistical models, caution should be exercised in inferring properties of the transition state, due to the unaccounted influence of final state interactions beyond the transition state.