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

Abstract

The unimolecular decomposition of expansion-cooled NO 2 at excess energies 0-3000 cm -1 is described with emphasis on the manifestations of overlapping resonances. When NO 2 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 NO 2 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.

Calculation of bound and resonance states of HO2 for nonzero total angular momentum

“…Although experimental data are still not available for this system, fluctuating resonance rates have been observed for several other dissociation systems including H 2 CO, CH 3 O, and NO 2 . [66][67][68] Finally, in Tables I-VI an unambiguous ⍀ assignment has been given for the low-lying bound states, supposing that the helicity conserving approximation holds ͑because there exist near degeneracies for the same ⍀ components from both symmetries, it is possible to assign them by comparing the calculated energies from even and odd symmetries͒. The purpose of ⍀ assignment is to investigate the importance of the Coriolis coupling for this system.…”

Fano interference and cross-section fluctuations in molecular photodissociation

“…For a classically chaotic system, these statistics can be derived from random matrix theory. In recent years, such a random matrix approach has been successfully used to model the statistics of resonances [4] and cross-section fluctuations in the photodissociation of classically chaotic molecules [8,9,10,11,12,13]. A semiclassical treatment was discussed in Refs.…”

Quantum Mechanical Studies of Photodissociation Dynamics Using Accurate Global Potential Energy Surfaces

“…98. NO 2 has been a benchmark molecule for spectroscopy and unimolecular dissociation for a long time.…”