The density of reactive levels in NO2 unimolecular decomposition

Abstract

Laser induced fluorescence spectra of expansion-cooled NO2/Ne samples (1 and 2 K) are reported for transitions that originate from the lowest rovibronic levels and terminate on levels near D0. At 1 K, nearly all transitions originate from N″=0. With the present resolution of 0.02 cm−1, the 1 K spectra are resolved rather well. The high density of transitions is due to couplings between rovibronic levels with different N and K quantum numbers and with electronic characters that borrow oscillator strength from bright B2 vibronic species of the mixed 2A1/2B2 electronic system. Just above reaction threshold, such rovibronic species comprise the manifold of levels sampled by optically prepared wave packets. However, at higher energies we argue that the density of B2 vibronic species is a more relevant parameter to describe the nature of unimolecular reactions. Nuances of the optical excitation process are discussed.