THE DENSITY OF REACTIVE LEVELS IN NO2 UNIMOLECULAR DECOMPOSITION

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 D-0. 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 B-2 vibronic species of the mixed (2)A(1)/B-2(2) 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 B-2 vibronic species is a more relevant parameter to describe the nature of unimolecular reactions. Nuances of the optical excitation process are discussed.