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 lowes t rovibronic levels and terminate on levels near D-0. At 1 K, nearly a ll 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 diff erent N and K quantum numbers and with electronic characters that borr ow oscillator strength from bright B-2 vibronic species of the mixed ( 2)A(1)/B-2(2) electronic system. Just above reaction threshold, such r ovibronic 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 descri be the nature of unimolecular reactions. Nuances of the optical excita tion process are discussed.