Molecular beams studies of the dissociation of highly excited NO2 induced by molecular colliders

NO2 in high vibrational levels was prepared in a pulsed molecular beam by laser excitation of the mixed 1(2)A(1)/2(2)B(2) state to energies h nu below dissociation threshold D-0, D-0 - h nu = 0-500 cm(-1). The beam of excited molecules was crossed with pulsed, neat molecular beams of HCl, CO2, N2O, and NH3 at relative collision energies of similar to 2000 cm(-1), and the NO produced by collision-induced dissociation (CID) was detected state-selectively. The CID yield spectra obtained by monitoring specific NO rotational levels while scanning the excitation wavelength show spectral features identical with those in the fluorescence excitation spectrum of NO2. The yield of the CID products, however, decreases exponentially (compared with the fluorescence spectrum) with the increase of the amount of energy required to reach the threshold of appearance of the monitored NO state. The average energy transferred per activating collision with polyatomic colliders is in the range 130-200 cm(-1), having values similar to or lower than those for diatomic and atomic colliders. This is in contrast to deactivating collisions, in which polyatomic colliders are in general more effective. The results are discussed in terms of a mechanism in which the NO2 molecules are activated by impulsive collisions creating a distribution of molecules in quantum states above D-0 whose populations diminish exponentially with energy. The collisional activation is followed by unimolecular decomposition. The differences between the activation and deactivation pathways are rationalized in terms of the number of degrees of freedom available for energy transfer in each channel.