State selected removal of vibrationally excited NH2[X 2B1(0,ν2,0)] radicals

Abstract

The influence of vibrational excitation on the decay rate of NH2 radicals in the presence of selected substrates has been studied using the laser-induced fluorescence (LIF) technique. The NH2 radicals were generated by infrared multiple photon dissociation (IRMPD) of selected precursors (N2H4, CH3NH2), and the state selected NH2(v″2 =0,1) decay was observed by means of the sensitive LIF measurement of [NH2]. The reactions studied were of the type: NH2(v]2 =0,1)+R→products, with R=NO, CH3NH2, N2H4. The reaction rates were determined under pseudo-first-order conditions, and were found to be strongly dependent on the vibrational state of the NH2 radical, e.g., kNO(v″2 =0)=1.4×10−11 cm3 molecule−1 s−1 and kNO(v]2 =1)=3.2×10−11 cm3 molecule−1 s−1. This influence of vibrational excitation on the total decay rates is discussed and compared with previously reported data on thermal excitation. In thermal studies of these same chemical reactions, a negative temperature coefficient was found for the reaction rate constants. The contribution of diffusion and of vibrational relaxation processes is also considered, as well as the relative importance of the two-body vs the three-body collision reaction channels: NH2+NH2→NH3+NH and NH2+NH2+M→N2H4+M for M=O2, N2, and Ar.