Mechanism of thermal electron attachment to NO2

Abstract

The mechanism of thermal electron attachment to NO2 has been reexamined by observing the dependence of the attachment rates on the nature and the pressure of the environmental gases. Measurements for mixtures of NO2 with rare gases, H2, D2, N2, CO2, and n-C4H10 all showed two-body pressure dependence of the attachment rates at buffer-gas pressures of about 10 to 100 Torr. They gave the same two-body rate constant of (1.13±0.07)×10−10 cm3 molecule−1 s−1. The latter result disagrees with the data reported by Mahan and Walker in 1967. The present results indicate that the collisional electron detachment process introduced previously to interpret the effect of the nature of environmental gases should be negligible. We have also observed the decrease of the two-body rate constants at pressures below about 10 Torr for all the mixtures studied. This strongly suggests that the attachment mechanism is an ordinary two-step three-body process. The three-body rate constants then obtained are mostly of the orders of 10−27 cm6 molecule−2 s−1 and do not differ much with nature of the third bodies. An autoionization lifetime of 1×10−8 s has been estimated for the transient-negative ion of NO2. It has been found that even room light could cause appreciable decrease of the rate constants, probably through decomposition of NO2 molecules. The discrepancy between the present results and the previous ones may be due to such an effect.