Temperature dependence of electron attachment to NO2

Thermal electron attachment to NO₂ in He, Ar, and N₂ as buffer gases has been studied at various temperatures between 257 and 354 K with the pressure range of a few to 900 Torr. At buffer-gas pressures from a few to several tens Torr, the effective two-body attachment rate constant (k_eff) initially increases and then shows a saturation at all temperatures for three buffer gases. Further increase of the pressure up to near 1 atm gives a gradual increase of k_eff for Ar and N₂ systems, whereas no such an increase is seen for He at three different temperatures. These behaviors are explained by a mechanism of a combination of the two-step three-body attachment and the attachment to van der Waals molecules containing NO₂. A simulation analysis supports this explanation. A remarkable finding is that in all systems studied k_eff's become larger at lower temperatures. An Arrhenius plot for the initial two-body attachment rate constant gives an activation energy of -0.17 eV. It has been suggested that an excited triplet state of NO^-₂ ion may be the species formed by the initial electron attachment.