Mechanism of thermal electron attachment to O2 as studied by observing isotope effects of attachment rates for 18O2 systems

Thermal electron attachment in pure 18O2 and its mixtures with H2, D2, N2, and CO2 has been studied at room temperature. The three-body attachment rate constants for pure 18O2, and its H2, D2, and CO2 mixtures are twice or more greater than those for the corresponding 16O2 systems, but slightly smaller than those expected from the Bloch–Bradbury mechanism. Analyses of the results suggest that the attachment in those systems mostly follow the Bloch–Bradbury mechanism but involve a partial contribution of the attachment to van der Waals molecules containing oxygen. The result for the 18O2–N2 mixture, in which the three-body rate constant increases only by a factor of 1.3 compared to that for the 16O2–N2 mixture, strongly suggests a significant role of the attachment to van der Waals molecule (O2⋅N2) in this case. The difference in the rate constant between the H2 and D2 mixtures may primarily be interpreted in terms of the difference in the collisional-stabilization probability between O−2*−H2 and O−2*−D2 collisions.