Mechanism of thermal electron attachment to O2: Isotope effect studies with 18O2 in rare gases and some hydrocarbons

Abstract

Thermal electron attachment to 18O2 has been studied at room temperature for mixtures with rare gases (He, Ne, Ar, Kr, and Xe) and hydrocarbons (CH4, C2H6, C3H8, and neo-C5H12) as third bodies (M), and the results are compared with those for the corresponding 16O2 systems. The obtained three-body attachment rate constants (k18M ) for rare gases and CH4 are nearly equal to, or even less than those for the 16O2 systems (k16M ), which strongly suggests that the attachment to van der Waals molecules (or the vdW-M mechanism) predominates in these systems. On the other hand, for C2H6, C3H8, and neo-C5H12 k18M ’s are about twice greater than k16M ’s, thus indicating the importance of the Bloch–Bradbury (or the B–B) mechanism. All the isotope-effect data imply that the well-known feature of the marked dependence of the three-body rate constants upon nature of third bodies originates mainly from the B–B mechanism, and also that the vdW-M mechanism becomes important only when the B–B rate constant is very small, as in the case of rare gases, N2, and CH4.