Interaction of O{sup −} and H{sub 2} at low temperatures

Reactive collisions between O{sup −} and H{sub 2} have been studied experimentally at temperatures ranging from 10 K to 300 K using a cryogenic radiofrequency 22-pole ion trap. The rate coefficients for associative detachment, leading to H{sub 2}O + e{sup −}, increase with decreasing temperature and reach a flat maximum of 1.8 × 10{sup −9} cm{sup 3} s{sup −1} at temperatures between 20 K and 80 K. There, the overall reaction probability is in good agreement with a capture model indicating efficient non-adiabatic couplings between the entrance potential energy surfaces. Classical trajectory calculations on newly calculated potential energy surfaces as well as the topology of the conical intersection seam leading to the neutral surface corroborate this. The formation of OH{sup −} + H via hydrogen transfer, although occurring with a probability of a few percent only (about 5 × 10{sup −11} cm{sup 3} s{sup −1} at temperatures 10–300 K), indicates that there are reaction paths, where electron detachment is avoided.