Reactions of Ar+ with H2, N2, O2, and CO at 20, 30, and 70 K

Abstract

Reactions of Ar+(2P3/2) ions with H2, N2, O2, and CO have been studied with the CRESU apparatus at 30 K in argon buffer gas and with the newly developed mass-selected ion injector (CRESUS configuration) at 20 and 70 K in helium buffer gas. The atom exchange reaction with H2 is rather fast, with a rate coefficient k showing a small energy dependence well represented by k=1.5×10−9E0.16 cm3 s−1 with the collision energy in the center-of-mass frame, E, in the range 2.5×10−3−0.65 eV. This result is more compatible with a frozen-rotor capture model rather than with a threshold model for endoergic reactions. Concerning the reaction with N2, a competition between the two exit channels involving the two first vibrational levels of the N+2 product ion explains the minimum of the rate constant suggested at about 140 K by previous SIFT experiments [D. Smith and N. G. Adams, Phys. Rev. A 23, 2327 (1981)] and presently confirmed. The rate coefficients for the charge transfer reactions with O2 and CO increase with decreasing energy, i.e., k=9.6×10−12E−0.51 and k=1.1×10−11E−0.47 cm3 s−1, respectively, for E in the range 2.5×10−3−0.3 eV. Both these reactions proceed via an intimate collision mechanism.