Vibrational specificity for charge transfer versus deactivation in N 2 + (υ = 0, 1, 2) + Ar and O 2 reactions

Abstract

Rate constants for charge transfer and vibrational deactivation in the N 2 + (X 2 Σ g + , υ = 0, 1,2) + Ar and O 2 reactions are directly measured by a state-resolved optical detection method. The novel, selected-ion flow tube, laser-induced fluorescence (SIFT–LIF) technique is used to study the vibrationally specific reactions at near-thermal collision energy. The total rate constant for N 2 + (υ = 1,2) + Ar increases by more than a factor of 40 relative to N 2 + (υ = 0). This enhancement is due exclusively to an increase in the charge transfer channel. The charge transfer rate constants for the N 2 + (υ) + Ar reaction are found to be almost identical for υ = 1 and υ = 2; this differs slightly from previous results at higher collision energies. The vibrational deactivation rate constant for the N 2 + (υ = 1) + Ar reaction is measured for the first time; the upper limit for the branching fraction is ≈3%, confirming that this reaction is a useful monitor for N 2 + (υ > 0). The total rate constant for N 2 + (υ = 1, 2) + O 2 increases by factors of 2.6 and 3.3, respectively, relative to N 2 + (υ = 0). In contrast to the N 2 + + Ar reaction, this enhancement is largely due to the occurrence of vibrational deactivation, which is found to be slightly faster for υ = 2 than for υ = 1. For N 2 + (υ = 2) + O 2 , the υ = 2 → 1 and υ = 2 → 0 vibrational deactivation channels are found to occur with comparable rates. The lack of substantial enhancement in the charge transfer channel in the N 2 + (υ) + O 2 reaction by vibrational excitation (up to υ = 2) is in contrast to the observed translational enhancement, which opens a higher lying, endothermic O 2 + (a 4 Π u ) product channel. These results are consistent with a short-range, curve-crossing mechanism that efficiently channels energy into the O 2 + (a 4 Π u ) state.