Vibrational enhancement of the charge transfer rate constant of N2+(v=0-4) with Kr at thermal energies

Abstract

The charge transfer reaction of N2+(v = 0-4)+Kr→N2+Kr+ is studied at thermal energy as a function of vibrational excitation in the reactant ion. The selected-ion flow tube technique coupled with laser-induced fluorescence detection is used to measure the vibrationally state specific rate constants. A dramatic vibrational enhancement is observed; measured rate constants are 1.0 (±0.6)×10-12, 2.8 (±0.3)×10-12, 2.1 (±0.2)×10-11, 5.1 (±0.2)×10-11, and 8.3 (±0.4)×10-11 cm3 molecule-1 s-1 for v = 0, 1, 2, 3 and 4, respectively. Mass spectrometric kinetics experiments are also performed to confirm that vibrational relaxation, N2+(v)+Kr→N2+(v′υ) + Kr, is a negligible process. The charge transfer for υ=0 is extremely slow in spite of the large exothermicity (e.g., 0.915 eV for the production of N2(υ′=0)+Kr+(2P1/2) states), yet the reaction is enhanced when the apparent energy mismatch is greater for the vibrationally excited reactant. A simple model is proposed to explain the experimental results at thermal energies (≪1 eV). The model assumes that only the most energy-resonant exothermic transitions, N2+(υ)+Kr→N2(υ+3)+Kr +(2P1/2), occur within the duration of the ion-molecule collision complex and that the charge transfer takes place with probabilities governed by the corresponding Franck-Condon factors. However, the Franck-Condon factors are modified by a trial displacement of 0.02 Å to account for the changes in vibrational wave functions of N2+ and N2 during a close approach of the (N2-Kr)+ pair; this method gives an excellent description of the experimental results. © 1996 American Institute of Physics.