Enhancement of charge-transfer reaction rate constants by vibrational excitation at kinetic energies below 1 eV

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Abstract
A number of molecular ion charge-transfer reactions with neutral molecules have been studied in a flow-drift tube system as a function of average kinetic energy from thermal to ∼1 eV. Varying the buffer gas gives an independent control of the vibrational state distribution of the ions. The reactions include the charge transfer of N2O+, NO2+, SO2+, and H2O+ ions with NO and CO2+ ions with NO, O2, CH4, and Xe. For CO2+ reaction with CH4 and Xe the charge transfer occurs at near the collision rate and has little energy dependence and no measureable vibrational dependence. In every other case, where the rate constants are less than the collision rate constants, large enhancements result from vibrational excitation of the ions. The presence of ion vibrational excitation is demonstrated by the addition of a quenching gas to the flow-drift tube. In the low energy (near-thermal) regime where reaction is presumed to occur via long-lived intermediate complex formation, the reaction rate constants are increased by vibrational energy and decreased by kinetic energy. In the higher energy range where the rate constants increase with increasing kinetic energy, vibrational energy also increases the rate constants and to a comparable extent.