Effect of Resonance on Vibrational Distributions in Charge Transfer Reactions

The emission from the ${\mathrm{A\; \hspace{0.17em}}}^2{\Sigma }^{+}{\mathrm{\to \; X\; \hspace{0.17em}}}^2{\Pi }_i$ transition in ${\mathrm{HBr}}^{+}$ and HCl⁺ has been used to monitor the vibrational distribution produced in these ions by charge transfer between the neutral molecules and Ar⁺ and Kr⁺ ions over the energy range from 5000 to 100 eV. In all cases the vibrational distribution at high ion velocities approached that for a Franck‐Condon excitation. The vibrational distribution from the (Ar⁺, HCl) and (Kr⁺, HBr) reaction showed appreciable change with ion velocity. The vibrational distributions were compared with distributions that were calculated using the Rapp and Francis theory of near‐resonant charge transfer, modified to take vibrational overlap into account. The (Kr⁺, HBr) reaction showed quantitative agreement with calculations for the variation of vibrational distribution with ion velocity but the predictions for the (Ar⁺, HCl) reaction showed only ``semiquantitative'' agreement. A large change was predicted for the (Ar⁺, HBr) reaction but no change was observed. A summary is made of available data on vibrational distributions resulting from near‐resonant and nonresonant charge exchange and some mechanisms are discussed.