Single collision ion–molecule reactions at thermal energy: Rotational and vibrational distributions from N++CO→N+CO+

Abstract

A new apparatus is developed and used to obtain nascent vibrational and rotational distributions in the ground electronic state of CO+ formed from the charge transfer reaction N+(3P)+CO (X 1Σ+)→N(4S)+CO+ (X 2Σ+,v,J)+0.52 eV, at approximately thermal energies. The device utilizes a flow tube for the production of thermal N+ ions in a helium buffer and a large diameter sampling orifice which delivers the ions via a mild free jet expansion into a low pressure chamber. The expansion is crossed by a stream of reactant CO molecules and the CO+ product states are probed by laser-induced fluorescence. Although the energy available is sufficient to populate CO+ vibrational states up to v″=2, the major vibrational channel in the CO+ product is v″=0. The relative vibrational distribution is found to be: Nv=0≳0.81 (observed under single collision conditions), Nv=1<0.15 (not observed), and Nv=2≊0.04 (observed only under nonsingle collision conditions). The rotational distribution in the v″=0 state is characterized closely by a Boltzmann distribution with a temperature of 410±50 K. This represents a fractional energy disposal into rotation of only 2%. Nearly all of the reaction exothermicity is therefore released into translational recoil. These results are considered in terms of simple dynamical models of the charge transfer process.