OH A 2-SIGMA+-]X2-PI CHEMILUMINESCENCE MEASUREMENTS OF N2+, AR+ +H2O HYDROGEN-ATOM-TRANSFER REACTIONS AT SUPRATHERMAL ENERGIES

Chemiluminescence attributable to OH A 2-SIGMA+ --> X 2-Pi emission is observed in hydrogen-atom transfer reactions of N2+ and Ar+ with H2O. High-resolution OH A --> X spectra [0.5 nm full width at half maximum (FWHM) ] are presented as a function of collision energy in the range E(c.m.) = 5-20 eV. The spectra clearly show an increase in rotational broadening with collision energy. To reasonably fit the spectral data, simulations require contributions from two Boltzmann distributions with different rotational temperatures. It is proposed that the OH A state is formed via two channels. In the dominant channel, A state OH is formed in both v' = 0 and v' = 1, with a collision energy-dependent rotational temperature ranging from 1 000 K at E(c.m.) = 6 eV to 3400 K at E(c.m.) = 19 eV. The rotationally excited products are proposed to be formed in a two-step reaction involving charge transfer, followed by the transfer of a proton to the "primary" species. The second channel is characterized by formation of A state OH primarily in the v' = 0 vibrational level with a rotational temperature of 300 K. Comparison of low-resolution (4 nm FWHM) OH A --> X and H2O+ A2A1 --> X2B1 emissions provides an estimate for the OH A state cross sections of sigma < 0.3 X 10(-16) cm2 for N2+ + H2O at E(c.m.) between 6 and 20 eV, representing less than 10% of the hydrogen atom pickup channel at these energies. For Ar+ + H2O at E(c.m.) between 11 and 32 eV, the estimated OH A state cross section is sigma < 0. 5 angstrom2, which may represent all of the hydrogen atom pickup channel at the higher energies.