NEAR-RESONANT VERSUS NONRESONANT CHEMILUMINESCENT CHARGE-TRANSFER REACTIONS OF ATOMIC IONS WITH HCL

Charge-transfer reactions of C+, O+, F+, Ar+ and some other atomic ions with hydrogen chloride were investigated at collision energies between < 1 eV and 1 keV. The electronically excited products HCl+ (A (2) Sigma(+)) were detected by means of the A (2) Sigma(+) --> X (2) Pi(i) optical emission. In some cases the spectra showed, at low collision energies, an enhanced excitation of specific vibrational HCl+ (A,v') levels: for C+, v' = 1; for O+, v' = 3 as well as v' = 1; and for F+, v' = 6. These levels are populated in near-resonant, slightly exothermic processes. Their rotational temperature was on the order of 600-700 K. For the other vibrational levels the excitation is off-resonance, mostly endothermic, and here the rotational temperature was 1000-4000 K. Corresponding data are also given for DCl. The selectivity for certain vibrational states is explained by crossings between the vibronic entrance and exit state energy surfaces, calculated from classical electrostatic multipole potentials. The cross sections for the near-resonant reactions decrease monotonically with increasing collision energy, while for the endothermic channels they rise steeply from threshold to a plateau, With argon ions the excitation function exhibits an unusual shape. Here the charge-transfer cross sections for all. vibrational levels go through a maximum just above threshold, which is followed by a distinct minimum at about 10 eV(c.m.). This may be due to formation of a long-lived collision complex (Ar-HCl)(+).