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 ion s with hydrogen chloride were investigated at collision energies betwe en < 1 eV and 1 keV. The electronically excited products HCl+ (A (2) S igma(+)) were detected by means of the A (2) Sigma(+) --> X (2) Pi(i) optical emission. In some cases the spectra showed, at low collision e nergies, an enhanced excitation of specific vibrational HCl+ (A,v') le vels: 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, mo stly endothermic, and here the rotational temperature was 1000-4000 K. Corresponding data are also given for DCl. The selectivity for certai n vibrational states is explained by crossings between the vibronic en trance and exit state energy surfaces, calculated from classical elect rostatic multipole potentials. The cross sections for the near-resonan t reactions decrease monotonically with increasing collision energy, w hile 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 l evels 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)(+).