MASS-DEPENDENT DYNAMICS OF THE LUMINESCENT EXCHANGE-REACTIONS C+(P-2), P+(P-3)+H-2, D-2, HD

Chemiluminescent ion/molecule reactions of ground state C+ and P+ ions with H-2, D-2 and HD have been studied in an ion beam/target gas cell arrangement. Emission spectra of CH+, CD+ (A (II)-I-1) and of PH+, PD+ (A (2) Delta) were observed with up to 1 Angstrom FWHM resolution and at collision energies from threshold (approximate to 3 eV) to 8 eV(c.m.) (centre-of-mass) and 15eV(c.m.), respectively. Very detailed computer simulations of the spectral contours were done, including ab initio transition moments and, in the case of PH+/PD+, the effects of predissociation. In simulating the spectra obtained with HD, the overlapped hydride and deuteride product ion spectra could be isolated by varying the respective weighting factors to achieve an optimum overall fit. In the case of C+ + HD, the two components were found to have very similar rovibrational distributions as with the products from C+ + H-2 and C+ + D-2 In the P+ case, however, the rotational, although not the vibrational, distributions were found to be significantly different for the isotopically mixed and the pure reactions. The cross-sections showed an intermolecular isotope effect only for C+ + H-2 VS. C+ + D-2 at high energies. However, both with C+ + HD and P+ + HD, a very strong intramolecular isotope effect, i.e. an energy-dependent branching ratio, was observed: at low energies deuteride formation prevails, at high energies hydride. This behaviour is discussed in terms of an impulsive collision model, assuming the ''pairwise'' relative kinetic energy between the reacting atoms to be the determining factor. On the basis of the measured cross-section curves for the H-2 and D-2 reactions, the energy-dependent hydride/deuteride ratio in the HD reaction can then be predicted. The agreement with the experimental results is excellent in the P+ case, but only moderate for the C+ reactions. Even the P+ reaction, however, does not occur via the spectator stripping mechanisms. The spectra show an energy-independent vibrational excitation, contrary to the stripping model assumption. The impulsive model was also extended to an analysis of the relative mean product angular momenta. Again it performs well for P+ + H-2, D-2, HD, but cannot fit the C+ + HD case.