Mass and orientation effects in dissociative collisions between rare gas atoms and alkali halide molecules

The collision induced dissociation of alkali halide molecules to ion pairs upon impact with hyperthermal rare gas atoms has been investigated using the crossed molecular beam method. Relative total cross sections for the dissociation of CsI, CsBr, RbI, and KI to ion pairs upon collision with xenon and krypton have been measured over a relative collision energy range from threshold to 10 and 8 eV, respectively. In addition, complete angular and energy distributions of both dissociated ions from Xe+CsI, CsBr, and RbI collisions and from Kr+CsI and CsBr collisions have been obtained at several collision energies within the above energy range. Mass, collision orientation, and energy dependence effects observed throughout this work define two limiting case dissociation mechanisms for the Xe(Kr)+MX→Xe(Kr)+M++X− processes. The dominant dissociation configuration consists of the rare gas atom incident on the light atom end of the alkali halide molecule in a near collinear collision. The less preferred dissociation mechanism results when the rare gas atom is incident in a near collinear configuration on the heavy atom end of the alkali halide molecule. Experimental measurements of the percentage of energy transfer from the relative kinetic energy between Xe(Kr) and MX to the relative motion of M+–X− range as high as 95%; these percentage energy transfers correlate well with the predictions of an impulsive collision model. Three-dimensional classical trajectory calculations using realistic interaction potentials have been performed and they verify the dynamical interpretation suggested by the experiments.