Dynamics of barium-bromine chemiionization reactions

The formation of BaBr{sup +} ions in reaction of Ba({sup 1}S{sub 0},{sup 1}P{sub 1}) with Br{sub 2} was studied as a function of laboratory scattering angle and product translational energy in a crossed-beam experiment. The contour map of BaBr{sup +} flux obtained for the ground-state reaction at 1.1-eV collision energy showed a backscattered angular distribution (relative to the barium beam) with a substantial fraction of the available energy appearing in translation. Laser excitation strongly inhibited this channel at 1.1 eV. These experimental observations suggest that for the chemiion reaction head-on, collinear collisions and proximal crossings of the potential energy surfaces are necessary to preclude escape into the dominant neutral pathways. At 1.6-eV collision energy potential energy surfaces are necessary to preclude escape into the dominant neutral pathways. At 1.6-eV collision energy a new laser-dependent source of BaBr{sup +} appeared. This laser-enhanced BaBr{sup +} showed a laboratory angular distribution substantially narrower than for the ground-state reaction, indicating a smaller translational energy release. The angular distribution was {approximately}70% backscattered and displayed a clear dip at the center of mass. This new BaBr{sup +}, produced from electronically excited barium at higher translational energy, is ascribed to secondary collisions of BaBr+Br initially formed inmore » low impact parameter collisions. Reaction of electronically excited barium with Br{sub 2} also yielded an associative ionization product BaBr{sub 2}{sup +} at both collision energies studied, with a cross section about 1/100th that of the chemiion channel.« less