Translational energy effects in the Ca*(D-1(2))+ HBr -> CaBr*(A,B) reaction. Experimental evidence of synchronization between electron transfer and rearrangement of the intermediate complex

The collision energy dependence of the Ca*(D-1(2)) + HBr --> CaBr*(A,B) + H reaction cross sections has been measured over the 210-290 meV range using the time-of-flight technique under crossed molecular beam conditions. Both reaction channels show a steep increase with pronounced maximum at low ene rgy, followed by a sharp decline as energy increases. A modified time-depen dent Landau-Zener model is employed to describe the transition probability from the entrance neutral to the exit ionic channels, directly leading to r eaction products. This model, which incorporates the influence on the react ion dynamics of the time necessary for the rearrangement of the intermediat e complex, satisfactorily accounts for the experimentally observed translat ional features of the excitation functions.