Energy and angular differential cross sections for electron transfer in the systems K+Br2 and K+Cl2

Doubly differential cross sections, in energy and angle, were measured in a crossed beam for the electron transfer reaction K+X2→K++X−2 (or X+X−) at relative energies of 4.4 to 29.8 eV for X=Br and 9.6 to 36.6 eV for X=Cl. The translational energy-loss spectra show maxima which we attribute to X2− ions formed in the ground state and an electronically excited state. The excited X−2 ions are scattered through wider angles than the ground state ions. Also, the cross section for excited X−2 production is strongly dependent on beam velocity and is only significant at the lower end of the energy ranges examined. Both effects are interpreted in terms of bond stretching of the halogen molecular ion during the collision.