Molecular beam study of the chemi-ionization in the reactive scattering of Ca and Sr with Cl2 and Br2 at collision energies Ec.m. s< 4.5 eV

Integral cross sections for the reactive scattering of the chemi-ionization M ÷ X 2 → MX + + X − (M = Ca, Sr: X = Cl, Br) have been measured in a crossed molecular beam experiment in the collision energy range E c.m. < 4.5 eV. The reactions are endothermic. Chemi-ionization products are observed just above their energetic threshold without indication of activation energies. It is assumed that the collision-parameter region (peripheral collisions versus central collisions) plays a key role for the reaction dynamics. There is evidence that the chemi-ionization products are preformed in central collisions. Ionization energies of the alkaline earth monohalides J (MX) and the dissociation energies of the corresponding ions D 0 (MX + ) are estimated. The experimental observation of the charge-transfer process Ca+Br 2 → Ca + + Br 2 − at higher collision energy ( E c.m. > 3 eV) yields an adiabatic electron affinity of 2.65 eV for Br 2 .