Ionizing collisions of fast alkali atoms with Cl2, Br2, and O2

A time-of-flight apparatus is described for measuring the inelastic energy loss incurred by fast (< 50–350 eV) alkali atoms in ionizing collisions with diatomic molecules. The inelastic energy losses were found to be nearly energy independent for collisions between K and Cs with Cl2, Br2, and O2, and it is furthermore shown that these energy losses are from about 8 to 10 eV, irrespective of the reactants. The collisions between alkali atoms and halogen molecules (Cl2 and Br2) yield as intermediate alkali ions (M+) and excited molecular ions, (X2−) in the 2Σg state, followed by dissociation to the ground-state halogen ions and excited halogen atoms. In the case of oxygen, excited molecular ions in the 2πu state are formed and these species either remain in the bound state of the 2πu or decay to vibrationally excited molecular states (bΣg+, aΔg). The excitation functions (change of cross sections with incident energy variations) are interpreted in terms of curve crossings near or inside the equilibrium distance of the M−X2 potential surfaces. A comparison of the results obtained at high energies (20–200 eV c.m.) is made to those known from collision energies between 1 and 20 eV (c.m.) and a composite excitation function for the three most probable processes over the whole energy range is constructed.