Dynamics of electron transfer collisions of fast Li atoms with several molecules

Abstract

The angular distributions of Li+ ions produced in electron transfer collisions of neutral lithium atoms with the molecules Br2, Cl2, HBr, HCl, O2, and SF6 have been measured. For each system, studied by the crossed molecular beam method, data were obtained at several collision energies, ranging from near threshold [ΔE = 5.390-target electron affinity (eV)] to approximately 14 eV in the center-of-mass (c.m.) system. The fast Li atoms were produced by surface sputtering and then velocity selected. An in-plane experimental geometry was used, with fixed beam sources and a rotatable quadrupole mass filter detection system. The laboratory angular distributions of Li+ ions fall into two groups. For molecules possessing high electron affinities, i.e., Br2 and Cl2, the distributions exhibit undulations, whereas for targets with low electron affinities, the angular distributions decrease monotonically. All show a strong peaking in the forward direction. Differential cross sections obtained by inversion to the c.m. system, with nominal collision velocities and ΔE, were well represented by σ(Eθ), a function of the c.m. collision energy times the scattering angle. By use of an iterative procedure, a single function σ(Eθ) was found to fit the data for each system at all E when averaged over the experimental velocity and angular spreads. Essential features of the scattering are interpreted in terms of classical motion of the heavy particles on potential curves representing the neutral and ionic collidants, using a central-force model of interaction. Electron transfer, which corresponds to a change in adiabatic surface, is considered to occur in a localized region of space where the surfaces are nearly degenerate. The Landau-Zener description of these transitions is followed. The results of the classical theory are discussed for Li–Br2 and Li–HBr, which are representative of the two distinct categories of collision systems studied-those having large electron affinities and thus long-range surface crossings, and those having small electron affinities with corresponding close collisions for electron transfer. A modification of the collision mechanism, which provides for dissociation of the negative molecular ion, leads to good agreement with the long-range crossing data. The influences on the angular distributions of several parameters in the classical scattering model, namely the asymptotic surface separation, the adiabatic well depth, the repulsion parameter, and the polarizability sum, are evaluated. However, for the short-range crossing cases, the classical model gives angular distributions in poorer agreement with the experimental results.