F−formation via simultaneous two-electron capture during grazing scattering ofF+ions from a LiF(001) surface

Abstract

For slow ${\mathrm{F}}^{+}$ ions $(v<0.05\mathrm{a}.\mathrm{u}.)$ scattered from a clean and flat LiF(001) surface under a grazing angle of incidence, large fractions of negative ${\mathrm{F}}^{\ensuremath{-}}$ ions have recently been observed in the reflected beam, while for neutral ${\mathrm{F}}^{0}$ projectiles no negative ${\mathrm{F}}^{\ensuremath{-}}$ ions are produced in the same velocity range [P. Roncin et al., Phys. Rev. Lett. 89, 043201 (2002)]. From detailed studies on projectile energy loss and charge fractions, the conclusion was drawn that the ${\mathrm{F}}^{\ensuremath{-}}$ ions are formed from ${\mathrm{F}}^{+}$ via a simultaneous capture of two electrons from adjacent ${\mathrm{F}}^{\ensuremath{-}}$ sites at the surface. We present a theoretical description of the double-electron-capture process leading to ${\mathrm{F}}^{\ensuremath{-}}$ formation from ${\mathrm{F}}^{+}$ projectiles grazingly scattered from the LiF(001) surface. We use quantum chemistry calculations to determine the relevant Hamiltonian matrix and close-coupling solution of the time-dependent Schr\"odinger equation. The theoretical results are in good agreement with experimental observations.