Quantum ricochets: surface capture, release and energy loss of fast ions hitting a polar surface at grazing incidence

Abstract

A diffraction mechanism is proposed for the capture, multiple bouncing and final escape of a fast ion (keV) impinging on the surface of a polarizable material at grazing incidence. Capture and escape are effected by elastic quantum diffraction consisting of the exchange of a parallel surface wave vector G = 2π/a between the ion parallel momentum and the surface periodic potential of period a. Diffraction-assisted capture becomes possible for glancing angles Φ smaller than a critical value given by Φc2 ≈ 2λ/a–|Vim|/E, where E is the kinetic energy of the ion, λ = h/Mv its de Broglie wavelength and Vim its average electronic image potential at the distance from the surface where diffraction takes place. For Φ < Φc, the ion can fall into a selected capture state in the quasi-continuous spectrum of its image potential and execute one or several ricochets before being released by the time reversed diffraction process. The capture, ricochet and escape are accompanied by a large, periodic energy loss of several tens of eV in the forward motion caused by the coherent emission of a giant number of quanta ħω of Fuchs–Kliewer surface phonons characteristic of the polar material. An analytical calculation of the energy loss spectrum, based on the proposed diffraction process and using a model ion–phonon coupling developed earlier (Lucas et al 2013 J. Phys.: Condens. Matter 25 355009), is presented, which fully explains the experimental spectrum of Villette et al (2000 Phys. Rev. Lett. 85 3137) for Ne+ ions ricocheting on a LiF(001) surface.