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 = 2 π / 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 Φ _c ^2 ≈ 2 λ / a –| V _im |/ E , where E is the kinetic energy of the ion, λ = h / Mv its de Broglie wavelength and V _im its average electronic image potential at the distance from the surface where diffraction takes place. For Φ < Φ _c , 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 http://dx.doi.org/10.1088/0953-8984/25/35/355009 25 http://dx.doi.org/10.1088/0953-8984/25/35/355009 ), is presented, which fully explains the experimental spectrum of Villette et al (2000 Phys. Rev. Lett. http://dx.doi.org/10.1103/PhysRevLett.85.3137 85 http://dx.doi.org/10.1103/PhysRevLett.85.3137 ) for Ne ^+ ions ricocheting on a LiF(001) surface.

Keywords

• surface channeling
• ion diffraction
• ion energy loss
• surface optical phonons
• ion skipping