FAST ATOM DIFFRACTION FROM METALS

M.S. GRAVIELLE; C.A. RÍOS RUBIANO; G.A. BOCAN; J.I. JUARISTI; N. BUNDALESKI; H. KHEMLICHE; P. RONCIN

Wirrina Cove

Workshop; 20th International Workshop on Inelastic Ion-Surface Collisions; 2014

IISC

Since the unexpected observation of grazing incidence diffraction of fast atoms (GIFAD) on crystal surfaces [1,2], extensive experimental and theoretical research has been devoted to the subject [3,4]. The first experimental evidences of this phenomenon were reported at insulator materials [1,2], where the presence of a wide band-gap helps to suppress inelastic processes. In these insulator surfaces the minimum electronic excitation amount to ~10 eV, corresponding to an exciton. When this exciton is populated, it has been shown that the coherence is strongly destroyed [5]. The situation is different for metals, where much weaker electronic excitations are possible. However, experiments for metal surfaces have shown diffraction effects [6,7], with a comparable total energy loss. The aim of this work is to investigate the diffraction patterns produced by fast He atoms grazingly impinging on a Ag(110) surface, focusing on the contribution of inelastic processes. This collision system corresponds to the first and simplest metallic case for which GIFAD effects were experimentally observed [6], in conjunction with considerable energy losses. To describe the elastic process we employ a distorted wave theory ? the surface eikonal (SE) approximation ? that makes use of the eikonal wave function to represent the quantum scattering with the surface, while the projectile motion is classically described using different initial conditions [8]. The SE approach has been applied to evaluate GIFAD distributions from insulator surfaces, providing results in good agreement with the experimental data [9]. Concerning the projectile-surface interaction, we use a precise description of the potential energy surface (PES) obtained from an accurate density functional theory calculation, where the PBE functional is used to approximate the exchange-correlation energy. This PES takes into account the projectile´s three degrees of freedom and it is built from a dense grid of ab-initio energies by means of a sophisticated interpolation technique. Projectile momentum distributions obtained with the SE approach are compared with the experimental spectra considering different incidence directions [10]. The contribution coming from inelastic processes, originated by electron-hole pair excitations, is evaluated by introducing a friction force in the calculation of the classical projectile trajectories. This dissipative force is expressed in terms of the transport cross section at the Fermi level by considering the scattering with an electron gas, whose density is evaluated from density functional theory calculations [11]. [1] A. Schüller et al., Phys. Rev. Lett. 98, 016103 (2007). [2] P. Rousseau et al., Phys. Rev. Lett. 98, 016104 (2007). [3] H. Khemliche et al., Appl. Phys. Lett. 95, 151901 (2009). [4] H. Winter et al., Prog. Surf. Sci. 86, 169 (2011). [5] M. Busch et al., Vacuum, 86, 1618 (2012). [6] N. Bundaleski et al., Phys.Rev. Lett. 101, 177601 (2008). [7] M. Busch et al., Surf. Sci. 603, L23 (2009). [8] M.S. Gravielle et al., Phys. Rev. A 78, 022901 (2008). [9] A. Schüller et al., Phys Rev. A 80, 062903 (2009). [10]C. Ríos Rubiano et al., Phys. Rev. A 87, 012903 (2013). [11] J. I. Juaristi et al., Phys. Rev. Lett. 100, 116102 (2008).