GIFAD for He/KVl(001): unexpected rainbow angle increase for low-E┴ incidence

G.A. BOCÁN; E.A. SANCHEZ; M.S. GRAVIELLE; H. KHEMLICHE

Buenos Aires

Encuentro; Encuentro Nacional de Dinámica Cuántica en la Materia; 2019

IAFE

In this contribution we will address Grazing Incidence Fast Atom Diraction (GIFAD) [1, 2]for He/KCl(001) [3, 4], with the focus on the rainbow angle and its behaviour for low perpendicular energies Eperp (associated to the 4He atoms motion in the plane normal to the channeling direction). In GIFAD the dynamics is ruled by the potential energy surface (PES), averaged along the channel (henceforth axial PES).The rainbow angle is the angle of maximum deflection which, within a hard corrugated wall(HCW) approach, is determined by the steepest slope of the Eperp equipotential curve of the axial PES. Intuitively, upon decreasing Eperp this slope should decrease, as He atoms are reflected farther from the surface. Intriguingly, experiments how the rainbow angle increases for decreasing low Eperp, this increase being particularly sharp when incidence is along the <110> direction (Fig.1). Simulations, performed as explained in Ref. [4] (PBE-PES + SIVR), very well reproduce the observed feature, in marked contrast to the pooraccord obtained with the HCW approach. This result points to soft potential eects asthe main mechanism behind the observed rainbow increase. The detailed analysis of the equipotential curves across the <100> and <110> channels, and the simulated trajectories that contribute to the rainbow conrms the latter conclusion (Fig. 2, with a = 6.381 A). The presence of a shallow ( 5 meV) potential well deviates the outgoing low-Eperp trajectories, the effect being stronger for <110> given its equipotentials beyond-the-well higher corrugation.Referencias[1] P. Rousseau et al, PRL 98, 016104 (2007)[2] A. Schuller et al, PRL 98, 016103 (2007)[3] U. Specht et al, NIMB 269, 799 (2011)[4] G.A. Bocan et al, NIMB 421, 1 (2018)