GIFAD for He/KCl(001): Unexpected rainbow angle increase for low- E⊥ incidence

G. A. BOCAN; M. S. GRAVIELLE; H. KHEMLICHE; E. A. SÁNCHEZ

Buenos Aires

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

Instituto de Astronomía y Física del Espacio.

In this contribution we will address Grazing Incidence Fast Atom Diffraction (GIFAD) for He/KCl(001), with the focus on the rainbow angle and its behaviour for low perpendicular energies E⊥ (associated to the He 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 E⊥ equipotential curve of the axial PES. Intuitively, upon decreasing E⊥ this slope should decrease, as He atoms are reflected farther from the surface. Intriguingly, experiments show the rainbow angle increases for decreasing low E⊥ , this increase being particularly sharp when incidence is along the <110> direction. Simulations very well reproduce the observed feature, in marked contrast to the pooraccord obtained with the HCW approach. This result points to soft potential effects as the 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 confirms the latter conclusion. Thepresence of a shallow (∼ 5 meV) potential well deviates the outgoing low-E⊥ trajectories, the effect being stronger for <110> given its equipotentials beyond-the-well higher corrugation.