Quantum regime for the nuclear energy loss of fast atoms above crystal surfaces.

To describe the grazing scattering of keV atoms at surface, a new quantum binary collision model have been proposed where the dynamical properties of the surface atoms are considered via the wave-function of the local Debye harmonic oscillator. This leads to a finite probability of elastic scattering where the momentum transferred during the successive binary collisions is not associated with a change of energy. This Lamb-Dicke regime of the multiple collisions at the surface produces the same coherence ratio as the modified Debye-Waller factor adapted to grazing angle fast atom diffraction (GIFAD) but with the additional ability to predict the spot shape of the inelastic diffraction profiles. In terms of energy loss, we show here that at low angle of incidence $\theta$, this Lamb-Dicke effect leads to a marked $\theta^7$ dependency progressively merging to the $\theta^3$ classical dependency. The analytic model presented is supported by numerical simulations for neon atoms scattered off a LiF surface and remains to be confirmed by experiment