  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
