  The momentum transfer between the normal components to an index direction in
the collision of an atom with a periodic surface is investigated. For fast
atoms with grazing angle of incidence there is an interval of azimuthal angles
around the index direction for which the energy transfer can be very efficient.
This effect is reflected in quantum diffraction patterns with large
non-specular peaks, associated with the parallel to the surface and normal to
the index direction momentum component, and can be described in terms of
quasiresonance. Although the classical dynamics does not reproduce the precise
quantum diffraction probabilities, indicating the quantum nature of this
effect, classical and quantum computations show that the span of the
quasiresonance region coincides in both dynamics and can be classically
estimated from the phase-space analysis.
