  Electronic excitations near the surface of water ice lead to the desorption
of adsorbed molecules, through a so far debated mechanism. A systematic study
of photon-induced indirect desorption, revealed by the spectral dependence of
the desorption (7 to 13 eV), is conducted for Ar, Kr, N2, and CO adsorbed on
H2O or D2O amorphous ices. The mass and isotopic dependence and the increase of
intrinsic desorption efficiency with photon energy all point to a mechanism of
desorption induced by collisions between adsorbates and energetic H or D atoms,
produced by photodissociation of water. This constitutes a direct and
unambiguous experimental demonstration of the mechanism of indirect desorption
of weakly adsorbed species on water ice, and sheds new light on the possibility
of this mechanism in other systems. It also has implications for the
description of photon-induced desorption in astrochemical models.
