We investigate both theoretically and experimentally an elementary Sis
yphus process occurring during the reflection of an atom onto a laser
evanescent wave propagating at the surface of a dielectric prism. Cesi
um atoms bouncing at normal incidence may undergo a spontaneous Raman
transition between their two hyperfine levers; this leads to an effici
ent cooling since those levels are light shifted by a different amount
by the laser-atom interaction thanks to the large hyperfine splitting
. We compare the measured final energy distributions after the bounce
with Monte Carlo simulations. A quantitative agreement is obtained whe
n the van der Waals interaction between the cesium atoms and the diele
ctric prism is taken into account.