We present an experimental study of non-linear selective reflection at
a dielectric/Cs vapour interface, in a three-level caseade-up scheme.
The non-linear reflection at the D2 resonance line is monitored in th
e presence of a resonant irradiation at 621 nm or 658 nm (6P3/2 --> 8D
5/2, 9S1/2). A novel type of two-photon-induced reflection resonances
specifically connected with atoms desorbing from the surface is observ
ed. We demonstrate that these resonances allow one to monitor the atom
ic-velocity distribution at vapour-solid interfaces.