  We present the first study of light induced atom desorption (LIAD) of an
alkali atom (Rb) in porous alumina. We observe the variation due to LIAD of the
rubidium density in a vapor cell as a function of illumination time, intensity
and wavelength. The simple and regular structure of the alumina pores allows a
description of the atomic diffusion in the porous medium in which the diffusion
constant only depends on the known pore geometry and the atomic sticking time
to the pore wall. A simple one-dimensional theoretical model is presented which
reproduces the essential features of the observed signals. Fitting of the model
to the experimental data gives access to the diffusion constant and
consequently the atom-wall sticking time and its dependence on light intensity
and wavelength. The non-monotonic dependence of the LIAD yield on the
illumination light frequency is indicative of the existence of Rb clusters in
the porous medium.
