  We study the two-photon laser excitation to the 5D$_{5/2}$ energy level of
$^{85}$Rb atoms contained in the interstices of a porous material made from
sintered ground glass with typical pore dimensions in the 10 - 100 $\mu$m
range. The excitation spectra show unusual flat-top lineshapes which are shown
to be the consequence of wave-vector randomization of the laser light in the
porous material. For large atomic densities, the spectra are affected by
radiation trapping around the D2 transitions. The effect of the transient
atomic response limited by time of flight between pores walls appears to have a
minor influence in the excitation spectra. It is however revealed by the
shortening of the temporal evolution of the emitted blue light following a
sudden switch-off of the laser excitation.
