  We present a systematic study of the photo-absorption spectra of various
Si$_{n}$H$_{m}$ clusters (n=1-10, m=1-14) using the time-dependent density
functional theory (TDDFT). The method uses a real-time, real-space
implementation of TDDFT involving full propagation of the time dependent
Kohn-Sham equations. Our results for SiH$_{4}$ and Si$_{2}$H$_{6}$ show good
agreement with the earlier calculations and experimental data. We find that for
small clusters (n<7) the photo-absorption spectrum is atomic-like while for the
larger clusters it shows bulk-like behaviour. We study the photo-absorption
spectra of silicon clusters as a function of hydrogenation. For single
hydrogenation, we find that in general, the absorption optical gap decreases
and as the number of silicon atoms increase the effect of a single hydrogen
atom on the optical gap diminishes. For further hydrogenation the optical gap
increases and for the fully hydrogenated clusters the optical gap is larger
compared to corresponding pure silicon clusters.
