  Rich populations of clusters have been observed after femtosecond laser
ablation of bulk silicon in vacuum. Size and velocity distributions of the
clusters as well as their charge states have been analyzed by reflectron
time-of-flight mass spectrometry. An efficient emission of both neutral silicon
clusters Sin (up to n = 6) and their cations Sin+ (up to n = 10) has been
observed. The clusters are formed even at very low laser fluences, below
ablation threshold, and their relative yield increases with fluence. We show
the dependencies of the cluster yield as well as the expansion dynamics on both
laser wavelength and laser fluence. The mechanisms of the cluster formation are
discussed.
