Silicon clusters produced by femtosecond laser ablation: non-thermal emission and gas-phase condensation

Neutral silicon clusters Si-n (up to n=7) and their cations Si-n(+) (up to n=10) have been produced by femtosecond laser ablation of bulk silicon in vacuum and investigated using time-of-flight mass spectrometry. Two populations of the Si-n(+) clusters with different velocity and abundance distributions in the ablation plume have been clearly distinguished. Possible mechanisms of cluster formation (Coulomb explosion, gas-phase condensation, phase explosion) are discussed.