We have studied the polarizabilities of neutral silicon clusters with 8 to 75 atoms at 300 K employing the molecular beam deflection technique. In the range of 8 to 30 atoms the polarizability is strongly size-dependent, whereas for clusters up to 75 atoms polarizability approaches the bulk value of 3.71 ?A3, estimated from the Clausius-Mossotti relation for α-Si. This indicates a transition in the growing pattern of silicon clusters in this range. For some clusters, i.e. Si18, we observe a significantly increased polarizability compared to the bulk value, which can be interpreted by taking a permanent dipole moment via a simple Langevin-Debye model into account.
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