Effective polarizabilities of water clusters

We report the measurement of electrical polarizabilities of water clusters [(H$_{2}$O)$_{n}$, n=3-18] by deflection of a supersonic beam in an inhomogeneous field. In all the clusters studied, the deflections are only towards higher fields, indicating a polarizability-like behavior. The highest polarizability per one water molecule is seen in the trimer (7.9 $\pm $ 0.3 {\AA}$^{3})$ and it quickly decreases for larger clusters to about 4 {\AA}$^{3}$ per molecule for n=6-18. These values are large and could be explained by the addition of two parts: an intrinsic electronic polarizability plus a contribution from a permanent dipole moment responding with a Langevin-like behavior. With a cluster temperature of $\sim $200 K, estimated from the evaporative ensemble picture, the permanent dipole moments are $\sim $1-2 Debye per cluster.