Polarizability ofKC60: Evidence for Potassium Skating on theC60Surface

We present the first measurement of the polarizability and the permanent dipole moment of isolated ${\mathrm{KC}}_{60}$ molecules by molecular beam deflection technique. We have obtained a value of $2506\pm 250{Å}^3$ for the polarizability at room temperature. The addition of a potassium atom enhances by more than a factor of 20 the polarizability of a pure ${\mathrm{C}}_{60}$ molecule. This very high polarizability and the lack of observed permanent dipole show that the apparent polarizability of ${\mathrm{KC}}_{60}$ is induced by the free skating of the potassium atom on the ${\mathrm{C}}_{60}$ surface, resulting in a statistical orientation of the dipole. The results are interpreted with a simple model similar to the Langevin theory for paramagnetic systems.