Polarizability of KC60: Evidence for potassium skating on the C-60 surface

We present the first measurement of the polarizability and the permanent di pole moment of isolated KC60 molecules by molecular beam deflection techniq ue. We have obtained a value of 2506 +/- 250 Angstrom(3) for the polarizabi lity at room temperature. The addition of a potassium atom enhances by more than a factor of 20 the polarizability of a pure C-60 molecule. This very high polarizability and the lack of observed permanent dipole show that the apparent polarizability of KC60 is induced by the foe skating of the potas sium atom on the C-60 surface, resulting in a statistical orientation of th e dipole. The results are interpreted with a simple model similar to the La ngevin theory for paramagnetic systems.