Electric dipole moment and charge transfer in alkali-C-60 molecules

We have measured the electric dipole moment of isolated alkali-C-60 molecul es (with alkali = Li, Na, K, Rb, and Cs) by molecular beam deflection exper iments. The dipole increases from 12.4 D for LiC60 to 21.5 D for CsC60. The se results are compared to those deduced from an analytical polarizable-ion model. This comparison shows that there is a strong electron transfer from the alkali atom to the C-60 cage, which is almost complete for KC60 and Rb C60.