Strong long-range forces between C₆₀ and Na atoms and microclusters

We describe a measurement of the long-range van der Waals interaction V=-C₆/r⁶ between free neutral sodium atoms and clusters (Na_n,1<~n<~20) and C₆₀ fullerenes. The C₆ coefficients were derived from absolute integral cross sections for low-energy scattering of a supersonic beam of metal particles by C₆₀ vapor. Their values are extremely high, ranging from ∼10⁴a.u. up to more than 10⁵a.u., corresponding to center-of-mass scattering cross sections of up to ∼6000Ų (which exceeds the hard-sphere dimensions of the particles by a factor of 20). These experimental values of C₆ are in excellent quantitative agreement with the predictions of the London theory of dispersion forces based on atom and cluster spectral properties. The strength of the long-range potential is due to high cluster polarizabilities.