Strong long-range forces between ${\mathrm{C}}_{60}$ and Na atoms and microclusters

We describe a measurement of the long-range van der Waals interaction $V=-C_6{/r}^6$ between free neutral sodium atoms and clusters $({\mathrm{Na}}_n,1\mathrm{}<~n<~20)$ and ${\mathrm{C}}_{60}$ fullerenes. The $C_6$ coefficients were derived from absolute integral cross sections for low-energy scattering of a supersonic beam of metal particles by ${\mathrm{C}}_{60}$ vapor. Their values are extremely high, ranging from $\sim {10}^4\mathrm{a}.\mathrm{u}.\mathrm{}$ up to more than ${10}^5\mathrm{a}.\mathrm{u}.,$ corresponding to center-of-mass scattering cross sections of up to $\sim 6000{\mathrm{\AA }}^2$ (which exceeds the hard-sphere dimensions of the particles by a factor of 20). These experimental values of $C_6$ 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.