Strong long-range forces betweenC60and Na atoms and microclusters

Abstract

We describe a measurement of the long-range van der Waals interaction $V=\ensuremath{-}{C}_{6}{/r}^{6}$ between free neutral sodium atoms and clusters $({\mathrm{Na}}_{n},1<~n<~20)$ 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 $\ensuremath{\sim}{10}^{4}\mathrm{a}.\mathrm{u}.$ up to more than ${10}^{5}\mathrm{a}.\mathrm{u}.,$ corresponding to center-of-mass scattering cross sections of up to $\ensuremath{\sim}6000{\mathrm{\AA{}}}^{2}$ (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.