LONG-RANGE VAN-DER-WAALS FORCES BETWEEN ALKALI CLUSTERS AND ATOMS

We study the long-range attractive force between neutral sodium cluste rs Na-n (2 less than or equal to n less than or equal to 20) and the a lkali atoms Li, Na, and K. Absolute cross sections for the scattering of a beam of clusters by atomic vapor are measured, and are shown to a rise from the van der Waals dispersive interaction V = -C-6/r(6). Thes e cross sections are extremely large (up to thousands of Angstrom(2) i n the center-of-mass frame). Their magnitudes an in good agreement wit h predictions based on the London theory of dispersion forces; the lar ge strength of the interaction is a consequence of the high cluster po larizabilities. In addition, we evaluate the contribution of the highe r-order potential term -C-8/r(8) to the scattering cross section and s how that it can become quite significant for collisions involving larg e clusters (n similar to 10(2)-10(3)). (C) 1998 American Institute of Physics.