Polarization forces in collisions between low-energy electrons and sodium clusters

We study low-energy (E similar to 0-5 eV) electron collisions with free neu tral Na-n, clusters (n = 20,40,57,58,70). The measured inelastic cross sect ions range from several hundred Angstrom(2) at E>1 eV up to well over a tho usand Angstrom(2) for E --> 0, signifying the action of strong long-range p olarization forces. For collision energies below 1 eV, the cross sections r ise strongly with decreasing energy, and are in good agreement with values predicted by the mechanism of electron capture by the polarization field of the cluster. At higher energies, the cross sections become approximately i ndependent of the collision energy, implying efficient energy transfer at l arge impact parameters. This may proceed via excitation of collective and s ingle-particle resonances of the cluster electron cloud by the incoming ele ctron, followed by fragmentation. [S1050-2947(99)02910-8].