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 neutral Na-n, clusters (n = 20,40,57,58,70). The measured inelastic cross sections range from several hundred Angstrom(2) at E>1 eV up to well over a thousand Angstrom(2) for E --> 0, signifying the action of strong long-range polarization forces. For collision energies below 1 eV, the cross sections rise 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 independent of the collision energy, implying efficient energy transfer at large impact parameters. This may proceed via excitation of collective and single-particle resonances of the cluster electron cloud by the incoming electron, followed by fragmentation. [S1050-2947(99)02910-8].