ELECTRON-CAPTURE BY LARGE HELIUM DROPLETS

The attachment of electrons to large helium droplets containing up to 10(8) atoms produced in supercritical liquid free jet expansions has b een investigated in a crossed beam scattering experiment. Negative clu ster ions were formed in collisions with electrons from a nearly monoe nergetic (delta E approximate to 0.25 eV) electron beam with energies E-el = 1.0-100 eV and were subsequently size selected by electrostatic deflection. Depending on the droplet size up to seven distinct resona ncelike maxima in the negative-ion signal with peak widths Delta E-1/2 proportional to the droplet radius could be resolved. The lowest ener gy peak at E-e1 = 1.8-2.3 eV depending on size, is attributed to a dir ect localization of the electron which subsequently creates a bubble i nside the He droplet. The sharp additional peaks at energies above 20 eV are explained by the thresholds for single or successive electronic excitations of the droplets which result in a zero-kinetic-energy ele ctron which then also localizes in an internal bubble. (C) 1998 Americ an Institute of Physics.