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.