ELECTRON-SCATTERING FROM HE-4 AND NE CLUSTERS - DETERMINATION OF THE CLUSTER DENSITY FROM THE ELECTRONIC SURFACE-BARRIER POTENTIAL

A monoenergetic beam of electrons is scattered from a beam of He-4 clusters. The angular distribution of the scattered electronically excited atoms and clusters is measured by an open Venetian-blind multiplier, which is rotatable in the plane of both beams. The threshold electron energy for excitation of the clusters shows a shift to higher energies with respect to the atom component by between 0.6 and 1.1 eV depending on the cluster source temperature T0 (P0 = 20 bar in all experiments). The observed potential shift E(b) is attributed to the surface barrier for penetration of the electrons into the cluster. From the known dependence of E(b) on density the core densities of the helium clusters is estimated to vary between n = 1.2 x 10(22) cm-3 and n = 2.2 x 10(22) cm-3 for T0 = 14.9 and 11 K, respectively. The latter values agree with those for bulk liquid helium. For Ne clusters, E(b) = 0.7 +/- 0.3 eV independent of the source conditions.