Abstract
The inelastic scattering of fast electrons in ≈ 10 nm diameter aluminum spheres has been observed using 1 nm diameter probes of 100 keV electrons. Contrary to expectations, significant bulk scattering occurs even in spheres with 5 nm diameters. Furthermore, the bulk inelastic scattering probability displays an oscillating dependence on the sphere diameter, depending strongly on the spatial frequency content of the incoming electron probe. The surface plasmon scattering has been obtained as a function of impact parameter for single spheres, for clusters of spheres, and for a flat surface to guide the theoretical understanding. The scattering probability corresponds closely to the expected behavior predicted by simple electrostatic boundary-value calculations. Anomalously high scattering very close to the surface (< 1 nm) may relate to non-ideal boundary conditions in the real system. However, the complex shape of the probe on this scale must be accurately included in the theory to determine if this is true. Additional scattering at the oxide/vacuum interface of the oxide layer on a metal indicates that an induced surface charge exists there which follows the surface plasmon charge at the metal/oxide interface. When two spheres lie within about 20 nm of each other, their surface fields couple, producing a resonant mode with bispherical symmetry and with an excitation energy within the optical range. With the very small electron probe, the electric fields due to this mode may be mapped to demonstrate its large dipole symmetry. It appears possible that this coupled surface mode is partly responsible for the large enhancement in Raman scattering observed on rough surfaces.