ON THE ELECTRON-ENERGY LOSS SPECTRA OF SMALL PARTICLES

Electron energy loss spectroscopy experiments have very recently confirmed that at clean planar metallic surfaces, the smooth decay of the ground-state electronic density induces additional surface modes above the classical one tending to omega p/ square root 2 for q to 0 (in the non-retarded regime). It is shown here that similar modes-i.e. induced by surface diffuseness, should also be observable in the electron energy loss spectra of small particles of free-electron-like metals. More precisely, these spectra show an additional maximum above the classical modes of a sphere-which are given by omega p/ square root (2l+1)/l, l=1,. . ., infinity -and below omega p. This maximum is due to the superposition of the contributions of the aforementioned additional modes. Recent experimental reports of the electron energy loss spectra of potassium small particles, showing anomalous modes, can be understood from this point of view.