6.2.2.1.2 Surface plasmons

This chapter discusses surface plasmons which were first observed with electron energy loss spectrum (EELS) in transmission experiments through thin foils. They are self-sustained plasma oscillations localized at the surface which decay exponentially both towards the bulk and the vacuum. Surface-plasmon dispersion was measured in transmission for Al, In and Mg. More recently surface-plasmon frequency and dispersion were investigated at low impact energy for Al(111), for thick alkali films, for Mg(001) and for the single crystal faces of Ag. Calculations were performed using the random phase approximation (RPA) and using the local density approximation (LDA) and predicted a positive induced charge density for jellium. This chapter contains figures which illustrate 1) induced charge density for jellium with an effective density r_s = 5 a.u. at the frequencies of the surface plasmon and of the multipole mode; 2) sample electron energy loss (EEL)-spectra of Ag(001)<001>, Cs, K, Na film, and Mg(0001); 3) inelastic intensity of the surface-plasmon loss vs. wavevector transfer for electron scattering of Ag(001); 4) surface plasmon dispersion for Ag(001), Ag(111), Al(111), Cs, K, Li, and Na films; 5) dependence of the surface-plasmon frequency on Ag(001) with crystal temperature; 6) surface plasmon line width for thick Cs, K, and Na films; and 7) dispersion of the surface-plasma loss of polycrystalline In measured with 50 keV electrons. Surface-plasmon, volume plasmon and multipole mode frequencies, surface-plasmon loss width, and multipole mode dispersion are tabulated for simple-metal like materials.