Surface plasmon and electron-hole structures in the excitation spectra of thin films.

Surface excitation spectra are calculated, including both collective and single-particle modes, and examined in detail. This is achieved by calculating the non-local dielectric function ε(p)(Q,z,z('),ω) of the thin jellium film within the random phase approximation (RPA) (using local density approximation wavefunctions which actually takes us beyond the RPA), from which we then derive the spectral function. The high precision of the calculations enables us to analyse not only the collective (surface plasmon) modes and their dependence on the film thickness, but also the intra-band electron-hole excitations, and for the first time oscillatory structures due to inter-band transitions. The spectra are then analysed with special attention to their dependence on the slab thickness, and the periodic peaks observed due to single-particle excitations in the spectra.