Onset of 3D collective response in bimetallic jellium films: Surface and mode sensitivity

We use experimental and calculated excitation spectra of bimetallic jellium films with widths of up to a few Fermi wavelengths to explore how excitations of a thin metal with internal density gradients evolve into surface plasmons of a three-dimensional layer. We find that the onset of 3D plasmons is mode specific, and sensitive to surface potentials, even for small surface wave vectors.