EXCITATION MODES OF NEUTRAL JELLIUM SLABS

The excitation modes of electrons in symmetric, neutral jellium slabs are studied within the time-dependent local density and related approx imations, in the regime when there are several bound states below the Fermi level. Unlike most previous calculations, the present ones do no t require all single-particle wave functions to vanish at some small d istance from the slab. This modification of the boundary conditions si gnificantly affects the excitation spectra, generally reducing the num ber of peaks that appear. In particular, from our calculations modelin g excitation by longitudinal near fields with nonzero surface-parallel wave vector, we do not find resolvable peaks above the bulk plasmon f requency omega(p) which could be interpreted as standing plasma waves resonating across the neutral slab. This is in contrast to the case of a symmetric but non-neutral jellium slab (wide parabolic quantum well or embedded electron gas) where we do predict distinct (but weak) sta nding plasmon resonances above omega(p) at nonzero surface-parallel wa ve vector, even for a well with only a few occupied bound states. Alth ough bulk plasmon resonances seem to be unobservable in the relatively narrow, symmetric, neutral jellium slabs studied here, we do find the following peaks consistently at low surface-parallel wave vector: an ''intraband'' mode near the two-dimensional-plasmon frequency and a '' multipole surface plasmon'' mode near 0.8omega(p) the latter being ame nable also to other interpretations for very thin slabs. Further peaks , including a cluster near omega(p), are harder to interpret. They can be related to single-particle transitions in the case of thin slabs. However, in the intermediate-thickness regime it is difficult to assig n a unique physical cause to each peak in a spectrum, and various ways to help sort out ambiguities are illustrated. These do not always res olve matters since for neutral jellium slabs one has a relatively wide transition region between quantum and classical size effects.