  We study strongly confined plasmons in ultrathin gold and silver films by
simulating electron energy-loss spectroscopy (EELS). Plasmon dispersion
relations are directly retrieved from the energy- and momentum-resolved loss
probability under normal incidence conditions, whereas they can also be
inferred for aloof parallel beam trajectories from the evolution of the plasmon
features in the resulting loss spectra as we vary the impinging electron
energy. We find good agreement between nonlocal quantum-mechanical simulations
based on the random-phase approximation and a local classical dielectric
description for silver films of different thicknesses down to a few atomic
layers. We further observe only a minor dependence of quantum simulations for
these films on the confining out-of-plane electron potential when comparing
density-functional theory within the jellium model with a phenomenological
experimentally-fitted potential incorporating atomic layer periodicity and
in-plane parabolic bands of energy-dependent effective mass. The latter shows
also a small dependence on the crystallographic orientation of silver films,
while the unphysical assumption of energy-independent electron mass leads to
spurious features in the predicted spectra. Interestingly, we find electron
band effects to be more relevant in gold films, giving rise to blue shifts when
compared to classical or jellium model simulations. In contrast to the strong
nonlocal effects found in few-nanometer metal nanoparticles, our study reveals
that a local classical description provides excellent quantitative results in
both plasmon strength and dispersion when compared to quantum-mechanical
simulations down to silver films consisting of only a few atomic layers, thus
emphasizing the in-plane nearly-free conduction-electron motion associated with
plasmons in these structures.
