We demonstrate the existence of quantized “bulk” plasmons in ultrathin magnesium films on Si(111) by analyzing plasmon-loss satellites in core-level photoemission spectra, recorded as a function of the film thickness $d$. Remarkably, the plasmon energy is shown to vary as $1/d^2$ all the way down to three atomic layers. The loss spectra are dominated by the $n=1$ and $n=2$ normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. With decreasing film thickness, spectral weight is gradually transferred from the plasmon modes to the low-energy single-particle excitations. These results represent striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures.

• Received 9 October 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.197601