MgO/Ag(100): confined vibrational modes in the limit of ultrathin films

The vibrational modes of clean MgO films are investigated vs film thickness by means of high-resolution electron energy-loss spectroscopy. For thin films (20\char21{}30 monolayers) we observe, in accord with the literature, the Fuchs-Kliewer phonon at $677{\mathrm{cm}}^{\ensuremath{-}1}$ and the Wallis mode at $524{\mathrm{cm}}^{\ensuremath{-}1}.$ For ultrathin films standing wave optical phonons confined in the overlayer are present, whose frequencies depend strongly on film thickness. Comparison with theoretical calculations for MgO slabs on a perfect conductor shows that the experimental frequencies are lower than expected, indicating the presence of compressive stress. At and below one-…