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--30 monolayers) we observe, in accord with the literature, the Fuchs-Kliewer phonon at 677cm^-1 and the Wallis mode at 524cm^-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-monolayer thickness an intense loss peak at 427cm^-1 is observed.