Eels of Surface Phonons and Vibrations of Adsorbed Species

Abstract

By improving the resolution of electron energy loss spectrometers, surface vibrations on platinum and nickel single crystal surfaces could be observed by EELS. The observed phonons are in good agreement with recent theoretical results. On a stepped Pt(111) surface a phonon localized near the step edge and with a frequency slightly above the maximum bulk frequency has been found. The phonon is only observed on the clean surface and is caused by relaxation of the step atoms. The step phonon is excited by dipole scattering and the necessary dynamic dipole moment is provided by the particular electronic properties of the step atoms. On the flat Ni(111) and Ni(100) surface, nickel surface phonons at certain points of the two-dimensional Brillouin zone have been observed when suitable submonolayer amounts of gases were adsorbed. The adsorbates provide the appropriate coupling with the slow electrons. The same phonon frequencies are observed after adsorbing H2, O2, C2H2, or NO. Sideband- or multiple excitations including the metal phonons are observed. Examples show that the Ni-surface phonons can be excited by both dipole and impact scattering. Implications of these results for the interpretation of vibrational spectra of adsorbed species are outlined.