title: Two-Phonon States and Overtone Delocalization in One-Dimensional Systems of Oscillators: Vibrational Spectroscopy of a NO Adsorbate Row Structure.
abstract: We report on a strong overtone delocalization in one-dimensional systems due to interaction with the two-phonon state. Whereas this interaction leads to small continuous shifts in comparable two-dimensional systems we find a discontinuous shift with bond anharmonicity in one dimension. As a consequence no overtone features can be observed in the spectral function. Experimentally, the phonon dispersion for a quasi-one-dimensional adsorbate system, molecular NO rows in the well-ordered (NO+O)-(2x1) phase on Ru(001), has been mapped out using electron energy loss spectroscopy. The internal NO mode shows a strong band dispersion of 110 cm(-1) along the molecular row. In the region of its overtone the spectra show a peak structure and a nonvanishing background which is interpreted as an overtone-modified two-phonon state.
authors.affiliation: Physik-Department E20, Technische Universität München, D-85747 Garching, Germany.
authors.initials: T
authors.lastname: Moritz
authors.initials: W
authors.lastname: Widdra