Adsorption of lithium on Mo(110): an EELS study of the adsorbate vibrations and substrate phonons

Electron energy loss spectroscopy (EELS) was used to investigate the vibrations of lithium adsorbed on Mo(110) at T=110 K. The frequency of the perpendicular Li vibration was found to be 312 cm−1 at low coverage, reaches a maximum of 342 cm−1 at a coverage of 1/3, then drops to 307 cm−1 at 1 ML coverage. With the completion of the second monolayer the frequency shifts upwards to about 350 cm−1. The loss intensity (relative to the elastic peak) was found to have a maximum at 0.2 ML. At 1 ML coverage, the loss peak shows an asymmetric Langreth line shape indicative of a strong coupling to electron–hole pair excitations. For the same coverage range the dispersion curve of the Rayleigh wave along the direction displays an anomalous, broad dip. The dip is possibly a Kohn anomaly due to Fermi surface nesting, as found for the hydrogen-covered Mo(110) surface.