Fermi contours and adsorbate phonon anomalies for Li/Mo(110) and Li/W(110)

Angle-resolved photoemission was used to measure the Fermi contours of surf ace-localized states on the Mo(110) and W(110) surfaces with varying amount s of adsorbed lithium. In analogy with recent results for hydrogen adsorbed on the same surfaces, we find that the contours may have sufficient nestin g to drive a surface phonon (Kohn) anomaly. The nesting vector along <(<Gam ma>)over bar>-->(H) over bar (parallel to the <(<Sigma>)over bar> azimuth) on Mo(110) at monolayer coverage is measured to be 1.06 +/-0.05 Angstrom (- 1), which is compared to recent phonon measurements by electron energy-loss spectra. We also report the variation of this nesting vector as a function of coverage, and show that a more complete understanding of the electron-p honon coupling on these surfaces might be obtained from coverage-dependent measurements of surface phonon dispersion relations. Finally, our results s uggest there is even better nesting along the <(<Gamma>)over bar>--> (S) ov er bar direction and that a more pronounced Kohn anomaly might be observed there. (C) 2001 American Vacuum Society.