Coupling of an adsorbate vibration to a substrate surface phonon: H on Si(111)

We report high-resolution infrared--reflection-absorption measurements of an unreconstructed, ideally H-terminated Si(111) surface. The marked width and frequency variations of the Si-H stretching vibration with temperature are completely accounted for by a weak anharmonic coupling of this mode to a Si surface phonon band centered at 210\ifmmode\pm\else\textpm\fi{}25 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. A decrease in Si-H stretch intensity, observed as the temperature is increased above 300 K, suggests a strong anharmonic coupling between the Si-H stretching and bending modes.