Adsorbate−Surface Phonon Interactions in Deuterium-Passivated Si(111)-(1 × 1)

The vibrational spectra of atomically flat hydrogen- and deuterium-passivated Si(111)-(1 × 1) surfaces are investigated using theoretical and experimental techniques. An unexpected isotopic shift is observed for the Si−H bending mode when hydrogen is replaced with deuterium, with δ(Si−H) = 626 cm-1 and δ(Si−D) = 537 cm-1. The Si−H stretching mode, in contrast, behaves as expected with ν(Si−H) = 2083 cm-1 and ν(Si−D) = 1516 cm-1. Density functional studies reveal that the mode observed at 537 cm-1 is mostly a phonon mode, which results from the coupling of near-surface phonons with the lower frequency bending mode of the deuterated surface. This coupling causes a shift in the ordering of the expected vibrational modes when hydrogen is replaced by deuterium. We also suggest that such a mode around 535 cm-1 should be seen for many other surfaces and is a general feature of many monovalently terminated Si(111) surfaces.