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 v(Si-H) = 2083 cm^-1 and v(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.