Siloxene, a two-dimensional sheet polymer, is essentially a three-dimension
al stack of hydrogenated Si(111) surfaces. It can therefore act as a model
system which allows to investigate the properties of this surface with bulk
sensitive spectroscopy methods. We present here a comparative study of the
vibrational properties of hydrogenated and deuterated siloxene using Raman
scattering, FTIR spectroscopy and neutron scattering as well as force-cons
tant modeling. A coupling of the Si-H bending mode and the in-plane Si phon
on (both type E) is observed with all three experimental methods, but is mo
st notable in Raman scattering, where the level anti-crossing results in an
"anomalous" hardening of the dominant line from 496 cm(-1) in hydrogenated
to 575 cm-l in deuterated siloxene. A coupling strength of 56 cm(-1) is fo
und, which is about 10% of the phonon energy.