Vibrational anti-crossing in siloxene

Siloxene, a two-dimensional sheet polymer, is essentially a three-dimensional 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-constant modeling. A coupling of the Si-H bending mode and the in-plane Si phonon (both type E) is observed with all three experimental methods, but is most 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^-1 in deuterated siloxene. A coupling strength of 56 cm^-1 is found, which is about 10% of the phonon energy.