Knowledge Bank

Polarized infrared spectroscopy of dielectric surfaces has proven to be a sensitive probe of adsorbate orientation and thermodynamics. For simple systems, such as CO/NaCl(100), one absorbance band is observed for each infrared active normal mode.$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ As more complex systems are studied, however, multiple bands for a single normal mode are observed. These bands arise from a variety of surface-absorbate and inter-assorbate interactions. For methane absorbed on NaCl(100) two bands are observed in both the $v3$ stretching and $v4$ bending regions at 27K and monolayer coverage.$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ Below 17K, however, four absorbance bands are observed in the $v4$ region. This data is presented and the origin of these bands is examined. An adsorbate geometry is proposed and the possibility of cogwheeling motion of the adsorbates on the surface is discussed.