INFRARED-SPECTROSCOPY OF MONOLAYER CH4 ON NACL(100)

Polarized infrared spectra of CH4 absorbed on NaCl(100) in the temperature range 4-47 K are reported. Coverage-dependent data indicate that there is only one kind of adsorption site. The adsorbate layer is also seen to grow in the form of constant density islands. A partial degeneracy breaking into doublets is observed in both the nu(3) and nu(4) infrared-active vibrations. Analysis of the nu(4) doublet reveals one transition dipole component perpendicular to the (100) face of NaCl and the others in the plane of the face. Isotopic dilution experiments of CH4 in CD4 show that some of the nu(4) doublet separation can be attributed to resonant or dynamic adsorbate-adsorbate interactions. The remaining splitting cannot be specifically assigned to any particular interaction but must involve static adsorbate-adsorbate coupling and surface-adsorbate interactions. A structural analysis consistent with the spectroscopic data and the symmetry of slab groups is developed. Two possible adsorbate geometries are considered. A unique determination of the adsorbate geometry will have to await other surface measurement techniques.