Davydov and site symmetry splitting in polarized infrared spectra of ordered monolayers. A quantitative theoretical analysis of vibrational frequencies and integrated absorptions for CO 2 on NaCl(100)

Abstract

In the polarized infrared (IR) spectra of ordered physisorbates on insulator single crystal surfaces we observed absorption multiplets, which were assigned to correlation field or Davydov splittings, in some cases combined with site symmetry splittings. In our analysis of the infrared absorptions, based on the classical theory of dynamic dipole coupling, a two-layer model is used. The electric field of the incident light at the surface is dependent on the refractive index of the substrate and the angles of incidence and polarization, as manifested in the Fresnel equations. Analytical expressions are presented for the vibrational frequencies and the s- and p-polarized integrated absorptions of the correlation field and site symmetry multiplets for unit cells with two molecules and the two-dimensional space group pg. Many adsorbates belong to this class. The expressions presented are useful in determining the orientational order of adsorbates. As an example the monolayer CO 2 on NaCl(100) is treated, and the calculated data are compared with the measured spectra.