FOURIER-TRANSFORM INFRARED-SPECTRA OF CO ADSORBED ON NACL(100) - STRUCTURAL-CHANGES AT LOW-TEMPERATURES

The monolayer of CO adsorbed on NaCl(100) forms a p(1 x 1) structure with CO aligned normal to the surface at temperatures above 35 K. This has been revealed from polarized infrared spectra, measured with a high-resolution Fourier-transform spectrometer. At temperatures above 35 K at monolayer coverage a single sharp absorption is observed with p-polarized light only, the wavenumber being 2155.21 cm-1 at 40 K. Below 35 K a second absorption at approximately 2149 cm-1 is observed with p- and even stronger with s-polarized radiation. The p(1 x 1) structure described above is not in concert with these spectra. Monolayer spectra of isotopic mixtures of (C16O)-C-12/(C16O)-C-13 adsorbed on NaCl(100) at 19 K showed a decrease of the splitting with decreasing partial coverage. The data are most readily explained by the assumption of a correlation field splitting. The spectra of submonolayers show a temperature dependence similar to that of the full monolayer. The frequency shift and the observation of two bands suggest island formation.