INFRARED-SPECTROSCOPY OF CO ON NACL(100) .3. SUBMONOLAYERS AND ISOTOPIC MIXTURES

In this paper we explore lateral interactions within the CO adsorbate on NaCl(100) through its infrared spectroscopy. The infrared absorption of a monolayer of CO on NaCl(100) at 55 K presents a symmetric band that can be fitted to a Lorentzian profile. As the coverage is reduced the integrated absorbance decreases, the band shifts to higher wavenumber, and its profile broadens and becomes less symmetric. These observations are consistent with a random arrangement of the molecules for submonolayer coverages. This spectroscopic behavior is likely due to myriad electric multipolar interactions plus dispersion and repulsion contributions that become diminished as coverage is reduced. Heterogeneities at the surface can also give rise to coverage dependent spectroscopic profiles. Finally, increased mobility of the adsorbate for decreasing coverages can effect the spectroscopic response. Because of these complications the quantitative behaviour of the submonolayer absorption frequency and bandshape has not been successfully modeled. Isotopic composition of the monolayer also affects the spectroscopy. Dilution of (CO)-C-12-O-16 with (CO)-C-13-O-16 shifts the absorption to lower wavenumber and its band profile becomes broader and asymmetric. A similar behavior is exhibited by the (CO)-C-13-O-16 band on dilution with (CO)-C-12-O-16. Models based on dynamic dipole coupling can account for the spectroscopic behavior of the isotope mixtures in the monolayer.