Vibrations of Molecules Adsorbed on Alkali Halides. Theory and Infrared Measurements of CO₂ on NaCl Films

Abstract

Abstract The vibrational frequency shifts and splittings of polyatomic molecules upon adsorption are discussed in the harmonic approximation. Equations relating the shifts and splittings of the normal modes of the adsorbed molecule to its position and orientation on the surface are derived on the basis of interactions built up by summation over all atoms of the vibrating molecule. As an example, the shifts and splittings of the vibrational energy levels of CO 2 on the {100} face of NaCl are calculated. The calculated values approach very well those observed. The infrared transmission spectra of CO 2 adsorbed on NaCl films were recorded at ‐ 160°C. Upon adsorption new bands were observed due to the symmetric v 1 vibration and one component of the 2v 2 transition, which are in Fermi resonance and infrared inactive in the free molecule, and a well‐resolved splitting of the v 2 transition probably caused by the removal of its degeneracy. As are all other frequencies, those of the v 3 transition of both 12 CO 2 and 13 CO 2 are shifted to lower values.