Anharmonicity and Fermi resonance in the vibrational spectra of a CO2 molecule and CO2 molecular crystal: Similarity and distinctions

The vibrational spectra of a molecule and related molecular crystal are systematically analyzed theoretically with the account of the influence of anharmonic interactions between the vibrations. The specific anharmonism-induced features in the spectra of a molecule and crystal are revealed, and spectral lineshapes at varied strength of anharmonic interaction are simulated. The proposed general theoretical approach is used for fitting the experimental vibrational spectra of CO2 substance in gas and molecular crystal phases. Pretty good coincidence of experimental data with theoretical results is demonstrated, and the constants of anharmonic interaction are obtained. The proposed theoretical approach can be used to interpret the bands in the Raman and infrared absorption spectra of various complex molecules and crystals.