Tuning of the Fermi resonance of CO2 and CS2 by temperature, pressure, and matrix material

Abstract

The influence of temperature, pressure, and matrix material on the Fermi resonance interaction of the molecular crystals CO2 and CS2 is investigated. The anharmonic constant κ122 and the unperturbed energy difference Δ are calculated from the experimental energy and intensity data. The sharp Fermi resonance of CO2 is mainly influenced by the change in unperturbed energy difference with a constant anharmonicity, while the tuning of the weak resonance of CS2 is also driven by changes in κ122. Moreover, the variation of the Fermi resonance quantity Δ is changed by the usual solid, temperature, pressure, and matrix shifts of the energy levels. Although the fundamental ν2 should be independent of the Fermi resonance interaction it shows an unusual behavior, which is influenced by its overtone 2ν2 and driven by the tuning of the Fermi resonance. A decrease of Fermi resonance is observed on the gas–solid transition, when increasing temperature and on increasing pressure. An increasing Fermi resonance, produced by an elevation of environmental symmetry and increasing lattice size, realized by matrix-isolated studies, is only significant for the smaller resonance of CS2. For matrix-isolated CO2 a decrease of Fermi resonance is noticeable only in the nitrogen matrix.