Fermi resonance in solid CO2 under pressure.

The symmetric-stretching fundamental (ν1) and the bending first overtone (2ν2) of CO2, which are accidentally degenerate with the same symmetry, undergo a Fermi resonance and give rise to two Raman bands with a frequency difference of 107 cm(-1) and an intensity ratio of 2.1. Both the frequency difference and intensity ratio can be varied by pressure applied to CO2 in condensed phases, which has been utilized as a spectroscopic geobarometer for minerals with CO2 inclusion. This study calculates the pressure dependence of the Fermi dyad frequency difference and intensity ratio by combining the embedded-fragment second-order Mo̸ller-Plesset perturbation calculations of harmonic frequencies of solid CO2 under pressure and the coupled-cluster singles and doubles with noniterative triples and vibrational configuration-interaction calculations of anharmonic frequencies of molecular CO2. It reproduces frequency difference quantitatively and intensity ratio qualitatively up to 10 GPa. The analysis of the results is shown to render strong support for one particular order of unperturbed frequencies, ν1 > 2ν2, in both the gas and solid phases, which has been a matter of controversy for decades.