Pressure tuning of Fermi resonance in crystal CO2

A solid state theory on pressure tuning of Fermi resonance in crystals is presented and applied to the CO2 case. The theory relies on consideration of delocalized ω2+ω2 two-phonon states anharmonically coupled to ω1 and it is developed along the formalism of the Green function technique. This gives a straightforward method to split bound states (or biphonons) out of the continuum. Explicit expressions for their energy separation and intensity are obtained. Calculations are performed on the pressure dependence of the main parameters governing the Fermi resonance and discussed in terms of the crystal approach. In contrast with the more limited molecular treatment, W is seen to decrease with pressure. The effect is specifically related to the spread of the two-phonon continuum over a finite energy interval. Other quantities, for instance the intensity ratio between bound states, do not suffer such a big discrepancy between the two models. Limits of applicability of our approach are also sketched.