HIGH-RESOLUTION RAMAN-SPECTRA OF THE FERMI DIAD IN DISORDERED CRYSTALLINE CO2

Using high-resolution Raman spectroscopy, very weak features in the biphonon spectra of disordered materials were investigated. The analysis was focused on the bound states due to the Fermi resonance in crystalline CO2. Two systems were studied: the mixed isotopic ((CO2)-C-12)(x)-((CO2)-C-13)(1-x) crystals and the molecular alloys (N2O)(x)-((CO2)-C-12)(1-x) and (N2O)(x)-((CO2)-C-13)(1-x). In the first system, it was shown that the wavenumbers and band-widths of biphonon states at K approximate to 0 follow a concentration dependence identical with that encountered for one-phonon states which obey the separated band limit model in mixed isotopic crystals. In the second system, the occurrence of a fine structure in the biphonon band is discussed on the basis of a strong interaction between non local matrix biphonons and quasi-local biphonons erected by the disorder.