FERMI RESONANCE STATES IN THE NATURAL ISOTOPICALLY MIXED CS2 CRYSTAL

High-resolution Raman spectroscopy (HRRS) is used to analyze the anharmonic interactions between the modes nu(1) and nu(2) in the CS2 crystal and its isotopic aggregates. Statistics of aggregates (mainly formed with molecules containing S-34 and S-33 isotopes) are calculated. Using the exciton theory formalism, a unified description of the Fermi resonance going from the molecule to the crystal is provided and used to calculate the various eignmodes of aggregates. HRRS allows an accurate analysis, in the nu(1) frequency domain, of the complex band structure due to the various cofigurations of isotopic aggregates (monomers, dimers, trimers,...). In the crystal, the weak Fermi resonance interaction (nu(1) 2(nu)2)) leads to the appearance of bound states nu+ and nu-. HRRS allows one to clearly separate the two Raman active Davydov components (A(g) and B3g) in both the nu- and nu+ bands. Their thermal evolution is also given and compared to other incoherent as well as coherent Raman data.