Pressure-tuning resonance between the vibron and the libron in molecular solid dibromomethane

The Raman spectra of liquid and pressure-induced crystalline ${\mathrm{CH}}_2$ ${\mathrm{Br}}_2$ have been measured up to 100 kbar at 300 K in a gasketed diamond anvil cell. The ${\nu }_4$ symmetric ${\mathrm{CBr}}_2$ bending vibration and the pressure-sensitive librational mode cross near 43 kbar. Drastic changes in the intensity of the peaks as well as the frequency behavior with pressure are analyzed by the theory of resonance interaction between two states of the same symmetry. It is believed that this is the first observation of pressure-tuning resonance between the vibron and the libron in a molecular solid.