Pressure-tuning resonance between the vibron and the libron in ${\mathrm{CH}}_2$${\mathrm{Br}}_2$ and ${\mathrm{CD}}_2$${\mathrm{Br}}_2$ molecular solids

The Raman spectra of ${\mathrm{CH}}_2$ ${\mathrm{Br}}_2$ and ${\mathrm{CD}}_2$ ${\mathrm{Br}}_2$ molecular solids have been measured up to 100 kbar in a diamond-anvil cell. The pressure-tuning resonance between the vibron and the libron has been observed for the first time around the crossover pressures of 43 and 66 kbar for ${\mathrm{CH}}_2$ ${\mathrm{Br}}_2$ and ${\mathrm{CD}}_2$ ${\mathrm{Br}}_2$ molecular solids, respectively. By using the theory of resonance interaction between two states of the same symmetry, we analyzed the frequency behavior of observed vibrational modes as well as the drastic changes in the intensity with pressure: By deuteration (${\mathrm{CD}}_2$ ${\mathrm{Br}}_2$) the frequency of the unperturbed libron for ${\mathrm{CH}}_2$ ${\mathrm{Br}}_2$ was shifted down by about 21 ${\mathrm{cm}}^{\mathrm{-}1}$ at all pressures. This effect is due to the increase in moment of inertia involving mainly the motion of deuteron atoms, and is the chief cause for the change of the crossover pressure. The coefficients of mixing between the vibron and the libron were calculated at various pressures, and the transfer phenomenon for the Raman intensity was investigated by comparison to the experimental intensity ratio.