Raman and piezomodulated Raman spectroscopy of molecular crystals

The intermolecular potential energy of the 1,2,4,5-tetrabromobenzene (TBB) molecular crystal has been studied using Raman and piezomodulated Raman spectroscopy and lattice dynamical calculations.;Polarized Raman spectra of single crystals of TBB were measured in the lattice mode energy region in both the beta phase at room temperature and the gamma phase at 330 K. Symmetry assignments for the modes in both phases have been determined from the experimental data. Assignments of the Raman active intramolecular modes in TBB as well as the lattice mode in 1,2,4,5-tetrachlorobenzene (TCB) were also obtained from polarized Raman spectra.;Piezomodulated Raman (PR) spectroscopy has been employed to determine the extent of anharmonicity in crystal vibrations in TBB. Strain-induced coupling constants have been derived from the relative intensities of the spectral bands observed for the various modes. Measurements at room temperature indicated that among all crystal vibrational modes the lattice modes exhibit the greatest amount of strain-induced coupling and the response is dependent on the direction of the applied stress. PR data for the molecular modes showed these vibrations to be less anharmonic, particularly for modes at high frequencies. PR spectra measured below the TBB phase transition at temperatures between 300 and 313 K have demonstrated that the most anharmonic mode in the crystal is also most affected by the phase transition. The strain produced in the crystal during the PR experiments is observed to drive the crystal toward the high temperature phase at temperatures just below the phase transition.;The PR spectra have been able to determine the molecular motions involved in the Raman active lattice modes in TBB. This is the first experimental technique known that directly probes lattice vibrations to obtain information about their eigenvectors.;Polarized Raman spectra of single crystals of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ...