K. M. White

Single-crystal reflection and Raman spectra of the polydiacetylene THD

Abstract The 298 K. Raman and reflection spectra of the polydiacetylene THD, a close structural variant of DCHD, are presented. The polymer double- and triple-bond stretch frequencies are 1485 and 2111 cm −1 , respectively, and the lowest electronic transition peaks at 17600 cm −1 . The reflection spectrum is more reminiscent of the polydiacetylene TCDU than DCHD.

Assignment of the lattice modes in TCNQ0 single crystals

A complete assignment of the six librational modes of the TCNQ0 crystal has been made employing both polarized Raman spectroscopic measurements and lattice dynamical calculations. Agreement between theory and experiment is good and modes at 40.5, 74.5, and 96 cm−1 are assigned as Ag active and those at 63, 75.5, and 104 cm−1 are Bg active. The lattice modes are found to be clearly separated from the molecular modes. The study emphasizes the importance of crystal optics and quality in the measurement of the polarized Raman spectra of biaxial crystals.

Assignment of the Raman active lattice vibrations in various phases of 1,2,4,5-tetrabromobenzene and 1,2,4,5-tetrachlorobenzene crystals

Results of polarized Raman spectroscopy are reported and used to obtain complete assignment of the optical lattice modes of 1,2,4,5‐tetrabromobenzene crystals for both the beta and gamma phases. Similar information is provided for the beta phase of 1,2,4,5‐tetrachlorobenzene crystal. The results are compared with lattice‐dynamics calculations. Discrepancies with previously reported measurements and calculations are discussed and rationalized.

Semiempirical determination of molecular motions associated with phonon modes in molecular crystals

An approach using piezomodulated Raman scattering is presented which permits determination of the molecular motions associated with the eigenmodes of molecular crystals. When coupled with a lattice dynamical calculation for the eigenmodes, the experiment can be used to validate the calculation. The case of 1,2,4,5‐tetrabromobenzene is treated and it is shown that the phase relations of the piezomodulated Raman signals confirm the calculated eigenmodes.

Single crystal Raman spectra of 1,2,4,5-tetrabromobenzene: Calculational and experimental assignment of the internal modes

A detailed study of the fundamental internal modes of the 1,2,4,5-tetrabromobenzene crystal has provided assignments of Raman-active modes consistent with polarized single-crystal measurements. A normal-mode calculation based on a valence force field derived from brominated and chlorinated benzenes is found to be in excellent agreement with the assignments. Additional bands arising from anharmonic interactions of the fundamentals are also assigned.

Anisotropic anharmonicity of lattice and molecular vibrations of 1,2,4,5‐tetrabromobenzene determined by piezomodulated Raman spectroscopy

Strain‐induced coupling constants for the anharmonicity of Raman‐active lattice and certain molecular modes of vibration in 1,2,4,5‐tetrabromobenzene (TBB) crystals have been determined using piezomodulated Raman spectroscopy. These constants, which are directly related to the first anharmonic term in the potential energy expansion for lattice dynamical calculations, are a quantitative measurement of the modal anharmonicities in the TBB molecular crystal. Application of uniaxial stress in the experiments permits the anisotropy of the anharmonicity to be determined as well as its magnitude. The TBB lattice modes are significantly coupled by the induced strains and the effects of coupling were observed to be dependent on the direction and symmetry of the strains. The molecular modes investigated were, by comparison, less coupled by the acoustic phonons and generally exhibited less anharmonic response with increasing frequency.

Molecular Motions in Organic Crystals

Abstract The piezomodulation spectroscopy of molecular crystals is discussed with particular reference to its application to lattice dynamics and its extension to the understanding of the motions of the molecules in crystls through their displacement by the strain. Results arising from the investigation of the proposed triclinic intergrowth phase in anthracene and the triboluminescence of acenaphthene are presented.