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Abstract. The anomalous dispersion found by inelastic neutron scattering in one of the vibron bands in solid tetracyanoethene appears to be due to the electrostatic interactions that couple the infrared-active intramolecular vibrations via the transition dipole resonance mechanism. This is demonstrated by way of both numerical lattice dynamics calculationsand a simple mathematical analysis, which relates the Fourier components of the dispersion curves in specific directions of the wavevector k to two-dimensional lattice sums over the dipole-dipole interactions between layers perpendicular to k. Only for specific orientations of the molecular transition dipole moments in a solid and for specific directions of k will this lead to a dispersion curve of irregular shape.

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Using the H 2-H 2 potential from ab initio calculations and the isotropic H 2-graphite potential from selective adsorption measurements, we have calculated the stability and lattice dynamics of o-H 2 and p-D2 monolayers on graphite. An anisotropic H 2-graphite potential was modelled such that either the herringbone or the pinwheel ordering of the adsorbed layers is stable, at very low temperature. For both these orientationally ordered overlayers we have computed the phonon and libron band structure by the time-dependent Hartree method. We find small coupling between the translational phonons and the librons, which can be well taken into account by renormalisation of the appropriate coupling constants. In the pinwheel structure, observed for o-H 2 monolayers by NMR, the differences between the axial and the wheel molecules are considerably smaller than in the corresponding structure for N-, layers on graphite.

Abstract. Lattice dynamics calculations of the lattice modes (phonons) and the internal modes (vibrons) and Raman spectra at liquid helium temperatures are presented for the / 3 a-and y-phase of 1,4-dichlorobenzene (d c b ). It follows from the calculations and it is confirmed by the Raman spectra that these three phases are characteristically different with respect to the following properties: vibron band dispersion, Davydov splitting, 35CI/37C1 isotopic effects on the bandstructure and vibron-phonon mixing. Only the low temperature y-phase shows a nice one-dimensional band structure for some vibrations. The vibron band dispersion in all phases of d cb is caused by the repulsive exponential terms in the intermolecular potential. This is in contrast to 1,2,4,5-tetrachlorobenzene, where the elec trostatic interactions are responsible for the dispersion. Some of the ungerade vibron modes become visible in the Raman spectra of dcb due to the (random) occurrence of molecules with different distributions of 35C1/37C1 isotopes. A few of the Raman lines observed could not be uniquely assigned; they are probably caused by Fermi resonances.