High-Resolution Inelastic Neutron Scattering from Some Aromatic Molecular Polycrystals for Study of Crystal Excitations and Anharmonic Effects

Abstract

Inelastic neutron scattering spectra have been obtained at high resolution from a number of aromatic polycrystals, mainly halogenated benzenes, in the range 4 cm⁻¹‐1600 cm⁻¹, at room temperature and below. Since the spectra observed reflect only nuclear motion (in contrast to optical spectroscopy), analysis reveals direct information on the harmonicity. In para‐dichlorobenzene the angular dependence of the intensity of incoherent scattering, both elastic and inelastic, the relation between the total inelastic and total elastic intensities, and the temperature dependence of these spectra (and those for other substances) are all consistent with a weakly anharmonic model of the dynamics (phonons which interact with one another to give a temperature dependent lifetime and frequency). Assuming that multiphonon effects are negligible, this weakly anharmonic model is valid in p–C₆H₄Cl₂ up to room temperature, 30°K below the melting point. At low temperatures (100°K) the phonon lifetimes are long enough to allow the observed critical points to be interpreted using quasiharmonic models. At low temperatures many critical points are observed at this high resolution (e.g., 19 for p–C₆H₄Cl₂). In the ``intermolecular'' region in p–C<sub>6</sub>H<sub>4</sub>Cl<sub>2</sub> and 1, 2, 4, 5–C<sub>6</sub>H<sub>2</sub>Cl<sub>4</sub> at 200 cm<sup>−1</sup> and above no dispersion in the modes is observed. In the ``intermolecular'' region, less than 150 cm⁻¹, spectra for p–C₆H₄Cl₂, p–C₆H₄Br₂ and C₁₀H₈ were observed where absolute intensity and main features are well reproduced by Pawley's rigid‐molecule model of lattice motion, but less well than Brillouin Zone center optical data is reproduced by this model.