Inelastic neutron scattering from molecular and crystal excitations in aromatic molecular crystals

Abstract

High resolution incoherent neutron scattering spectra from polycrystalline para-dichlorobenzene and 1,2,4,5-tetrachlorobenzene have been obtained. Scattering from the hydrogen motions is dominant and the relationship between the neutron spectra and optical spectra is described using the extensive normal coordinate and force field calculations available for the molecular motions. Simulation of the neutron spectra from optical data tests the harmonic approximations used in both the force field calculations and in expanding the neutron scattering law. The conclusions give a guideline for interpreting the sometimes strong angular dependence of the neutron elastic and inelastic peak intensities in terms of molecular mode vibrational amplitudes.The crystal vibrations observed by neutron scattering for these systems have been fitted by a rigid molecule, single force constant per mode scheme as a first approximation and comparisons with the lattice spectrum of para-diiodobenzene have been used to assign modes in the density of states spectrum. The use of combined neutron and optical spectroscopy to obtain vibrational band assignments is discussed.