Probing vibrational excitations in molecular crystals by inelastic scattering: From neutrons to X-rays

Inelastic X-ray scattering spectra of single crystalline benzoic acid have been measured as a function of the momentum transfer vector, Q, in the frequency range of 300-1800 cm−1. These measurements are confronted with dispersion curves that have been calculated using force constants derived from periodic DFT calculations in a (2, 2, 1) supercell. Even though limited by the signal-to-noise ratio and the resolution of the spectra, all spectral features and their Q-vector dependence are reproduced by the calculations. A comparison of this technique with inelastic neutron and light (Raman) scattering is made.