Probing Vibrational Excitations in Molecular Crystals by Inelastic X-ray Scattering


Benzoic acid is a model system for studying molecular vibrations and proton transfer in hydrogen-bonded dimers. An inelastic X-ray experiment was intended to complement neutron scattering and Raman measurements and ab initio phonon calculations in the high frequency range (> 1000 cm-1).

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IXS spectra of single crystalline benzoic acid were therefore recorded as a function of the momentum transfer vector, Q, in the frequency range 300 to 1800 cm-1. The measurements were compared with results from density functional theory. Despite the limited signal-to-noise ratio, all spectral features and their Q-vector dependence are well reproduced. Considerable improvements can be envisaged by relaxing the Q-resolution, and most importantly, by increasing the solid angle of detection of the scattered photons. These developments are within reach, and will render IXS a viable technique for the study of molecular vibrational modes, especially in the frequency range above 1000 cm-1, where neutron techniques are limited in terms of energy resolution. It may even become possible to measure the O-H stretching modes and their dispersion, which are a direct probe of the hydrogen bond.

IXS spectra of single crystalline benzoic acid

Fig. 1: Calculated (top panels) and experimental (bottom panels) IXS spectra of single crystalline benzoic acid for Q=(0, 3+x, 0). Left: x=-0.2, 0, 0.19, 0.38, 0.58, from bottom to top. Right: x=-0.14, 0.06, 0.25, 0.44, 0.64, from bottom to top.

M. Plazanet (a), A. Beraud (b), M. Johnson (a), M. Krisch (b), H.P. Trommsdorff (c)
(a) ILL
(b) ESRF
(c) Laboratoire de Spectrométrie Physique, Université J. Fourier - CNRS (UMR 5588), Grenoble (France)