Lattice vibrations of para-hydrogen impurities in a solid deuterium matrix: An inelastic neutron scattering study

In the present study, we report inelastic neutron scattering measurements from para-hydrogen defects in solidnormal deuterium at three different concentrations (between 3% and 11%) using the time-of-flight spectrometerTOSCA-II. The measured double-differential cross sections give access to the self-inelastic structure factors forthe H2 centers of mass. Corrected experimental data, analyzed through the Young-Koppel model and theGaussian approximation, are transformed into defect densities of phonon states, which come out to be broad,structured, and nearly concentration independent. Two experimentally determined Bose-corrected spectral momentsare found to be in agreement with independent estimates, providing a strong validation of our datareduction procedure. Subsequently, experimental phonon spectra are compared to three calculations, namely, asimple harmonic model at infinite dilution, a more advanced harmonic model with concentration effects, andfinally a lattice dynamics simulation based on self-consistent phonon and coherent potential approximations.However, while the first part of the defect spectral density, attributed to the propagating modes, turns out to beroughly explained, the localized part is properly described by none of these models, except for its meanfrequency position. The large overall width appears so far impossible to be reproduced, representing a challengefor the physicists involved in quantum dynamics simulations.