In the present study, we report inelastic neutron scattering (INS) measurements from 95% pure solid parahydrogen as function of pressure using the VISION Spectrometer at the Spallation Neutron Source in Oak Ridge National Laboratory. We observe small but appreciable changes in the rotational signal of parahydrogen, around 14.7 meV, however, the effects are very striking on the translational modes (i.e. the vibrational motion of the center of mass of the molecule around its equilibrium position) The INS changes with pressure, the peak rotational peak around 14.7 meV, broadens and can be deconvoluted as an unhindered contribution and a hindered contribution that increases with temperature. More interestingly a feature appearing at 29 meV cannot be accounted for using a single scattering event. The origin of that feature can be an unusual quantum event of one neutron interaction with one molecule and that molecule interacting with a neighboring one. We also quantify the stiffening of the crystal as function of pressure in the shift of phonon to higher frequencies. I will present the results and use them to evaluate a measure of equivalent pressure in confined hydrogen as is the case of molecular hydrogen in Mg(BH4)2 where there is evidence of very short distance between hydrogen molecules. This equivalent pressure can be used to extract an additional independent measure of the distance between these hydrogen molecules.

Study of molecular hydrogen at kbar pressures using inelastic neutron scattering and the case of hydrogen in Mg(BH4)2

255th National Meeting (2018)

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