Hydrogen self-dynamics in liquid H(2)-D(2) mixtures studied through inelastic neutron scattering.
We have measured the dynamic structure factor of liquid para-hydrogen mixed with normal deuterium (T=20 K) at two different concentration levels using incoherent inelastic neutron scattering. This choice has been made since the presence of D(2} modifies the self-dynamics of H(2) in a highly nontrivial way, acting both on its pseudophononic and its diffusive parts in a tunable way. After an accurate data reduction, recorded neutron spectra were studied through the modified Young and Koppel model and the H(2) center-of-mass self-dynamics structure factor was finally extracted for the two mixtures. Some physical quantities (i.e., self-diffusion coefficient and mean kinetic energy) were determined and compared with accurate quantum calculations, which, in addition, also provided estimates of the velocity autocorrelation function for the H(2) centers of mass. These estimates, in conjunction with the Gaussian approximation, were used to simulate the H(2) center-of-mass self-dynamics structure factor in the same range as the experimental one. The agreement between measured and calculated spectra was globally good, but some discrepancies proved the unquestionable breakdown of the Gaussian approximation in these semiquantum systems at a level comparable to that already observed in pure liquid para-hydrogen.
Consiglio Nazionale delle Ricerche, Istituto dei Sistemi Complessi, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy.
Daniele
D
Colognesi
Consiglio Nazionale delle Ricerche, Istituto dei Sistemi Complessi, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy.
Ubaldo
U
Bafile
Consiglio Nazionale delle Ricerche, Istituto dei Sistemi Complessi, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy.
Milva
M
Celli
Fakultät für Physik der Universität Wien, Strudlhofgasse 4, A-1090 Wien, Austria.
Martin
M
Neumann
Physics and Geology Department, University of Perugia, Via Pascoli, I-06123 Perugia, Italy.|Institut Laue-Langevin, 71 av. des Martyrs, F-38042 Grenoble Cedex 9, France.
Andrea
A
Orecchini