Microscopic structure of liquid hydrogen

Abstract

Hydrogen makes the simplest molecular liquid. Nonetheless, due to several different reasons, measuring its microscopic structure has been one of the most challenging tasks in neutron diffraction experiments. The recent development of modern pulsed neutron sources triggered a renewed experimental interest which, in turn, led to new knowledge and also to a more effective use of the classic reactor-based experimental data. The contemporary development of quantum mechanical computer simulation techniques, and a critical comparison among the results of different experiments using steady and pulsed neutron sources, resulted in a quantitatively reliable solution of the problem.

Liquid hydrogen structure factor to 5 GPa and evidence of a crossover between two density evolutions

“…2, the S(Q) measured here at the lowest density is in good agreement with previous neutron determinations obtained near the critical point, which show a maximum value of 2.17 and 2.4 for hydrogen and deuterium, respectively [1,4]. The larger intensity of the MDP of liquid deuterium was interpreted as a signature of a more extended structure and attributed to the smaller zero-point motion of the deuterium molecule with respect to that of hydrogen [22]. In contrast here, no difference in the shape of the main peak between the S(Q) of liquid H 2 and of liquid D 2 is detected within the accuracy of our measurements at similar P -T conditions.…”

“…An isotopic shift is also observed with slightly larger Q m for D 2 than for H 2 under the same P -T conditions. A similar isotopic shift was already reported in previous measurements performed at low temperature near the critical point [6,22]. In dense simple liquids, it is empirically found that Q m is related to the density through the relation Q m ≈ 4.4(4πρ/3) 1/3 [23].…”

Neutron diffraction determination of the microscopic structure of solid deuterium close to melting

“…37 The effective values used here were DW = 0.036 83 Å and d e = 0.741 04 Å. 41 In this way one obtains results virtually indistinguishable from those provided by the application of the well-known Young-Koppel model 42 for homonuclear diatomic molecules, which, however, requires slightly heavier computational efforts. In order to extract the CoM static structure factor, S͑Q͒, the intramolecular scattering has to be subtracted from the measured single-differential cross section and the division by the intermolecular form factor has to be carried out.…”

Quantum Molecular Dynamics Simulation of Condensed Hydrogens by Nuclear and Electron Wave Packet Approach

“…Although hydrogen is the simplest of all molecular species, its nuclear quantum effects (NQEs) dominate the structure and thermodynamical properties of condensed phases, making it difficult to elucidate them [1][2][3][4][5][6][7]. The NQEs of liquid hydrogen appear as broader radial distribution functions (RDFs), large self-diffusion coefficients even at low temperatures, and anomalous temperature dependence of thermal conductivity and shear viscosity [8][9][10][11][12][13][14][15].…”