Pressure-enhanced ortho-para conversion in solid hydrogen up to 58 GPa

Abstract

We measured the ortho-para conversion rate in solid hydrogen by using Raman scattering in a diamond-anvil cell, extending previous measurements by a factor of 60 in pressure. We confirm previous experiments that suggested a decrease in the conversion rate above about 0.5 GPa. We observe a distinct minimum at 3 GPa followed by a drastic increase in the conversion rate to our maximum pressure of 58 GPa. This pressure enhancement of conversion is not predicted by previous theoretical treatments and must be due to a new conversion pathway.high-pressure physics ͉ Raman spectroscopy ͉ solid hydrogen S olid hydrogen is a highly compressible quantum molecular solid whose behavior changes drastically over experimentally accessible pressures. Recently, a great deal of attention has been paid to the orientational ordering behavior of solid hydrogen at very high pressures (1-6). Driving this interest is the interpretation of several high-pressure phase transitions as involving orientational ordering, as well as the realization that the insulator-to-metal transition in solid hydrogen depends critically on the orientational properties of high-pressure hydrogen. Crucial to understanding these phenomena is a detailed knowledge of the ortho-para state of the solid at high pressures.The molecular wavefunction for any isolated homonuclear diatomic molecule must be symmetric or antisymmetric under nuclear exchange. For hydrogen the molecular wavefunction must be antisymmetric under proton exchange, which leads to two distinct types of hydrogen: ortho with total nuclear spin I ϭ 1 and para with I ϭ 0. If we restrict ourselves to ground electronic states, the total protonic wavefunction must be antisymmetric and can be written as a product of vibrational, spin, and rotational parts, mol ϭ vib spin rot . vib is always symmetric, and spin is symmetric for the triplet state I ϭ 1 and antisymmetric for the singlet state I ϭ 0. Thus rot must be antisymmetric for ortho-hydrogen (I ϭ 1) and symmetric for para-hydrogen (I ϭ 0). For an isolated hydrogen molecule rot is given by spherical harmonics, Y J mj , with the symmetry restrictions that J is odd for ortho-hydrogen and even for parahydrogen. These species are very stable and do not intermix for isolated molecules. For solid hydrogen the rotational states are only slightly perturbed from the gaseous state, and rot is still accurately described by spherical harmonics.The equilibrium ortho-hydrogen concentration is easily calculated by using the rotational energy of a diatomic simple rotator E rot ϭ BJ(J ϩ 1), where B ϭ 59.3 cm Ϫ1 is the rotational constant for hydrogen (7), and assuming a Boltzmann distribution. The equilibrium concentration of ortho-hydrogen drops from 75% at room temperature, to 61% at 100 K, to 0.1% at 20 K (7). While ortho-para conversion is strictly forbidden for isolated molecules, conversion does occur in the solid. At ambient pressure ortho-para conversion in the solid is very slow, taking weeks for a sample to equilibrate (7,8). Although there have been numerous e...

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

“…No difference is observed, reflecting the fact that, in the corresponding density range, the anisotropic interaction is too weak to structure an orientational order of the liquid. The present data have been obtained on liquids with the normal concentration since below 5 GPa the conversion rate between J = 0 and J = 1 states should be very slow [29,30]. It has been shown that the anisotropic interaction is dominated by the electric quadrupole-quadrupole interaction up to at least 40 GPa and so it should change rapidly under compression, as 5/3 [31].…”

Raman signal from a hindered hydrogen rotor

“…This assumes equilibration of the ort ho/para concentrations, however, the nuclear spins equilibrate on the timescale of a typical experiment [22], and so ort ho-peaks are initially visible even at 10 K. We account for this by redefining the density matrix aŝ…”

Intensity of Raman modes as a temperature gauge in fluid hydrogen and deuterium

“…The quantum mechanical character of H 2 (D 2 ) assures that nuclear spins can be parallel or anti-parallel, resulting in two different species of H 2 (D 2 ), referred to as ortho and para states. 26 Both pressure and temperature have an important role in controlling the population of the different ortho/para species, 29,30 and thus controlling the intensity ratio between the ortho/para rotons. To avoid any influence on the proposed roton thermometry method, intensity ratios between ortho/ortho and para/para rotons were only considered.…”