Observation of Librational Waves in the Ordered State of Solid Hydrogen and Deuterium by Raman Scattering

Abstract

conduction band. Each time this happens, as the temperature is raised for example, a magnetic ion is formed in the lattice. At higher temperature the extra entropy associated with the magnetic ion degeneracy favors its formation and the Sm becomes predominantly trivalent. While such a model is conceptually quite clear, its quantitative application to magnetic behavior is complicated by the uncertainty of the crystalfield splittings of the low-lying levels of the trivalent Sm ion. It may be surprising that the two configurations are so close in energy but actually we were more surprised that it was possible to form polycrystalline SmB 6 so well by simply reacting the elements that its intrinsic properties were not masked.We are grateful to R. C. Sherwood and G. W. Hull, Jr., for help with the magnetic measurements and W. A. Reed for the use of his laboratory. We have profited from useful conversation with J. H. Wernick, H. J. Levinstein, and R. M. White.tThe research at Stanford was sponsored by the U. S, We have observed 1 by laser Raman scattering the theoretically predicted 2 " 5 librational excitations of the ordered state of solid orthohydrogen and paradeuterium. The observations agree only qualitatively with existing theories and two possible explanations for the discrepancies are proposed. The first implies that the rigid-lattice, nearest-neighbor quadrupole-interaction model is inadequate in the present case. The second alternative explanation requires a crystal structure with symmetry lower than the previously assumed 2 " 5 PaZ space group.The ordered state of hydrogen is one in which the molecular axes assume preferential orientations with respect to the crystalline axes, analogous to the ordering of spins in an antiferromagnet. Because the anisotropic interactions are Air Force November 1968 (unpublished).small compared with the separation of the J levels and because the temperature is low, only the J=l manifold need be considered in solid o-H 2 and />-D 2 and therefore it is the axes of the J=l molecular rotational wave functions that are ordered.X-ray and neutron data 6 have shown that the molecular centers form an fee lattice. Theories 7 based on such a rigid fee lattice and electric quadrupole-quadrupole inter molecular coupling predict the structure of the ground state to be Pa3; the neutron-diffraction data are also consistent with this space group. In this structure the axes of the dumbbell-shaped raj = 0 wave functions are aligned along the four fee body diagonals. The crystal is divided into four sublattices with the oriented molecules on a given sublattice The Raman spectra of high purity o-H 2 and £-D 2 at liquid-helium temperatures have been obtained. Features which appear below the ordering temperature are attributed to librational waves within the J~ 1 manifold of states. On the basis of a comparison with recent theories of the ordered state, a new structure is proposed.297

New Ortho-Para Conversion Mechanism in Dense Solid Hydrogen

“…By definition, E c is the difference between the conversion energy in the gas, E c0 2B 170.5 K, and the energy per J 1 molecule in the ground state. Using values deduced from measurements [21] (in particular, the libron frequencies obtained by Raman scattering at ambient pressure [22]), we find that the mechanism works from j 0 Ӎ 3.2 to j c Ӎ 4.9 (Fig. 2).…”

Observation of Libron-Libron Interactions in Solid Hydrogen

“…Accordingly, a distortion to a lower symmetry structure was suggested. 17 From x-ray work 18 a similar distortion has been suggested for solid N 2 . However, the distortion in solid N 2 has not been confirmed by subsequent optical data, 19 and hence its existence is uncertain.…”

“…17 However, the interpretation of this spectrum has been unclear for two reasons. First, there were observed five lines in the Raman spectrum instead of three as predicted from theory.…”

Far-infrared absorption by phonons and librons in solid H2 and D2 in the ordered state

“…Both experiments suggest that the order-disorder transition is first order in D2. 8. By comparing the infrared spectra of J = 1 and J = 0 hydrogen we have been able to define and separate out a broad apparent absorption which does not arise from the EQ-induced mechanism.…”