Transitions between the rotational states J = 1 and J = 0 and the densities of phonon and libron states are investigated by incoherent inelastic neutron scattering at 4.2 and at 1.15°K for several concentrations c of the ortho component. By extrapolation of the observed c‐dependent energy of the J = 1 state to c = 0 and c = 1 the effective quadrupole‐quadrupole coupling constant is determined. The observations on the densities of phonon and libron states are compared to theoretical predictions.

Phonons, Librons, and the Rotational State J = 1 in HCP and FCC Solid Hydrogen by Neutron Spectroscopy

A physically meaningful analogy is drawn between the ordering and dynamical behavior of molecular rotations in solid hydrogen and the ordering and dynamical behavior of magnetic spin systems. In solid hydrogen the magnitude J of the rotational angular momentum is a good quantum number. Molecules having J = 1 resemble spin one magnetic moments, whereas those with J = 0 are ``non‐magnetic.'' Solid solutions of these two species are analogs of magnetic alloys. The thermodynamic properties, in particular the order parameter, the specific heat, and the nuclear magnetic resonance properties are similar to those of magnetic systems. As for the dynamics, the small librations about the ground state are analogs of spin waves, and the interactions between the librational excitations are similar to, but larger than, the usual spin‐wave interactions. This circumstance enables the direct observation of the two‐libron spectrum and also leads to large anharmonic shifts in the single‐libron spectrum.

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Solid Hydrogen: The Ideal Analog of an Antiferromagnet

Publisher Summary This chapter focuses on the spectra of molecular solids, explaining intensities of intra-molecular vibrational spectra The original incentive to the development of the field of infrared spectroscopy of molecular crystals was twofold On one hand, it seemed promising to study the absorption spectra of single-oriented crystals in polarized light to obtain information helpful for the assignment of intra-molecular vibrations Because the molecules in the crystal are rigidly fixed in space, it was thought that it should be possible to relate the results of polarized light investigations to the symmetry of the vibration observed The vibrational exciton theory was based on the electronic exciton theory of molecular solids In this case, the first attempts at quantitative calculations were made for an intermolecular potential that was expanded in a multipole series retaining the first term, the dipole–dipole term It is concluded that for intense infrared transitions, the splitting energies observed are to a large degree accounted for by the dipole interaction approximation

The Spectra of Molecular Solids

We report a systematic study of the longitudinal relaxation timeT
1 for solid H2 samples with ortho concentrationX between 0.15 and 0.70 and for 0.1≲T≲2 K. It is over this temperature region that substantial orientational ordering takes place, which drastically affectsT
1. The measurements were made using the solid echo technique at 9 and 27 MHz. By Fourier-transforming the echo decay, it was possible to investigate the recovery from saturation of the various parts of the NMR absorption line as a function of their distance ν−ν
 L
 from the center at the Larmor frequency ν
 L
 .The dependence ofT
1 on ν−ν
 L
 is particularly noticeable in the hcp disordered (para-orientational) phase at the lowest temperatures, where the NMR absorption is broad, and this observation is qualitatively explained. An attempt is made to understand the general features ofT
1 in the cubic ordered phase and in the hcp para-orientational phase in terms of certain relaxation mechanisms. A comparison with previous work is presented.

Pulsed NMR studies in solid H2. III. Spin-lattice relaxation times

NMR line shape and spin lattice relaxation time of solid hydrogen are measured at 22 MHz for the fractional ortho content of 0.55> X >0.08 and temperature range of 1 K> T >0.2 K. As the sample temperature decreases below 0.35 K, the absorption line changes from singlet to doublet. The phenomenon is observed at ortho contents as small as 0.08. The critical temperature ( T * ), below which the line exhibits the doublet structure, weakly depends on X and is expressed as T * =(0.3±0.1)( X +0.5) K (0.55> X >0.08). The spin lattice relaxation time ( T 1 ) is found to show an anomalous increase by a factor of 10 2 at T ≃ T * . The increase of T 1 with decreasing temperature below T * gives Δ L =0.25±0.05 K for the lower edge of the excited state with J z =±1. Temperature dependence of the field separation of the doublet line is analyzed by using the order parameter formulated by Ueyama and Matsubara. The result gives Δ =1.0±0.2 K for the mean energy gap between the rotational substate with J z =0 and J z =±1.

New Anomalous Phenomena in NMR Absorption and Spin Lattice Relaxation of Solid Hydrogen

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Theory of Rotational Excitations in Solid Ortho-Hydrogen

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Nuclear Magnetic Spin-Lattice Relaxation in Solid Ortho-Hydrogen below T_λ

Low-Lying Rotational Excited States in Solid Ortho-Hydrogen

In this note we point out the difference of the nuclear relaxation mechanism between solid ortho-H2 and para-D2 below T,. It has been investigated theoretically by a number of authors1> that the low-lying rotational excited states of solid ortho-H2 or para-D2 can be described in terms of a quasi-particle of librational excitation; this quasi-particle may be called a libron. The present author estimated the nuclear magnetic spin-lattice relaxation time T 1 of solid ortho-H2 below TA on the basis of the inelastic scattering of librons by nuclear spins.2> The relaxation mechanism assumed is the intramolecular magnetic dipole coupling and 1-J coupling modulated by the intermolecular electric quadrupole interaction which gives rise to librons. However, for para-D1 another relaxation mechanism has to be taken into account in addition to the mechanism assumed for ortho-H2 • This mechanism arises from the coupling between the electric quadrupole moment of deuteron and the field gradient at its position; The perturbation Hamiltonian lf' responsible for the nuclear relaxation for a homonuclear 1 I diatomic molecule in the subspace J=1 can be written as8> (c-1.5d)l·J+3d(l·J)2, (1)

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Shigeo Homma

Papers

Theory of Rotational Excitations in Solid Ortho-Hydrogen

Nuclear Magnetic Spin-Lattice Relaxation in Solid Ortho-Hydrogen below T_λ

Low-Lying Rotational Excited States in Solid Ortho-Hydrogen

Nuclear Magnetic Relaxation in Solid Para-Deuterium