Shoji Hirokawa

Theory of Polycrystalline Electron Paramagnetic Resonance Spectra of the Oxygen Molecule in Librational Motion

The energy levels of the O 2 molecule subject to a hindering potential are calculated on the basis of the Hamiltonian \(\mathscr{H}=BN^{2}+CP_{2}(\cos\theta)+D\left({S_{z}}^{2}-\frac{1}{3}S(S+1)\right)+g\mu_{B}\mbi{S}\cdot\mbi{H}\). Here, the z axis is chosen as the internuclear axis of the molecule; N is the angular momentum of nuclear rotation; θ is the angle between the z axis and the axis of the molecule at equilibrium. It is shown that the coupling of the spin to the librational motion largely affects the magnetic splitting of energy levels. The resonance peaks in polycrystalline line shape at X band are obtained as a function of c , where c =(-3 C /2 B ) 1/2 . The temperature dependence of the line shape is discussed, and a comparison with EPR experiments on O 2 in solid N 2 is tried.

Quantum Effects on the Orientational Ordering of H2S and D2S Molecules Enclosed in β-Quinol Clathrate

A system of planar rotators representing H 2 S and D 2 S molecules is investigated quantum-statistically, using the mean-field approximation based on an eight-sublattice model. The equilibrium molecular orientation is determined by competition among the dipole-dipole interaction between guest molecules, the host-guest interaction and the quantum kinetic energy. The system has a complex order parameter describing an average direction of dipoles. Both R\bar1 and R\bar3 structures and the effect of the depolarization field are studied. A comparison with experiment by use of theoretical phase diagrams gives: the low-temperature phase has the R\bar1 structure with an antiferroelectric order of dipoles; the direction and arrangement of dipoles in equilibrium for H 2 S significantly differ from those for D 2 S because of the quantum nature of the rotational motion.

Theory of the Electron Paramagnetic Resonance of Oxygen Impurity in Solid Nitrogen

The effective zero-field splitting parameters, \(\tilde{D}_{lm}\), of the O 2 molecule in the states of librational motion, \(\varPsi_{lm}\), are calculated on the assumption that the molecule is subjected to a hindering potential C P 2 (cos θ). According to experiments on specific heats, C =-306.6 K, \(\tilde{D}_{1,0}{=}107\) GHz and \(\tilde{D}_{1,1}{=}95.0\) GHz. The temperature dependence of the polycrystalline absorption line shape is investigated on the basis of the isotropic g -factor. The main-peak position is 11.56 kG at X band, and 19.33 kG at K band. Small peaks owing to the O 2 molecules in \(\varPsi_{1,1}\) appear if T > 11 K: the positions at X- and K-bands are 11.0 kG and 18.4 kG, respectively. Calculated results account for paramagnetic resonance experiments.

Orientational Order of Guest H2S Molecules in β-Quinol Clathrate

The orientational ordering of guest H 2 S molecules is studied using a mean-field theory on the basis of quantum statistical mechanics; it is assumed that the guest system is a quenched one consisting of a normal mixture of ortho and para H 2 S, and that the guest molecules which are influenced by a crystalline field, of which strength is estimated from far-infrared spectra, couple with one another via dipolar interactions. A phase diagram based on an effective dipole moment p is obtained. It shows: If p =0.800D, a second-order transition between a partially disordered and a disordared phase occurs at T =15.2 K, and the temperature dependence of the heat capacity well coincides with experiment; if p <0.778D, no cooperative order is expected.

Effects of a Crystal Field on the Orientational Phase Transition in a System of Dipoles: A Mean-Field Study of SO2-β-Hydroquinone Clathrate

The orientational ordering in a system of dipolar molecules located on a trigonal lattice is studied in the eight-sublattices approximation. Our concrete example is SO 2 molecules enclosed in β-hydroquinone clathrate. It is assumed that the molecules interact with each other via effective dipole-dipole interactions, and that the crystal field acting on those molecules is of the form W A = C cos[ ν( φ- δ A ) ], where φ is a rotational angle and A is a sublattice number. If ν is an odd integer, we assume that δ A may be different for different A . The mean electric field which depends on the depolarization field is obtained. Various cases of C , ν, δ A are investigated. Results are as follows: the possible dipole arrangement is antiferroelectric in all cases; the phase transition for ν=2, 4, 5, 6 is of second order; if ν= 1, no disordered phase occurs; if ν= 3, the phase transition is weakly of first order except for special cases; the C dependence of the transition temperature is weak unless ν= 2; δ A chie...

On The Nuclear Pure Quadrupole Resonance in Solid α-Nitrogen

The 14 N– 14 N PQR spectrum obtained by Brookeman, McEnnan and Scott [Phys. Rev. B 4 (1971) 3661] is interpreted as the superposition of the spectra by both nuclear spin species, o - and p -N 2 . The effect of the correlation-corrections to the Hartree approximation for molecular libration on the electric field gradient tensor is studied in the α-phase of p -N 2 . The study is based on the electric quadrupole-quadrupole intermolecular potential with molecular quadrupole moment, e Q m . The predicted value of | e Q m | agrees with the value obtained from Raman frequencies.