Diffusion and recombination of H atoms were studied in solid hydrogen containing ortho-H2 molecules at relative concentration Xo=0.001→0.75 using electron spin resonance (ESR), electron-nuclear double resonance (ENDOR), and electron spin echo (ESE) methods at around 4 K. When the rate-determining step for recombination is assumed to be the diffusion of H atoms, the rate constant for recombination at Xo⩾0.1 is consistent with the diffusion coefficient estimated from the analysis of ENDOR spectra and longitudinal spin relaxation behaviors. The recombination rate constant at Xo<0.1, however, is too slow to be explained using the diffusion coefficient estimated from longitudinal spin relaxation and forbidden spin-flip satellite transition studies. This result suggests that, even if one H atom finds another H in its immediate neighborhood, these H atoms do not react to form a H2 molecule at Xo≪0.1. The absence of recombination of H atoms is due to lack of the energy dispersion path required for the recombination of diatomic molecules. Since the absence of recombination becomes less significant at higher Xo, ortho-H2 molecules are found to play an important role in the energy dispersion which accompanies the recombination reaction.

Time-and frequency-resolved coherent anti-Stokes Raman scattering is used to carry out systematic measurements of vibrational dephasing on I 2 (v ¼ 1-19) isolated in solid Kr, as a function of temperature, T ¼ 7-45 K. The observed quantum beats, o v 0 ,v 00 allow an accurate reconstruction of the solvated molecular potential, which is well represented by the Morse form: o e ¼ 211.56 AE 0.14, o e x e ¼ 0.658 AE 0.006. Near T ¼ 7 K, the coherence decay rates g v,0 become independent of temperature and show a linear v-dependence, indicative of dissipation, which must be accompanied by the simultaneous creation of at least four phonons. At higher temperatures, the T-dependence is exponential and the v-dependence is quadratic, characteristic of pure dephasing via pseudo-local phonons. A normal mode analysis suggests librations as the principle modes responsible for pure dephasing.

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Dynamic nuclear polarization (DNP) at low temperature (1.2 K) and high magnetic field (3.3 T) was applied to a contrast variation study in small-angle neutron scattering (SANS) focusing on industrial rubber materials. By varying the scattering contrast by DNP, time-of-flight SANS profiles were obtained at the pulsed neutron source of the Japan Proton Accelerator Research Complex (J-PARC). The concentration of a small organic molecule, (2,2,6,6-tetramethylpiperidine-1-yl)oxy (TEMPO), was carefully controlled by a doping method using vapour sorption into the rubber specimens. With the assistance of microwave irradiation (94 GHz), almost full polarization of the paramagnetic electronic spin of TEMPO was transferred to the spin state of hydrogen (protons) in the rubber materials to obtain a high proton spin polarization (P H ). The following samples were prepared: (i) a binary mixture of styrene-butadiene random copolymer (SBR) with silica particles (SBR/SP); and (ii) a ternary mixture of SBR with silica and carbon black particles (SBR/SP/CP). For the binary mixture (SBR/SP), the intensity of SANS significantly increased or decreased while keeping its q dependence for P H = À35% or P H = 40%, respectively. The q behaviour of SANS for the SBR/SP mixture can be reproduced using the form factor of a spherical particle. The intensity at low q ($0.01 Å À1) varied as a quadratic function of P H and indicated a minimum value at P H = 30%, which can be explained by the scattering contrast between SP and SBR. The scattering intensity at high q ($0.3 Å À1 ) decreased with increasing P H , which is attributed to the incoherent scattering from hydrogen. For the ternary mixture (SBR/SP/CP), the q behaviour of SANS was varied by changing P H . At P H = À35%, the scattering maxima originating from the form factor of SP prevailed, whereas at P H = 29% and P H = 38%, the scattering maxima disappeared. After decomposition of the total SANS according to inverse matrix calculations, the partial scattering functions were obtained. The partial scattering function obtained for SP was well reproduced by a spherical form factor and matched the SANS profile for the SBR/SP mixture. The partial scattering function for CP exhibited surface fractal behaviour according to q À3.6 , which is consistent with the results for the SBR/CP mixture.

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By combining two methods of selective doping of paramagnetic species into a microdomain and small‐angle neutron scattering (SANS), the spatially inhomogeneous proton polarization created by dynamic nuclear polarization (DNP) has been precisely evaluated. A lamella‐forming diblock copolymer composed of polystyrene (PS) and polyisoprene (PI) block chains (PS‐b‐PI) was employed, the SANS profile of which clearly shows scattering peaks up to the third order due to interlamellar interference. As a source of electron spin for DNP, 2,2,6,6‐tetramethylpiperidine 1‐oxyl (TEMPO) was doped into one or other of the microdomains; samples with PS or PI microdomains selectively doped with TEMPO are designated PS.‐b‐PI and PS‐b‐PI., respectively. The SANS intensity at the first‐ and third‐order peaks is well reproduced by assuming that the proton polarization is homogeneous throughout the sample, but that at the second‐order peak cannot be explained by this assumption. This anomaly regarding the second‐order peak was successfully explained by a model postulating that proton polarization in a doped microdomain decreases with increasing distance from the interface with a neighbouring doped microdomain. The decrease in proton polarization at the centre of a doped microdomain was estimated to be 0.07 (2) for PS‐b‐PI. and 0.05 (1) for PS.‐b‐PI, relative to constant proton polarization in a doped microdomain. The inhomogeneous proton polarization results from two competing dynamic processes, i.e. spin diffusion from doped to undoped microdomains, and spin lattice relaxation occurring on the pathway of proton spin diffusion.

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