Polarized Neutrons Observed Nanometer-Thick Crystalline Ice Plates in Frozen Glucose Solution.

Spin-contrast-variation (SCV) small-angle neutron scattering (SANS) is a technique to determine the nanostructure of composite materials from the scattering of polarized neutrons that changes with proton polarization of samples. The SCV-SANS enabled us to determine structure of nanoice crystals that were generated in rapidly frozen sugar solutions by separating the overlapped signals from the nanoice crystals and frozen amorphous solutions. In the frozen glucose solution, we found that the nanoice crystals formed a planar structure with a radius larger than several tens of nanometers and a thickness of 2.5 ± 0.5 nm, which was close to the critical nucleation size of ice crystals in supercooled water. This result suggests that the glucose molecules were preferentially bound to a specific face of nanoice crystals and then blocked the crystal growth perpendicular to that face.

Heterogeneous Aggregation of Humic Acids Studied by Small-Angle Neutron and X-ray Scattering.

Aggregation of humic acids (HAs) was studied by small-angle neutron and X-ray scattering techniques. The combination of these techniques enables us to examine the aggregation structures of HA particles. Two HAs with distinctive compositions were examined: a commercial HA (PAHA) and a HA extracted from deep sedimentary groundwater (HHA). While macroscopic coagulation tests showed that these HAs were stable in solutions except for HHA at pH < 6, small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) revealed that they formed aggregates with sizes exceeding the sub-micrometer length scale. The SAXS curves of PAHA remarkably varied with pD = log aD+, where aD+ stands for the activity of deuterium ions, whereas the SANS curves did not. With the help of theoretical fittings, it was revealed that PAHA aggregates consisted of two domains: poorly hydrated cores and well-hydrated proton-rich shells. The cores were (dis)aggregated with pD inside the aggregates of the shells. The SANS and SAXS curves of HHA resembled each other, and their intensities at low q, where q stands for the scattering vector, increased with a decrease of pD, indicating the formation of homogeneous aggregates within the spatial resolutions of SANS and SAXS. This study revealed that distinctive aggregation behaviors exist in humic substances with nm-scale heterogeneous structures like PAHA, which is important for their roles in the fate of contaminants or nutrients in aqueous environments.

Contrast variation by dynamic nuclear polarization and time-of-flight small-angle neutron scattering. I. Application to industrial multi-component nanocomposites.

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 (PH). 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 PH = -35% or PH = 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-1) varied as a quadratic function of PH and indicated a minimum value at PH = 30%, which can be explained by the scattering contrast between SP and SBR. The scattering intensity at high q (∼0.3 Å-1) decreased with increasing PH, 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 PH. At PH = -35%, the scattering maxima originating from the form factor of SP prevailed, whereas at PH = 29% and PH = 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-3.6, which is consistent with the results for the SBR/CP mixture.

Frequency-resolved optical gating for characterization of VUV pulses using ultrafast plasma mirror switching.

We propose and experimentally demonstrate a method for characterizing vacuum ultraviolet (VUV) pulses based on time-resolved reflection spectroscopy of fused silica pumped by an intense laser pulse. Plasma mirror reflection is used as an ultrafast optical switch, which enables us to measure frequency-resolved optical gating (FROG) traces. The VUV temporal waveform can be retrieved from the measured FROG trace using principal component generalized projections algorithm with modification. The temporal profile of the plasma mirror reflectivity is also extracted simultaneously.

Development of high-efficiency etalons with an optical shutter for terahertz laser pulses.

A high-efficiency etalon operated in the terahertz (THz) frequency region has been proposed to generate a THz pulse train. To achieve high-conversion efficiency to the pulse train, an optical shutter is employed in this etalon. The etalon is composed of a silicon (Si) plate as an input coupler and an indium-tin-oxide (ITO)-coated glass plate as an output coupler. After THz light is introduced into the etalon through the Si plate, the optical shutter pulse irradiates the Si surface to generate a photoconductive layer that acts as a highly reflective mirror for THz light. A THz pulse train and its comb-shaped spectrum have been realized by the use of the proposed etalon with the optical shutter. A finesse F of 9.04 was achieved at the free spectral range of 75 GHz in this etalon.

Dynamic nuclear polarization of electron-beam irradiated polyethylene by pairs of alkyl free radicals.

We compared dynamic nuclear polarization (DNP) behavior of electron-beam irradiated polyethylene (e-PE) with that of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-doped polystyrene (T-PS). Growth and decay rates of nuclear polarization P(n) of e-PE increased linearly with increasing concentration C(e) of free radicals, whereas those of T-PS increased with square of C(e). From these results, we suggest that the pair of alkyl radicals produced by the radiolysis, -CH(2)-CH(2)-CH(2)-→-CH(2)-CH()-CH(2)-+H(), and the subsequent hydrogen abstraction reaction, H()+-CH(2)-CH(2)-CH(2)-→H(2)+-CH(2)-CH()-CH(2)- in e-PE plays an important role on DNP by the cross effect, whereas T-PS is polarized by a pair of unspecified TEMPO radicals accidentally located nearby. We propose that higher P(n) would be expected by optimizing the inter-radical distance within the pair in e-PE and other irradiated samples.

Mesoscale spatial distribution of electron spins studied by time-resolved small-angle and ultrasmall-angle neutron scattering with dynamic nuclear polarization: a case of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) doped in high-density polyethylene.

We carried out time-resolved small-angle neutron scattering (SANS) and ultrasmall-angle neutron scattering (USANS) studies of dynamically polarized high-density polyethylene (HDPE) doped with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) persistent free radicals. We observed a remarkable enhancement of the scattering intensity shortly after a switching of microwave frequency from positive (negative) to negative (positive) dynamic nuclear polarization (DNP). The enhancement was found to be due to spatially heterogeneous proton-spin polarization generated as a result of heterogeneously distributed TEMPO in the HDPE sample. The spatial fluctuation of the polarization ranged up to the length-scale of > or = 100 nm. This result strongly suggests that the TEMPO free radicals are localized more in nonfibrils but less in fibrils of HDPE. In this way, we propose that the time-resolved DNP-SANS and DNP-USANS be general techniques to determine mesoscale spatial distribution of electron spins in dielectric materials.

Negative and positive ion trapping by isotopic molecules in cryocrystals in case of solid parahydrogen containing electrons and H(6) (+) radical cations.

We performed electron spin resonance studies of trapped electrons and H(6) (+) radical cations produced by radiolysis of solid parahydrogen (p-H(2)), p-H(2)-ortho-D(2) (o-D(2)), and p-H(2)-HD mixtures. Yields of trapped electrons, H(6) (+) radical cations, and its isotopic analogs H(6-n)D(n) (+) (4>or=n>or=1) increased with increasing o-D(2) and HD concentrations in solid p-H(2). Electrons were found trapped near an o-D(2) or an HD in solid p-H(2) due to the long-range charge-induced dipole and quadrupole interactions between electrons and isotopic hydrogen molecules. H(6) (+) radical cations diffuse in solid p-H(2) by repetition of H(6) (+)+H(2)-->H(2)+H(6) (+) and are trapped by ortho-D(2) or HD to form H(6-n)D(n) (+) (4>or=n>or=1) as isotope condensation reactions. Decay behaviors of these cations by the repetition, isotope condensation, and geminate recombination between electrons and H(6-n)D(n) (+) (4>or=n>or=0) were reproduced by determining the corresponding reaction rate constants k(1), k(2), and k(3). Values of 0.045 and 0.0015 L mol(-1) min(-1) were obtained for k(1) (H(6) (+)+D(2)-->H(2)+H(4)D(2) (+)) and k(2) (H(4)D(2) (+)+D(2)-->H(2)+H(2)D(4) (+)), respectively, and the value was quasinull for k(3) (H(2)D(4) (+)+D(2)-->H(2)+D(6) (+)). These rate constants suggest that hole mobility drastically decreased in the repetition reaction when H(6) (+) radical cations acting as hole carriers formed H(4)D(2) (+) or H(2)D(4) (+). HD and D(2) molecules, therefore, act as electron and hole acceptors in irradiated solid p-H(2)-o-D(2) and p-H(2)-HD mixtures.

Electron spin resonance spectroscopy of molecules in large precessional motion: a case of H6(+) and H4D2(+) in solid parahydrogen.

We have measured electron spin resonance (ESR) spectra of H6+ and H4D2(+) ions produced in gamma-ray irradiated solid parahydrogen. Anisotropic hyperfine-coupling constants for H6(+) and H4D2(+) determined by the analysis of ESR lines at 4.2K were -0.06 and -0.12 mT, respectively, which were opposite in sign to and much smaller than theoretical results of 1.17-1.25 mT. Although no change was observed in H6(+), the constant for H4D2(+) increased to be 1.17 mT at 1.7 K, which is very close to the theoretical value. We concluded that H6+ both at 4.2 and 1.7 K and H4D2(+) at 4.2K should be in a large precessional motion with the angle of 57-59 degrees, but the precession of H4D2(+) is stopped at 1.7 K.

Electron spin resonance study on H6+, H5D+, H4D2+, and H2D4+ in solid parahydrogen.

We carried out an electron spin resonance (ESR) study on hydrogen ion radicals produced by radiolysis of solid para-H(2). In addition to quartet ESR lines proposed to be H(2) (+)-core H(6) (+) (D(2d)) ions in solid para-H(2) [T. Kumada et al., Phys. Chem. Chem. Phys. 7, 776 (2005)], we newly observed totally more than 50 resolved lines in gamma-ray irradiated solid para-H(2)-ortho-D(2) (1 mol %) and para-H(2)-HD (1 mol %) mixtures. We assigned these lines to be isotope substituents of H(2) (+)-core H(6) (+) ions such as H(5)D(+), H(4)D(2) (+), and H(2)D(4) (+) throughout the comparison of their ESR parameters with theoretical results. These results provide a conclusive evidence that H(2) (+)-core H(6) (+) ions are generated in irradiated solid hydrogens. Analysis of the EPR spectrum and ab initio calculations predicts D(2d) symmetry of the H(6) (+) ions, whereas a lowering symmetry (D(2d)-->C(2v)) induced by asymmetric nuclear wave function is observed in H(5)D(+) and H(4)D(2) (+). We also observed isotope-substitution reactions such as H(6) (+)+D(2)-->H(4)D(2) (+)+H(2) and H(6) (+)+HD-->H(5)D(+)+H(2), which are analogous to the well-known isotope-condensation reactions of H(3) (+) in dark nebula, H(3) (+)+HD-->HD(2) (+)+H(2) and HD(2) (+)+HD-->D(3) (+)+H(2).

Tunneling chemical reactions D + H2 --> DH + H and D + DH --> D2 + H in solid D2-H2 and HD-H2 mixtures: an electron-spin-resonance study.

Tunneling chemical reactions D + H2 --> DH + H and D + DH --> D2 + H in solid HD-H2 and D2-H2 mixtures were studied in the temperature range between 4 and 8 K. These reactions were initiated by UV photolysis of DI molecules doped in these solids for 30 s and followed by measuring the time course of electron-spin-resonance (ESR) intensities of D and H atoms. ESR intensity of D atoms produced by the photolysis decreases but that of H atoms increases with time. Time course of the D and H intensities has the fast and slow processes. The fast process, which finishes within approximately 300 s after the photolysis, is assigned to the reaction of D atom with one of its nearest-neighboring H2 molecules, D(H2)n(HD)(12-n) --> H(H2)(n-1)(HD)(13-n) or D(H2)n(D2)(12-n) --> H(HD)(H2)(n-1)(D2)(12-n) for 12 > or = n > or = 1. Rate constant for the D + H2 reaction between neighboring D atom-H2 molecule pair is determined to be (7.5 +/- 0.7) x 10(-3) s(-1) in solid HD-H2 and (1.3+/-0.3) x 10(-2) s(-1) in D2-H2 at 4.1 K, which is very close to that calculated based on the theory of chemical reaction in gas phase by Hancock et al. [J. Chem. Phys. 91, 3492 (1989)] and Takayanagi and Sato [J. Chem. Phys. 92, 2862 (1990)]. This rate constant was found to be independent of temperature up to 7 K within experimental error of +/-30%. The slow process is assigned to the reaction of D atom produced in a cage fully surrounded by HD or D2 molecules, D(HD)12 or D(D2)12. This D atom undergoes the D + DH reaction with one of its nearest-neighboring HD molecules in solid HD-H2 or diffuses to the neighbor of H2 molecules to allow the D + H2 reaction in solid HD-H2 and D2-H2. The former is the main channel in solid HD-H2 below 6 K where D atoms diffuse very slowly, whereas the latter dominates over the former above 6 K. Rate for the reactions in the slow process is independent of temperature below 6 K but increases with the increase in temperature above 6 K. We found that the increase is due to the increase in hopping rate of D atoms to the neighbor of H2 molecules. Rate constant for the D + DH reaction was found to be independent of temperature up to 7 K as well.

H6+ in irradiated solid para-hydrogen and its decay dynamics: reinvestigation of quartet electron paramagnetic resonance lines assigned to H2-.

The quartet electron paramagnetic resonance (EPR) lines observed in gamma- and X-ray irradiated solid para-H2, which have previously been assigned to H2-, are reinvestigated. We have reassigned the quartet lines to H6 rather than H2- mainly due to comparison of experimentally obtained EPR parameters to theoretical results. Based on the new assignment, trapping site, rotation, ortho-para conversion, quantum diffusion and isotope effect of H+ have been reinterpreted by the precise reanalysis as follows. The H6+ ion is composed of the collinearly aligned H2+ core at the center and two H2 rotors at both ends, occupies a single substitutional site, and has a precession motion around a crystalline axis with the angle of approximately 57 degrees. The ortho-para conversion of H2+ core of H6+ is completed within the time-scale of hours, whereas ortho-H2 molecules near H6+ convert much faster. H6+ diffuses quantum mechanically by the repetition of H6+ + H2 --> H2 + H6+ reaction. The diffusion terminates by the reaction, H6(+) + HD --> H5D(+) + H2, with a HD impurity contained in the para-H2 sample at natural abundance. Finally, we will propose a possible reason why H6+ is produced instead of H3+ in the irradiated solid H2.

Vibrational dissipation and dephasing of I2 (v = 1-19) in solid Kr.

Time- and frequency-resolved coherent anti-Stokes Raman scattering is used to carry out systematic measurements of vibrational dephasing on I2 (v = 1-19) isolated in solid Kr, as a function of temperature, T = 7-45 K. The observed quantum beats, omega(v', v") allow an accurate reconstruction of the solvated molecular potential, which is well represented by the Morse form: omega(e) = 211.56 +/- 0.14, omega(e)chi(e) = 0.658 +/- 0.006. Near T = 7 K, the coherence decay rates gamma(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.