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We are developing time-resolved grazing-incidence small-angle scattering (GI-SAXS) system using a soft X-ray laser. As a first step, we carried out GI-SAXS measurements of optical grating and craters produced on gold surface by femtosecond laser ablation . We succeeded in observing the diffraction spots from the grating as expected, but the diffuse scattering from the craters was hidden in thermal noises of a soft X-ray charge-coupled device camera.

We propose and numerically simulate a method of laser isotope separation based on field-free alignment of isotopologues, utilizing an intense “switched” nanosecond (ns) laser field which is slowly turned on and rapidly turned off at the peak with the falling time of 200 fs. The femtosecond (fs) laser-induced alignment of isotopologues including heavy isotopes is severely disturbed by ionization because of their small ionization potential. Our simulations for I79Br and I81Br isotopologues demonstrate that the switched ns laser field can make isotopologues well-aligned with the reduced ionization probability at the laser intensity [I 0 = 2 TW/cm2 (= 2 × 1012 W/cm2)] which is an order-of-magnitude lower than a typical intensity (I 0 = 20 TW/cm2) for field-free alignment induced by a fs laser field. In our simulations, I i Br isotopologues (i = 79 or 81) aligned with the switched ns laser field are selectively ionized by an intense fs laser pulse at an appropriate time delay from the alignment pulse.

We demonstrate that the methodology of frequency-resolved optical gating (FROG) is applicable to time-resolved reflection spectroscopy of a plasma mirror in the vacuum-ultraviolet (VUV) region. Our recent study [R. Itakura et al. Opt. Express 23, 10914 (2015)] has shown that a VUV waveform can be retrieved from a VUV reflection spectrogram of a plasma mirror formed on a fused silica (FS) surface by irradiation with an intense femtosecond laser pulse. Simultaneously, the increase in the reflectivity with respect to the Fresnel reflection of the unexcited FS surface can be obtained as a time-dependent reflectivity of the plasma mirror. In this study, we update the FROG analysis procedure using the least-square generalized projections algorithm. This procedure can reach convergence much faster than the previous one and has no aliasing problem. It is demonstrated that a significantly chirped VUV pulse as long as 1 ps can be precisely characterized.

We demonstrate laser nitrogen isotope separation, which is based on field-free alignment and angular-dependent ionization of 14N2 and 15N2 isotopologues. A linearly polarized short laser pulse (λ ~ 795 nm, Δτ ~ 60 fs) creates rotational wave packets in the isotopologues, which periodically revive with different revival times as a result of different moments of inertia. Another linearly polarized short laser pulse (λ ~ 795 nm, Δτ ~ 60 fs) ionizes one of the isotopologues selectively as a result of their different angular distributions. In the present experiments, the ion yield ratio R [=I(15N2 +)/I(14N2 +)] can be changed in the range from 0.85 to 1.22, depending on the time delay between the two laser pulses.

We carried out a dynamic nuclear polarization (DNP) study of high- and low-crystallinity polyethylene films defined as HPE and LPE, respectively, which were doped with TEMPO (2,2,6,6-tetra-methyl-piperidine-1-oxyl) by the same amount (1×10 19 spins/cm 3 ). The proton polarization of 32±3% was obtained for LPE and 23±4% for HPE at 3.35 T and 1.4 K. With increase in the concentration of TEMPO, the electron spin resonance (ESR) linewidth of TEMPO in HPE increased more steeply than that in LPE, indicating that TEMPO did not incorporated into the crystalline part, and was localized only at the amorphous part, hence the local concentration of TEMPO in the amorphous regions of HPE varied more steeply than that of LPE with the net amount of TEMPO fed into the film. We suggest that the smaller nuclear polarization for HPE is related to the localization of TEMPO in the smaller amorphous regions.

Behavior of the hydrogen isotope in recombination reactions, H+H→H2 H+T→HT and T+T→T2 under pressurized conditions at 1.6 K were investigated using liquid helium as cryogenic reaction media by radiochemical methods. Pressure effects were observed in changes of isotope ratio HT/T2. It became clearer that the balance of changes in bubble energy effective mass associated with energy and chemical potential contributed to the recombination reactions through bubble states. It was suggested that bubble radius was decreased significantly at higher pressure.

We carried out electron spin resonance (ESR) study of irradiated solid H 2 and para -H 2 . H-atom, e − , and H 6 + lines were observed. The H-atom radicals diffuse in solid H 2 by the tunneling chemical reaction H+H 2 →H 2 +H below 4 K, and by the thermally activated process above 5 K. These decay via H–H recombination. Yield of trapped electrons e − produced by the radiolysis of solid para -H 2 increases with increase in the concentration of added isotopic hydrogen, HD or D 2 . These decay via geminate recombination with positive ions with a half-life of 8 h at 4 K. H 6 + ion is an ionic hydrogen radical whose spectrum has firstly been observed in irradiated solid para -H 2 . H 6 + ions diffuse in solid H 2 by sequence of H 6 + +H 2 →H 2 +H 6 + reactions, and terminate by the isotope-exchange reaction H 6 + +HD→H 5 D + +H 2 or H 6 + +D 2 →H 4 D 2 + +H 2 . Unlike H 6 + , isotopically substituted H 5 D + and H 4 D 2 + cannot diffuse in solid para -H 2 . Rate-determining step of their decay is charge recombination with e − .

We have measured electron spin resonance (ESR) spectra of H 6 + and H 4 D 2 + ions produced in γ-ray irradiated solid parahydrogen. Anisotropic hyperfine-coupling constants for H 6 + and H 4 D 2 + determined by the analysis of ESR lines at 4.2 K 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 H 6 + , the constant for H 4 D 2 + increased to be 1.17 mT at 1.7 K, which is very close to the theoretical value. We concluded that H 6 + both at 4.2 and 1.7 K and H 4 D 2 + at 4.2 K should be in a large precessional motion with the angle of 57–59°, but the precession of H 4 D 2 + is stopped at 1.7 K.

The behavior of recombinations of H+H→H2, H+T→HT and T+T→T2 in liquid helium at cryogenic temperature of 1.6 K was investigated under pressurized condition using radiochemical methods. It was found that the isotope ratios of H2, HT and T2 were changed with the pressure. The changes will contribute to the bubble formation, chemical potential and the effective masses of H and T. This work is a preliminary information to carry out chemical reactions in liquid helium.

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.

The recombination reactions of thermal H and T atoms, H+T→HT and T+T→T2, have been investigated using radiochemical methods at 1.4-2.5 K in a mixture of 3He-4He. The reactive species were produced via 3He(n,p)T by neutron irradiation at the upgraded Japan Research Reactor No. 3 (JRR-3M). Nonstatistical formation of ortho-T2 was observed over the entire range of 3He atomic fraction at 1.4-2.5 K. The relative yield of T2 decreased with an increase in superfluid component, which is a second liquid phase observed only in liquid helium, at 1.6 K.

X-band electron spin resonance (ESR) spectroscopy has been applied to the study of molecular rotation of O2 molecules in isotopic solid hydrogen, D2 and HD. ESR signal of the O2 molecules in hindered rotational states has been observed, and its pressure dependence has been measured up to 19 MPa. Although molar volume of solid hydrogen decreases, the O2 rotation has become less hindered with the increase in pressure and isotope substitution from HD to D2. These effects of pressure and isotope substitution suggest that the potential barrier for the O2 rotation is mainly produced by distortion of O2 cages in D2 and HD. Since solid hydrogen becomes less compressible with the increase in pressure and the isotope substitution from HD to D2, the O2 rotation becomes less hindered in the less distorted O2 cages.

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

During the course of an investigation on chemical behaviors of hydrogen isotopes in quantum media, non-statistical formation of ortho -T 2 was observed in recombination reaction of T + T + 3,4 He → T 2 + 3,4 He in liquid 3 He– 4 He mixture at 1.42–2.50 K. The experimental ortho -T 2 fractions are larger than 0.9. The values are considerably higher than the equilibrium fraction of 0.75 at room temperature. Similar results have been reported in H–H recombination at low temperature. These facts suggest that similar reaction mechanism would be applied to both H and T atoms, which have the same nuclear spin number of 1/2.

High-resolution electron spin resonance (ESR) spectrum of ethyl radicals isolated in solid parahydrogen (>99.5 mol %) has been measured in the temperature range between 3.1 and 6.7 K. This study was carried out by UV photolysis of ethyl iodide doped in solid parahydrogen. Although ESR linewidth of the spectrum is broadened due to anisotropic terms of hyperfine interactions, the linewidth measured in parahydrogen remains comparable to that of the isotropic spectrum measured in liquid ethane [R. W. Fessenden, J. Chem. Phys. 37, 747 (1962)]. Small splittings were found in the resolved ESR signals and assigned as ethyl radicals with A(A′,A″) and E(E′,E″) symmetries, respectively. The ratio in the ESR intensities is not proportional to that of the degeneracy of each symmetry but varies as a function of temperature. We measured the signal intensities as a function of temperature and determined the rotational energy level splitting of methyl groups between the lowest with the A(A′,A″) symmetry and the second-low...

The tunneling abstraction reaction: H+H2→H2+H in γ-irradiated solid hydrogen has been studied using electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) spectroscopy and gas-chromatography. The rate constant for the tunneling reaction in solid hydrogen was found to decrease with the increase in the concentration of ortho-H2 molecules in solid hydrogen. We concluded that the decrease in the rate constant is due to the energy level mismatching between reactant species of H+H2 trapped and product species of H2+H trapped in the reverse induced by inhomogeneous intermolecular interactions between ortho-H2 molecules in solid hydrogen. This result indicates that resonance effects play an important role in tunneling reactions in solids.

The effect of pressure on yields of H atoms produced by X-irradiation of solid hydrogen has been studied using electron spin resonance (ESR) spectroscopy at around 4 K. The yields of H atoms have been found to decrease with an increase in pressure. We have concluded that the above finding is due to cage effects on the reaction producing H atoms, H2→2H, H3++e−→3H, and so on, which should occur during X-irradiation of H2 molecules in solid hydrogen. The pressure dependence of the yields of H atoms can be explained by a large compressibility of solid hydrogen. This also indicates that the compressibility is one of the important parameters in radiation-chemical processes in the solid phase.

Electron spin resonance (ESR) studies have been made of lithium metatitanate (Li 2 TiO 3 ) ceramics irradiated by γ -rays of 60 Co source at ambient and liquid nitrogen temperatures (LNT). The ESR spectra have been found strongly dependent on irradiation dose and temperature. The radiation defects induced by γ -irradiation at LNT were found to disappear almost completely by heating up to 225 K. In contrast to this, the radiation defects produced by irradiation at ambient temperature showed tolerance of elevated temperatures recovering at about 600 K.

When H atoms are produced by the photolysis of HI or the radiolysis of neo-C5H12 in the neo-C5H12−i-C4H10(2 mol %) mixtures above 40 K, the H atoms react selectively with the solute i-C4H10. The selective reaction is suppressed below 30 K. The reactions have been studied by electron spin resonance and electron spin echo measurements at 4−100 K. The drastic temperature effect on the reaction by H atoms suggests that the H atoms concerned are not hot atoms, but thermal atoms. The thermal H atoms react with the solute alkane by quantum tunneling at low temperature. Temperature effects on the ESR spectra and the spin−lattice relaxation times of the solvent neo-C5H11 radicals and the solute t-C4H9 radicals indicate that local motions of the solute i-C4H10 molecules take place above 40 K, while motions of the solvent neo-C5H12 molecules are severely restricted even at 100 K. It is concluded that the local motions of the solute i-C4H10 assist the tunneling abstraction reaction by H atoms, resulting in the select...

Decay processes of H2- anions in γ-ray-irradiated solid parahydrogen were studied by using an ESR spectrometer. The following interesting results were obtained. First, the initial yields of ortho-H2- anions in the γ-irradiated solid parahydrogen were 3 times as large as those of para-H2- anions. Second, the yields of para-H2- anions decrease faster than those of ortho-H2- anions upon storage of the irradiated samples at 4.2 K. Third, the decay rate of H2- anions is accelerated by the addition of HD and D2 molecules. Fourth, H2- anions at 2.2 K decay faster than at 4.2 K. According to the parity conservation rule in a homonuclear diatomic molecule, the energy of ortho-H2- anions at the ground state is lower than that of para-H2- anions, whereas that of ortho-H2 molecules is higher than that of para-H2 molecules at low temperatures. The first and second results are ascribed to para → ortho conversion of H2- anions. The third result indicates that nearby HD and D2 accelerate the decay of H2- anion or broaden...

The time dependence of the numbers of H and D atoms, produced by γ-radiolysis of a D 2 H 2 (1 mol%) mixture, was measured upon storage of the sample at 4.5 K by ESR. The concentration of D atoms gradually decreases for up to 190 min, while that of H atoms increases in 10 min and then approaches a plateau value. The results were interpreted in terms of competition between the tunneling reaction H 2 + D → H + HD with combination reactions of D and H atoms. The results cannot be explained by a single rate constant for the tunneling reaction used in homogeneous kinetics but by time evolution of the rate constant.

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.

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.

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).

The dynamics of femtosecond laser ablation of transparent polymers were examined using time-resolved reflectivity. When these polymers were irradiated by a pump pulse with fluence above the ablation threshold of 0.8–2.0 J/cm2, we observed the oscillation of the reflectivity caused by the interference between the reflected probe pulses from the sample surface and the thin layer due to the non-thermal photomechanical effects of spallation. As the fluence of the pump pulse increased, the separation velocity of the thin layer increased from 6 km/s to the asymptotic value of 11 km/s. It is suggested that the velocities are determined by shock-wave velocities of the photo-excited layer.

The electron spin resonance g-factor of H2- in γ-irradiated crystalline H2 was found to be not 2.003 but 2.0023. A novel model has been proposed for explaining the formation and stabilization of H2- in crystalline H2. The model assumes H2- to be located at the center of a cavity that has previously been occupied by a trapped electron. The excess electron of H2- is forced to be attached on one H2 in the cavity due to strong repulsive interactions between the excess electron and H2 molecules surrounding the cavity. A quantum-mechanical calculation shows that an electron trapped in the vacancy of crystalline H2 automatically converts to H2- by drawing one of the surrounding H2 molecules into the center of the vacancy.

The pressure effect on the recombination rate constant of H atoms in solid ${\mathrm{H}}_{2}$ has been studied by electron spin resonance spectroscopy. It has been found that the rate constant was independent of pressure up to 13 MPa in the temperature range below 4 K, while it significantly decreases with an increase in pressure in the range above 5 K. The low temperature behavior observed indicates that H atoms diffuse in solid ${\mathrm{H}}_{2}$ ``chemically'' through the $\mathrm{H}+{\mathrm{H}}_{2}\ensuremath{\rightarrow}{\mathrm{H}}_{2}+\mathrm{H}$ reaction not by the physical exchange mechanism.

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.