Yasuyuki Aratono

A remarkable temperature dependence of the rate constant for the tunneling reaction HD + D → H + D2 in the low temperature region 2.6–6.5 K

Abstract Rate constants for the tunneling reaction, HD + D → H + D 2 , in solid HD have been determined by using ESR in the very low temperature range of 2.6–6.5 K. The rate constants increase steeply with increasing temperature above 5 K, while they are almost independent of temperature below 5 K. The results above 5 K do not coincide with theoretical values calculated by the gas phase model. A phonon-assisted mechanism is proposed to account for the remarkable temperature dependence of the tunneling reaction in solid HD.

Radiolytic hydrogen gas formation from water adsorbed on type A zeolites

Abstract γ-Ray irradiation was performed on molecular sieve 3A and 4A zeolites adsorbing various amounts of water. The enhancement of hydrogen formation was observed, and the yields of hydrogen gas due to energy transfer from the zeolites to adsorbed water were expressed as a function of p -value, a fraction of adsorbed water, in the same way as formulated in the molecular sieve 5A-water system. The results obtained in the 4A-water system showed good agreement with those in the 5A-water system, but the 3A-water system showed definite difference from the others. The present results indicate that anionic framework oxygen atoms of zeolite structure are active sites for energy transfer rather than cations contained in zeolite cavities, and that effectiveness of energy transfer process depends on the extent of mutual interaction between water molecules, cations and anionic framework oxygen atoms in cation-exchanged type A zeolites.

An ENDOR spectrum of H atoms in solid H2

Abstract An ENDOR spectrum of H atoms produced in the γ-ray irradiated solid H 2 was measured at 4.2 K in order to elucidate the structure of the local environment of the H atoms in solid H 2 . We found that the H atoms were not trapped in interstitial sites but in substitutional sites of the solid, and almost all the ortho-H 2 molecules at the first nearest sites from the H atoms converted into para-H 2 molecules. This result shows that the ortho–para conversion is induced by electron spins of the H atoms.

ESR studies of a new reactive species produced in γ-ray irradiated solid para-H2 at 4.2 K

Abstract A new reactive species in γ-irradiated solid p -H 2 has been studied by ESR. When p -H 2 is irradiated at 4.2 K with γ-rays, a small amount of the new reactive species which shows doublet ESR lines with a separation of 12.4 G is observed in addition to intense peaks of H atoms. The yield of the new species is 0.0001 G-unit. The new species decays in several hours upon storage of the irradiated solid p -H 2 at 4.2 K. The doublet lines were interpreted in terms of an ion-pair of an electron and a hole (e t − … p -H 2 + ) separated by 13 A.

Experimental observation of H2− anions by high-resolution ESR spectroscopy in γ-ray irradiated quantum solid parahydrogen at 4.2 K

Abstract High-resolution ESR spectroscopy by use of quantum solid parahydrogen has been applied to the observation of H 2 − anions. When solid parahydrogen is irradiated with γ-rays at 4.2 K, H 2 − anions produced show ESR spectra of quartet lines with a hyperfine splitting constant of 203 G. H 2 − anions consist of the mixture of p -H 2 − (25%) and o -H 2 − (75%), though the concentration of p -H 2 in the matrix exceeds 95%. It is suggested that electron bubbles assumed previously in irradiated solid hydrogen are probably the H 2 − anions observed here.

The reactions of recoil tritium atoms with water: Energetic and thermal reaction yields

Abstract In the reactions of recoil T with H 2 O or D 2 O, both containing 0·07 M LiNO 3 , hydrogen-t and water-t are formed in the ratio of 0·14 ± 0·01 for HT/HTO and 0·15 ± 0·01 for DT/DTO. Using H 2 S, acetone, and I 2 as additives, it was found that the energetic reaction yields in the reaction with H 2 O are 68% for T for H and 11% for T to HT reactions. The thermalized T atoms are mostly stabilized as HTO. The failure of a definite overall isotope effect in the energetic reaction yields results from the counterbalance of two different kinds of isotope effects, the moderator isotope effect, α H 2 O / α D 2 O , and the overall reactivity integral isotope effect, (I HT + I HTO )/(I DT + I DTO ). The ratio of acetone-t yield from H 2 O-acetone systems to that from D 2 O-acetone systems is extraporated to 0·65 ± 0·1 at zero m.f. of acetone. This means that the ratio of relative reactivities of H 2 O and D 2 O will be close to 1·5.

Diffusivity of tritium in Li-Al alloys

Abstract The diffusion coefficients of tritium in Li-Al alloys were measured. The observed diffusion coefficients were expressed by the following equations: D = 8.5 × 10 −5 exp[ −43.9± 14.8(kJ/mol)/RT] (cm 2 /s ), D = 9.3 × 10 −5 exp[ −48.5± 15.0(kJ/mol)/RT] (cm 2 /s ), and D = 5.3 × 10 −5 exp[ −62.7± 13.8(kJ/mol)/RT] (cm 2 /s ) for 0.02, 0.26 and 1.12% Li-Al alloys, respectively. Results showed that the diffusion coefficients as well as the apparent activation energy for diffusion depend upon the lithium concentration in the alloy.

Diffusivity of recoil-injected or thermally-doped tritium in aluminum

Abstract Bulk diffusion coefficients of both recoil-injected and thermally-doped tritium in aluminum were measured. The temperature dependence of these diffusion coefficients was expressed by D = 2 × 10 −3 exp [−42.6 ± 2.5( kJ/mol )/RT] ( cm 2 / s ) for the recoil-injected tritium, and D = (9 ± 1) × 10 −3 exp [−51.9 ± 2.9( kJ/mol )/RT] ( cm 2 / s ) for the thermally-doped tritium. A reasonable agreement between the two sets of results is very contrasted to the results found with UO2 as a host material, in which thermally-doped hydrogen diffusion much faster than recoil tritium by several orders of magnitude. Chemical forms of recoil-injected tritium released on heating were also investigated. Both gaseous and condensable components were released. The majority of gaseous component was found as HT, and ca. 2% of CH3T and ca. 0.1% of T2 were detected in the radio-gas Chromatographic measurements.

Decay of H-2 anions in solid parahydrogen via two-stage quantum tunneling diffusion processes

Decay mechanism of H2- anions in X(γ)-irradiated solid para-H2(p-H2) has been studied using high-resolution ESR spectroscopy in the temperature range between 1.3–6.6 K. The results are summarized as follows. First,the decay rate constant of the2- anions is not proportional to initial yields of reactive species such as cations and H atoms but proportional to the concentration of less-reactive species of HD molecules in p-H2. This result shows that the decay of the2- anion is due to the reaction neither with cations nor H atoms but with HD molecules originally contained in the p-H2sample at natural abundance. Second,the decay rate constant of the2- anions increases proportionally with the increase in temperature below 3 K and decreases with the increase in temperature between 3–5 K,although it increases exponentially with the increase in temperature above 5 K. This result indicates that the decay rate constant of the2- anions is controlled by the rate constant for the diffusion of the2- anions via one-phonon assisted quantum tunneling below 3 K,two-phonon scattered or assisted quantum tunneling between 3–5 K,and thermally-activated process above 5 K.

Trapping sites of hydrogen atoms in solid HD and D2: An electron spin echo study

Trapping sites of H and D atoms in solid HD and D2 have been determined using electron spin echo (ESE) spectroscopy. It was found that all the H and D atoms are trapped in substitutional sites and that the H atoms push back surrounding HD(D2) molecules to produce local lattice distortion around the atoms, whereas the D atoms do not. It is expected that the local lattice distortion is produced by zero-point motion of the H atoms whose amplitude is larger than that of host HD(D2) molecules and that the isotope effect is due to difference in the amplitude between the H and D atoms. The lattice distortion around the H atoms may induce the increase in rate constant for the tunneling reaction D+DH→D2+H with the increase in temperature in solid HD reported in Chem. Phys. Lett. 261, 463 (1996).