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Journal Articles

Nuclear spin conversion of hydrogen and methane molecules inside low-temperature condensed layers

Yamakawa, Koichiro

Bunko Kenkyu, 72(5), p.187 - 198, 2023/10

Molecules including hydrogen nuclei in the rotationally symmetric positions are classified into the nuclear spin isomers, according to their resultant spin. Owing to the symmetry restriction originating from the indistinguishability of the identical particles, each isomer occupies only specific rotational levels. The conversion between the isomers is promoted in condensed systems, where the excess rotational energy of a converting molecule is dissipated. In this review, I focus on the fundamental molecules of hydrogen and methane, and expound studies on their nuclear spin conversion inside condensed layers at low temperatures.

Journal Articles

Fast ortho-to-para conversion of molecular hydrogen in chemisorption and matrix-isolation systems

Ueta, Hirokazu; Fukutani, Katsuyuki*; Yamakawa, Koichiro

Frontiers in Chemistry (Internet), 11, p.1258035_1 - 1258035_7, 2023/08

Molecular hydrogen has two nuclear-spin modifications called ${it ortho}$ and ${it para}$. Because of the symmetry restriction with respect to permutation of the two protons, the ${it ortho}$ and ${it para}$ isomers take only odd and even values of the rotational quantum number, respectively. The ${it ortho}$-to-${it para}$ conversion is promoted in condensed systems, to which the excess rotational energy and spin angular momentum are transferred. We review recent studies on fast ${it ortho}$-to-${it para}$ conversion of hydrogen in molecular chemisorption and matrix isolation systems, discussing the conversion mechanism as well as rotational-relaxation pathways.

Journal Articles

An Isotope-labeling study of CO$$_2$$ formation pathways in molecular clouds; Ultraviolet photolysis of CO-H$$_2$$O ice

Hirayama, Akemi*; Arakawa, Ichiro*; Yamakawa, Koichiro

Astrophysical Journal, 951(2), p.132_1 - 132_12, 2023/07

With use of reflection-absorption infrared spectroscopy, we investigated the binary ice of CO and H$$_2$$O at 10 K under ultraviolet irradiation as an analogue of interstellar ice mantle. We employed H$$_2$$$$^{18}$$O instead of H$$_2$$$$^{16}$$O to distinguish the formation pathways of CO$$_2$$, and prepared mixed-ice samples under CO- and H$$_2$$O-rich conditions as well as bilayered one, where CO was condensed on H$$_2$$O ice. After the ultraviolet irradiation onto any sample, we detected infrared absorption bands of C$$^{16}$$O$$_2$$ and C$$^{18}$$O$$^{16}$$O, which were generated through the CO-CO and CO-H$$_2$$O reactions, respectively. We analyzed the irradiation-time dependence of the C$$^{16}$$O$$_2$$ and C$$^{18}$$O$$^{16}$$O column densities in CO-rich ice to determine the effective cross sections of the CO$$_2$$ formation through the CO-CO and CO-H$$_2$$O reactions simultaneously.

Journal Articles

Fast nuclear-spin conversion of H$$_{2}$$ trapped and polarized in a CO$$_{2}$$ matrix

Yamakawa, Koichiro; Ishibashi, Atsuki*; Namiyoshi, Toshinobu*; Azuma, Yuichi*; Arakawa, Ichiro*

Physical Review B, 102(4), p.041401_1 - 041401_5, 2020/07

 Times Cited Count:1 Percentile:13.29(Materials Science, Multidisciplinary)

Journal Articles

Development of apparatus for in-situ terahertz and infrared spectroscopy

Yamakawa, Koichiro; Nasu, Hirokazu*; Shimizu, Genki*

Bunko Kenkyu, 69(3), p.105 - 106, 2020/06

Terahertz and infrared spectroscopy is one of the powerful ways to elucidate the structures and vibrational modes of molecular clusters. We review an experimental apparatus for absorption spectroscopy in an ultra-high vacuum covering the wavenumber range of 60-7000 cm$$^{-1}$$, from the viewpoints of vacuum-, low-temperature-, and spectroscopy-techniques. We also introduce the terahertz and infrared spectrum of D$$_{2}$$O clusters isolated in an Ar matrix measured with this apparatus.

Journal Articles

Terahertz and mid-infrared spectroscopy of matrix-isolated clusters and matrix-sublimation ice of D$$_2$$O

Yamakawa, Koichiro; Nasu, Hirokazu*; Suzuki, Natsumi*; Shimizu, Genki*; Arakawa, Ichiro*

Journal of Chemical Physics, 152(17), p.174310_1 - 174310_13, 2020/05

 Times Cited Count:2 Percentile:11.8(Chemistry, Physical)

We have established an apparatus for terahertz and mid-infrared spectroscopy in an ultrahigh vacuum and have measured absorption spectra of D$$_2$$O clusters trapped in solid Ar. To assign terahertz absorption peaks due to the D$$_2$$O dimer, trimer, and tetramer, the dependence of the spectrum on the annealing temperature and D$$_2$$O dilution was analyzed. The assignment was also examined by ab initio calculations with use of the ONIOM method, where flexibility of surrounding Ar atoms was systematically incorporated. We identified all of the intermolecular fundamentals of the dimer and those with significant intensities of the trimer and tetramer, whose structural symmetries were revealed to be broken down. After isolating the D$$_2$$O clusters in solid Ar, we sublimated only Ar atoms to leave behind matrix-sublimation ice, which was found to be amorphous- or crystal-like depending on the formation conditions: the dilution and sublimation-temperature. Since the crystallinity got higher with raising the dilution and sublimation-temperature, the diffusion of the D$$_2$$O monomer on the surface of sublimating solid Ar was found to be crucial to the crystallization of the sublimation ice.

Journal Articles

Nuclear spin conversion of H$$_2$$, H$$_2$$O, and CH$$_4$$ interacting with diamagnetic insulators

Yamakawa, Koichiro; Fukutani, Katsuyuki

Journal of the Physical Society of Japan, 89(5), p.051016_1 - 051016_12, 2020/02

A molecule including two or more hydrogen nuclei in the rotationally symmetric positions exists in nuclear spin isomers according to their resultant spin. Owing to the quantum statistics, the isomers occupy specific rotational states, which enables us to distinguish them by rotationally-resolved spectroscopy. Whereas the interconversion between the isomers are strictly forbidden in the isolated state, it is highly promoted in condensed systems. Focusing on simple and astronomically important molecules of H$$_2$$, H$$_2$$O, and CH$$_4$$, we review recent theoretical and experimental studies on the nuclear spin conversion proceeding on or in diamagnetic insulators. The fundamental difference between the conversion models of diatomic and polyatomic molecules is also clarified.

Journal Articles

Spectroscopic determination of interconversion rates among three nuclear spin isomers of methane in crystalline II

Sugimoto, Takeru*; Nasu, Hirokazu*; Arakawa, Ichiro*; Yamakawa, Koichiro

Journal of Chemical Physics, 150(18), p.184302_1 - 184302_5, 2019/05

We measured infrared absorption spectra of crystalline II of methane and succeeded in detecting a prominent Q(2) peak by rapid cooling after annealing as well as previously reported rovibrational and librational-vibrational peaks. The integral intensities of the R(0), R(1), and Q(2) peaks were found to show biexponential dependence on time. This clearly demonstrates the interconversion among the three nuclear-spin isomers occupying low-lying rotational levels.

Oral presentation

Current status of research and development of supercritical water cooled fast reactor (super fast reactor) in Japan

Nakatsuka, Toru; Oka, Yoshiaki*; Ishiwatari, Yuki*; Okumura, Keisuke; Nagasaki, Shinya*; Tezuka, Kenichi*; Mori, Hideo*; Ezato, Koichiro; Akasaka, Naoaki; Nakazono, Yoshihisa*; et al.

no journal, , 

Pressure-vessel-type supercritical water cooled reactors (SCWRs) have been developed at the University of Tokyo (UT) since 1989 and are studied in Japan, Europe and other countries. UT's concepts of the reactor are termed "Super LWR" for thermal neutron spectrum and "Super Fast Reactor (FR)" for fast spectrum respectively. With the scope of developing an economical fast reactor system, a Japanese research project of the "Super FR" had been conducted since December 2005 to March 2010. UT, Kyushu University, Japan Atomic Energy Agency and Tokyo Electric Power Company have participated in the project. It consisted of three subjects: (1) development of the Super FR concept, (2) thermal-hydraulic experiments and (3) materials developments. All items have successfully finished. The present paper will summarize the results of the project.

Oral presentation

Nuclear spin conversion of H$$_2$$ trapped in solid CO$$_2$$

Yamakawa, Koichiro; Namiyoshi, Toshinobu*; Ishibashi, Atsuki*; Arakawa, Ichiro*

no journal, , 

We detected weak infrared absorption by H$$_{2}$$ trapped and polarized in solid CO$$_{2}$$ and evaluated the strength of the local electric field just in the solid. By analyzing the time evolution of the spectrum, we determined the conversion rates of H$$_{2}$$ to examine the effect of the electric field on the conversion.

Oral presentation

Terahertz- and infrared-spectroscopic study of intermolecular vibration in D$$_{2}$$O nanoclusters combined with ONIOM calculation

Yamakawa, Koichiro; Nasu, Hirokazu*; Arakawa, Ichiro*

no journal, , 

Using an apparatus for terahertz- (THz-) and infrared- (IR-) spectroscopy in an ultrahigh vacuum, we measured absorption spectra of D$$_2$$O nanoclusters isolated in an Ar matrix and examined dependence of the spectrum on the D$$_2$$O dilution and annealing temperature. The dilution of Ar/D$$_2$$O was in the range of 100-1000, and the annealing temperature was increased from 15 K to 25 K. Comparing the spectrum in the THz region with the IR range, where spectral assignments have been well established, we assigned the THz absorption peaks due to the intermolecular fundamentals of (D$$_2$$O)$$_n$$ with $$n$$ = 2-4. To support the assignments, we also performed the ab initio calculation with the "our own N-layered integrated molecular orbital and molecular mechanics (ONIOM)" method, where flexibility of surrounding Ar atoms was systematically incorporated.

Oral presentation

Development of an apparatus for terahertz spectroscopy in an ultrahigh vacuum; Temperature dependence of the spectrum of vapor-deposited D$$_2$$O ice

Yamakawa, Koichiro; Shimizu, Genki*; Suzuki, Natsumi*; Nasu, Hirokazu*; Tsuboi, Rei*; Kurahashi, Hiroyuki*; Arakawa, Ichiro*

no journal, , 

no abstracts in English

Oral presentation

Nuclear spin conversion of hydrogen trapped in condensed layers of carbon dioxide

Yamakawa, Koichiro; Ishibashi, Atsuki*; Namiyoshi, Toshinobu*; Arakawa, Ichiro*

no journal, , 

The mechanism of the interconvertion between the nuclear spin isomers has been attracted much interest recently. In the present work, we detected the weak infrared absorption by hydrogen molecules polarized in the condensed layers of carbon dioxide and measured the rate of the nuclear spin conversion to discuss its mechanism.

Oral presentation

Infrared-spectroscopic study of the photolysis in CO-X ice with use of isotopologues

Hirayama, Akemi*; Arakawa, Ichiro*; Yamakawa, Koichiro

no journal, , 

Molecular clouds are composed of gases and interstellar dust. The dust is covered with an ice mantle containing a variety of molecules such as H$$_2$$O and CO and is exposed to ultraviolet radiation. In the present study, we investigated the photochemical reaction in the ice mantle utilizing isotopologues.

Oral presentation

Terahertz and infrared spectroscopic investigation on nuclear spin isomerization and clusterization of molecules

Yamakawa, Koichiro

no journal, , 

I have investigated the proton-related quantum phenomena: nuclear spin isomerization and hydrogen-bond clusterization. With use of in situ terahertz and infrared absorption spectroscopy combined with ab initio vibrational calculation, I determined the nuclear spin conversion rates of H$$_{2}$$, H$$_{2}$$O, and CH$$_{4}$$ as well as the intermolecular vibrational modes of water clusters.

Oral presentation

Terahertz and infrared spectroscopy of hydrogen-bonded clusters in ultrahigh vacuum

Yamakawa, Koichiro; Nasu, Hirokazu*; Suzuki, Natsumi*; Shimizu, Genki*; Arakawa, Ichiro*

no journal, , 

Nano-sized molecular clusters have been intensively investigated in a spectroscopic way to reveal the nature of intermolecular interaction. Compared with the infrared (IR) range, only a few spectral measurements in the terahertz (THz) range have been reported, so that the assignment of the vibrational mode to THz absorption was not established even for the water dimer. In the present study, we constructed an experimental apparatus for THz-IR absorption spectroscopy in an ultrahigh vacuum and measured spectra of deuterated-water clusters formed in solid Ar to identify the THz absorption peaks of the dimer, trimer, and tetramer.

Oral presentation

Oxygen-isotope effect on the nuclear spin conversion of water molecules

Yamakawa, Koichiro; Kawabe, Hiroyuki*; Yamaguchi, Kiwa*

no journal, , 

Oral presentation

Nuclear spin conversion of methane in condensed layers at low temperatures

Yamakawa, Koichiro; Sugimoto, Takeru*

no journal, , 

Methane has three kinds of nuclear spin isomers: ortho (I = 1), meta (I = 2), and para (I = 0). Methane is a spherical top molecule, and its rotational energy is dependent on the rotational quantum number J. The J = 0 (J = 1) level is occupied only by the meta (ortho) isomer while the ortho and para states are degenerate at the J = 2 level. Because of this degeneracy, the conversion between ortho and para proceeds considerably fast, with the result that only the ortho-meta conversion has been observed in matrices of rare gases and para hydrogen. On the other hand, the nuclear spin conversion between three kinds of the isomers has recently been observed in the lower-temperature phase of crystalline methane. In this presentation, the infrared spectroscopic study on the ortho-meta-para conversion in crystalline methane is demonstrated, comparing with previous matrix-isolation studies.

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