Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Initialising ...
Yamakawa, Koichiro
Kotai Butsuri, 60(7), p.341 - 356, 2025/07
Molecules including identical nuclei with non-zero spin in the rotationally symmetric positions are classified into the nuclear spin isomers. The conversion between the isomers is prohibited in the isolated state, but is promoted in condensed systems, where molecules perform hindered rotation. In this review, I expound the coupling between the nuclear spin isomers and rotational states, and demonstrate some experimental studies on the nuclear spin conversion in molecular solids.
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.
Ueta, Hirokazu; Fukutani, Katsuyuki*; Yamakawa, Koichiro
Frontiers in Chemistry (Internet), 11, p.1258035_1 - 1258035_7, 2023/08
Times Cited Count：1 Percentile：1.54(Chemistry, Multidisciplinary)Molecular hydrogen has two nuclear-spin modifications called 
and 
. Because of the symmetry restriction with respect to permutation of the two protons, the and isomers take only odd and even values of the rotational quantum number, respectively. The -to- conversion is promoted in condensed systems, to which the excess rotational energy and spin angular momentum are transferred. We review recent studies on fast -to- conversion of hydrogen in molecular chemisorption and matrix isolation systems, discussing the conversion mechanism as well as rotational-relaxation pathways.
formation pathways in molecular clouds; Ultraviolet photolysis of CO-HO iceHirayama, Akemi*; Arakawa, Ichiro*; Yamakawa, Koichiro
Astrophysical Journal, 951(2), p.132_1 - 132_12, 2023/07
Times Cited Count：0 Percentile：0.00(Astronomy & Astrophysics)With use of reflection-absorption infrared spectroscopy, we investigated the binary ice of CO and HO at 10 K under ultraviolet irradiation as an analogue of interstellar ice mantle. We employed H
O instead of H
O to distinguish the formation pathways of CO, and prepared mixed-ice samples under CO- and HO-rich conditions as well as bilayered one, where CO was condensed on HO ice. After the ultraviolet irradiation onto any sample, we detected infrared absorption bands of CO and COO, which were generated through the CO-CO and CO-HO reactions, respectively. We analyzed the irradiation-time dependence of the CO and COO column densities in CO-rich ice to determine the effective cross sections of the CO formation through the CO-CO and CO-HO reactions simultaneously.
Yamakawa, Koichiro
Shototsu, 17(5), p.86 - 101, 2020/09
no abstracts in English
trapped and polarized in a CO matrixYamakawa, Koichiro; Ishibashi, Atsuki*; Namiyoshi, Toshinobu*; Azuma, Yuichi*; Arakawa, Ichiro*
Physical Review B, 102(4), p.041401_1 - 041401_5, 2020/07
Times Cited Count：2 Percentile：8.44(Materials Science, Multidisciplinary)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
, from the viewpoints of vacuum-, low-temperature-, and spectroscopy-techniques. We also introduce the terahertz and infrared spectrum of DO clusters isolated in an Ar matrix measured with this apparatus.
OYamakawa, 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：7.23(Chemistry, Physical)We have established an apparatus for terahertz and mid-infrared spectroscopy in an ultrahigh vacuum and have measured absorption spectra of DO clusters trapped in solid Ar. To assign terahertz absorption peaks due to the DO dimer, trimer, and tetramer, the dependence of the spectrum on the annealing temperature and DO 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 DO 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 DO monomer on the surface of sublimating solid Ar was found to be crucial to the crystallization of the sublimation ice.
, HO, and CH
interacting with diamagnetic insulatorsYamakawa, Koichiro; Fukutani, Katsuyuki
Journal of the Physical Society of Japan, 89(5), p.051016_1 - 051016_12, 2020/02
Times Cited Count：6 Percentile：40.16(Physics, Multidisciplinary)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, HO, and CH, 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.
Sugimoto, Takeru*; Nasu, Hirokazu*; Arakawa, Ichiro*; Yamakawa, Koichiro
Journal of Chemical Physics, 150(18), p.184302_1 - 184302_5, 2019/05
Times Cited Count：4 Percentile：15.94(Chemistry, Physical)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.
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.
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 HO and CO and is exposed to ultraviolet radiation. In the present study, we investigated the photochemical reaction in the ice mantle utilizing isotopologues.
Yamakawa, Koichiro; Kawabe, Hiroyuki*; Yamaguchi, Kiwa*
no journal, ,
formation pathways in CO-HO ice films under ultraviolet irradiationYamakawa, Koichiro; Hirayama, Akemi*; Arakawa, Ichiro*
no journal, ,
CO is one of the abundant molecules in the ice mantles, and the following two formation channels have been discussed: (1) reaction of two CO molecules, one of which is electronically excited by UV light; (2) reaction of CO with the OH radical which is a dissociation product of HO. We focused on the fact that these two channels can be distinguished from each other by isotope labeling, i.e., by employing H
O instead of H
O. In the present study, we investigated the UV photolysis of CO-HO ice and determined the effective rates of the CO formation channels with use of isotope labeling and infrared spectroscopy.
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.
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.
Yamakawa, Koichiro
no journal, ,
I will demonstrate the studies on the nuclear spin conversion and clustering of matrix-isolated water and hydrogen molecules. Also, I will introduce the nuclear spin conversion study with use of solid para-hydrogen as the matrix.
trapped in solid COYamakawa, Koichiro; Namiyoshi, Toshinobu*; Ishibashi, Atsuki*; Arakawa, Ichiro*
no journal, ,
We detected weak infrared absorption by H trapped and polarized in solid CO 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 to examine the effect of the electric field on the conversion.
O nanoclusters combined with ONIOM calculationYamakawa, 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 DO nanoclusters isolated in an Ar matrix and examined dependence of the spectrum on the DO dilution and annealing temperature. The dilution of Ar/DO 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 (DO)
with 
= 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.
O iceYamakawa, Koichiro; Shimizu, Genki*; Suzuki, Natsumi*; Nasu, Hirokazu*; Tsuboi, Rei*; Kurahashi, Hiroyuki*; Arakawa, Ichiro*
no journal, ,
no abstracts in English
