78 research outputs found
Behavior of Hydroxyl Radicals on Water Ice at Low Temperatures
Because chemical reactions on/in cosmic ice dust grains covered by amorphous
solid water (ASW) play important roles in generating a variety of molecules,
many experimental and theoretical studies have focused on the chemical
processes occurring on the ASW surface. In laboratory experiments, conventional
spectroscopic and mass-spectrometric detection of stable products is generally
employed to deduce reaction channels and mechanisms. However, despite their
importance, the details of chemical reactions involving reactive species (i.e.,
free radicals) have not been clarified because of the absence of experimental
methods for in situ detection of radicals. Because OH radicals can be easily
produced in interstellar conditions by not only the photolysis and/or ion
bombardments of H2O but also the reaction of H and O atoms, they are thought to
be one of the most abundant radicals on ice dust. In this context, the
development of a close monitoring method of OH radicals on the ASW surface may
help to elucidate the chemical reactions occurring on the ASW surface.Comment: 25 pages, 9 figures; Accepted for publication in Acc. Chem. Re
Penetration of Non-energetic Hydrogen Atoms into Amorphous Solid Water and their Reaction with Embedded Benzene and Naphthalene
Chemical processes on the surface of icy grains play an important role in the
chemical evolution in molecular clouds. In particular, reactions involving
non-energetic hydrogen atoms accreted from the gaseous phase have been
extensively studied. These reactions are believed to effectively proceed only
on the surface of the icy grains; thus, molecules embedded in the ice mantle
are not considered to react with hydrogen atoms. Recently, Tsuge et al. (2020)
suggested that non-energetic hydrogen atoms can react with CO molecules even in
ice mantles via diffusive hydrogenation. This investigation was extended to
benzene and naphthalene molecules embedded in amorphous solid water (ASW) in
the present study, which revealed that a portion of these molecules could be
fully hydrogenated in astrophysical environments. The penetration depths of
non-energetic hydrogen atoms into porous and non-porous ASW were determined
using benzene molecules to be >50 and ~10 monolayers, respectively (1 monolayer
~ 0.3 nm).Comment: 30 pages, 4 figures, 1 table; accepted for publication by Ap
Thermal decomposition of the HXeCl center dot center dot center dot H2O complex in solid xenon : Experimental characterization of the two-body decomposition channel
The thermal decomposition process of HXeCl···H2O in solid Xe is studied, and HCl···H2O is identified as a decomposition product. The production is due to the two-body (2B) decomposition of HXeCl moiety, in agreement with theoretical predictions. Two types of 2B decomposition paths are predicted: catalytic and unimolecular 2B decompositions, where water molecule plays different roles. In an experiment to selectively produce HXeCl···D2O, only HCl···D2O is observed as a thermal decomposition product, indicating the occurrence of unimolecular 2B decomposition, where water molecule serves as a spectator. The activation energy for this decomposition process is experimentally determined to be 15 kJ mol−1.The thermal decomposition process of HXeCl center dot center dot center dot H2O in solid Xe is studied, and HCl center dot center dot center dot H2O is identified as a decomposition product. The production is due to the two-body (2B) decomposition of HXeCl moiety, in agreement with theoretical predictions. Two types of 2B decomposition paths are predicted: catalytic and unimolecular 2B decompositions, where water molecule plays different roles. In an experiment to selectively produce HXeCl center dot center dot center dot D2O, only HCl center dot center dot center dot D2O is observed as a thermal decomposition product, indicating the occurrence of unimolecular 2B decomposition, where water molecule serves as a spectator. The activation energy for this decomposition process is experimentally determined to be 15 kJ mol(-1).Peer reviewe
Surface Diffusion of Carbon Atoms as a Driver of Interstellar Organic Chemistry
Many interstellar complex organic molecules (COMs) are believed to be
produced on the surfaces of icy grains at low temperatures. Atomic carbon is
considered responsible for the skeletal evolution processes, such as C-C bond
formation, via insertion or addition reactions. Before reactions, C atoms must
diffuse on the surface to encounter reaction partners; therefore, information
on their diffusion process is critically important for evaluating the role of C
atoms in the formation of COMs. In situ detection of C atoms on ice was
achieved by a combination of photostimulated desorption and resonance enhanced
multiphoton ionization methods. We found that C atoms weakly bound to the ice
surface diffused approximately above 30 K and produced C2 molecules. The
activation energy for C-atom surface diffusion was experimentally determined to
be 88 meV (1,020 K), indicating that the diffusive reaction of C atoms is
activated at approximately 22 K on interstellar ice. The facile diffusion of C
at T > 22 K atoms on interstellar ice opens a previously overlooked chemical
regime where the increase in complexity of COMs as driven by C atoms. Carbon
addition chemistry can be an alternative source of chemical complexity in
translucent clouds and protoplanetary disks with crucial implications in our
current understanding on the origin and evolution of organic chemistry in our
Universe.Comment: 33 pages (main + SI), 14 figures, 1 tabl
Diffusive hydrogenation reactions of CO embedded in amorphous solid water at elevated temperatures ~70 K
The surface processes on interstellar dust grains have an important role in
the chemical evolution in molecular clouds. Hydrogenation reactions on ice
surfaces have been extensively investigated and are known to proceed at low
temperatures mostly below 20 K. In contrast, information about the chemical
processes of molecules within an ice mantle is lacking. In this work, we
investigated diffusive hydrogenation reactions of carbon monoxide (CO) embedded
in amorphous solid water (ASW) as a model case and discovered that the
hydrogenation of CO efficiently proceeds to yield H2CO and CH3OH even above 20
K when CO is buried beneath ASW. The experimental results suggest that hydrogen
atoms diffuse through the cracks of ASW and have a sufficient residence time to
react with embedded CO. The hydrogenation reactions occurred even at
temperatures up to ~70 K. Cracks collapse at elevated temperatures but the
occurrence of hydrogenation reactions means that the cracks would not
completely disappear and remain large enough for penetration by hydrogen atoms.
Considering the hydrogen-atom fluence in the laboratory and molecular clouds,
we suggest that the penetration of hydrogen and its reactions within the ice
mantle occur in astrophysical environments. Unified Astronom
Tendon transfers for claw hand
We have recently operated on 56 cases of claw hand and described the method of tendon transfer in Hansen's disease which occupied the majority of the cases, and several problems have been discussed from our experiences.</p
Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon Surface: H2 Formation and Desorption
The interactions of atomic and molecular hydrogen with bare interstellar dust
grain surfaces are important for understanding H2 formation at relatively high
temperatures (>20 K). We investigate the diffusion of physisorbed H atoms and
the desorption energetics of H2 molecules on an amorphous diamond-like carbon
(DLC) surface. From temperature-programmed desorption experiments with a
resonance-enhanced multiphoton ionization (REMPI) method for H2 detection, the
H2 coverage-dependent activation energies for H2 desorption are determined. The
activation energies decrease with increasing H2 coverage and are centered at 30
meV with a narrow distribution. Using a combination of photostimulated
desorption and REMPI methods, the time variations of the surface number density
of H2 following atomic and molecular hydrogen depositions are studied. From
these measurements, we show that H2 formation on a DLC surface is quite
efficient, even at 20 K. A significant kinetic isotope effect for H2 and D2
recombination reactions suggests that H-atom diffusion on a DLC surface is
mediated by quantum mechanical tunneling. In astrophysically relevant
conditions, H2 recombination due to physisorbed H-atoms is unlikely to occur at
20 K, suggesting that chemisorbed H atoms might play a role in H2 formation at
relatively high temperatures.Comment: 33 pages, 8 figures, Accepted for publication in Ap
Coverage Dependent H Desorption Energy: a Quantitative Explanation Based on Encounter Desorption Mechanism
Recent experiments show that the desorption energy of H on a diamond-like
carbon (DLC) surface depends on the H coverage of the surface. We aim to
quantitatively explain the coverage dependent H desorption energy measured
by the experiments. We derive a math formula to calculate an effective H
desorption energy based on the encounter desorption mechanism. The effective
H desorption energy depends on two key parameters, the desorption energy of
H on H substrate and the ratio of H diffusion barrier to its
desorption energy. The calculated effective H desorption energy
qualitatively agrees with the coverage dependent H desorption energy
measured by the experiments if the values of these two parameters in literature
are used in the calculations. We argue that the difference between the
effective H desorption energy and the experimental results is due to the
lacking of knowledge about these two parameters. So, we recalculate these two
parameters based on experimental data. Good agreement between theoretical and
experimental results can be achieved if these two updated parameters are used
in the calculations.Comment: 6 pages,6 figures,2 tables, accepted for publication in MNRA
Chiral Ice Crystals in Space
We observed the formation of CO, CH3OH, and H2O ices using a cryogenic transmission electron microscope, to determine if chiral ice crystals could form under the conditions of interstellar molecular clouds and young stellar objects (protoplanetary disks) and to clarify the crystalline structure of these ices. Our results suggest that the following ice crystals are chiral: crystalline CO (α-CO) formed on amorphous H2O (a-H2O) grains in a 10-K molecular cloud, crystalline CH3OH formed by the heating of amorphous CH3OH on a-H2O grains at 40–60 K in young stellar objects, and several polymorphs of hydrogen-ordered cubic ice crystals formed by the heating of a-H2O at 80–100 K and direct condensation at 120–140 K in protoplanetary disks. We also investigated candidates for other chiral ices using published data. We found that NH3 I and NH3·H2O I are chiral at low temperature and pressure conditions. If one-handed circularly polarized light is irradiated during the nucleation of these chiral ice crystals, homochiral crystals can be formed. These results have important implications for the origin of interstellar organic molecule homochirality
- …