106 research outputs found
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
A new measurement of thermal conductivity of amorphous ice and its implications for the thermal evolution of comets
Very slowly deposited amorphous ice has a thermal conductivity about four orders of magnitude or more smaller than hitherto estimated. Using the exceedingly low value of the thermal conductivity of comets deduced from the properties of amorphous ice leads to the expectation that internal heating of comets is negligible below the outer several tens of centimeters
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
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
Diffusion activation energy and desorption activation energy for astrochemically relevant species on water ice show no clear relation
The activation energy for desorption (Edes) and that for surface diffusion
(Esd) of adsorbed molecules on dust grains are two of the most important
parameters for the chemistry in the interstellar medium. Although Edes is often
measured by laboratory experiments, the measurement of Esd is sparse. Due to
the lack of data, astrochemical models usually assume a simple scaling
relation, Esd = fEdes, where f is a constant, irrespective of adsorbed species.
Here, we experimentally measure Esd for CH4, H2S, OCS, CH3OH, and CH3CN on
water-ice surfaces using an ultra-high-vacuum transmission electron microscope
(UHV-TEM). Compiling the measured Esd values and Edes values from the
literature, we find that the value of f ranges from ~0.2 to ~0.7, depending on
the species. Unless f (or Esd) for the majority of species is available, a
natural alternative approach for astrochemical models is running multiple
simulations, varying f for each species randomly. In this approach, ranges of
molecular abundances predicted by multiple simulations, rather than abundances
predicted by each simulation, are important. We here run 10,000 simulations of
astrochemical models of molecular clouds and protostellar envelopes, randomly
assigning a value of f for each species. In the former case, we identify
several key species whose Esd most strongly affects the uncertainties of the
model predictions; Esd for those species should be investigated in future
laboratory and quantum chemical studies. In the latter case, uncertainties in
the Esd of many species contribute to the uncertainties in the model
predictions.Comment: Accepted for publication in ApJ
FORMATION OF CARBONIC ACID (H2CO3) BY SURFACE REACTIONS OF NON-ENERGETIC OH RADICALS WITH CO MOLECULES AT LOW TEMPERATURES
We present the experimental results of carbonic acid (H2CO3) formation through surface reactions of CO molecules with non-energetic hydroxyl (OH) radicals at 10-40 K. The formation of H2CO3 was clearly identified both in the IR spectra and in the thermally programmed desorption mass spectra. The H2CO3 yield was rather high, amounting to approximately 40%-70% relative to that of CO2 formed by the reaction of CO with OH. The structure of H2CO3 formed by reactions of CO with OH may differ from that formed by energetic processes such as UV irradiation, ion irradiation, and electron irradiation of H2O/CO2 binary ices. In this paper, we envisage some of the possible roles H2CO3 may have in the interstellar medium, such as enriching grain mantles of new molecules via acid-base reactions with basic species and contributing to the formation of the unidentified band at 6.8 μm; we suggest possible reasons for its non-detection yet and discuss the restoration of carbonic acid molecules in the gas phase
EXPERIMENTAL STUDY OF CO 2 FORMATION BY SURFACE REACTIONS OF NON-ENERGETIC OH RADICALS WITH CO MOLECULES
Surface reactions between carbon monoxide and non-energetic hydroxyl radicals were carried out at 10 K and 20 K in order to investigate possible reaction pathways to yield carbon dioxide in dense molecular clouds. Hydroxyl radicals, produced by dissociating water molecules in microwave-induced plasma, were cooled down to 100 K prior to the introduction of CO. The abundances of species were monitored in situ using a Fourier transform infrared spectrometer. Formation of CO2 was clearly observed, even at 10 K, suggesting that reactions of CO with OH proceed with little or no activation barrier. The present results indicate that CO2 formation, due to reactions between CO and OH, occurs in tandem with H2O formation, and this may lead to the formation of CO2 ice in polar environments, as typically observed in molecular clouds
H-D Substitution in Interstellar Solid Methanol: A Key Route for D Enrichment
Deuterium enrichment of interstellar methanol is reproduced experimentally
for the first time via grain-surface H-D substitution in solid methanol at an
atomic D/H ratio of 0.1. Although previous gas-grain models successfully
reproduce the deuterium enrichments observed in interstellar methanol molecules
(D/H of up to 0.4, compared to the cosmic ratio of , the models
exclusively focus on deuterium fractionation resulting from the successive
addition of atomic hydrogen/deuterium on CO. The mechanism proposed here
represents a key route for deuterium enrichment that reproduces the high
observed abundances of deuterated methanol, including multiple deuterations.Comment: 4 pages, 4 figures, Accepted to the ApJ
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