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

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

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

セッション 21セイキ ノ ニッチュウ カンケイ ニ ツイテ

日中台共同研究「現代中国と東アジアの新環境」 ②21世紀の日中関係 : 青年研究者の思索と対

THE MECHANISM OF SURFACE DIFFUSION OF H AND D ATOMS ON AMORPHOUS SOLID WATER: EXISTENCE OF VARIOUS POTENTIAL SITES

To understand elementary processes leading to H{sub 2} formation, and the hydrogenation and deuteration reactions of adsorbed species on dust grains in dense clouds, we experimentally investigated the diffusion of atomic hydrogen and deuterium on amorphous solid water (ASW) at temperatures of 8-15 K. The present study extended our previous study for selective detections of H and D atoms, and of H{sub 2} (J = 0 and 1) and D{sub 2} (J = 0 and 1) molecules adsorbed on ASW using both photo-stimulated desorption and resonance-enhanced multiphoton ionization, to investigate potential sites on ASW for diffusion, recombination dynamics, and the diffusion mechanism of H and D atoms. Our results demonstrate that the ASW surface contains various potential sites that can be categorized into at least three groups: very shallow, middle-, and deep-potential sites, with diffusion activation energies of { =}30 meV, respectively. The present study pictured the outline of H{sub 2} formation on cosmic ice dust at low temperatures: H atoms landing on the dust will diffuse rapidly at the abundant shallow and middle sites on ASW, and finally become trapped at deep sites. The H atoms that arrive next recombine with suchmore » trapped H atoms to yield H{sub 2} molecules. The small isotopic difference between the diffusion of H and D atoms on ASW indicates that the diffusion mechanism can be explained by thermal hopping, at least at middle-potential sites.« les

Revision total knee arthroplasty for unexplained pain after unicompartmental knee arthroplasty : a case report

In this report, we present a case of a 64-year-old woman who underwent revision of knee arthroplasty after failed unicompartmental knee arthroplasty (UKA). She underwent UKA (Biomet Oxford Phase 3) for right localized medial knee pain at the age of 53 and the postoperative course had been uneventful. Eight years after UKA, she had right knee pain that gradually worsened. Tenderness was present over the medial femorotibial and patellofemoral (PF) joints. Plain radiograph showed small osteophytes on the intercondylar eminence and in the lateral compartment. However, these findings were not severe. Although several causes of knee pain after UKA have been reported, none of those causes were found in this case, so the diagnosis of unexplained pain was made. We performed knee arthroscopy and it revealed severe osteoarthritis of the PF joint, bone attrition and exposure of subchondral bone of the medial part of the lateral condyle together with severe synovitis. Revision surgery was performed in the same operation. The postoperative course was excellent and the severe knee pain resolved after surgery. Several registries revealed that revision for unexplained pain was more common after UKA than after total knee arthroplasty. We pointed out the possible causes of unexplained pain including pathological conditions, which were present in our case. Revision surgery may be unsuccessful if the cause of failure is not adequately considered