Akira Kouchi

Reaction kinetics and isotope effect of water formation by the surface reaction of solid H2O2 with H atoms at low temperatures

We performed laboratory experiments on the formation of water and its isotopologues by surface reactions of hydrogen peroxide (H2O2) with hydrogen (H) atoms and their deuterated counterparts (D2O2, D) at 10-30 K. High-purity H2O2 (> 95%) was prepared in situ by the codeposition of molecular oxygen and H atoms at relatively high temperatures (45-50 K). We determined that the high-purity H2O2 solid reacts with both H and deuterium (D) atoms at 10-30 K despite the large activation barriers (-2000 K). Moreover, the reaction rate for H atoms is approximately 45 times faster than that for D atoms at 15 K. Thus, the observed large isotope effect indicates that these reactions occurred through quantum tunneling. We propose that the observed HDO/H2O ratio in molecular clouds might be a good tool for the estimation of the atomic D/H ratio in those environments.

Statistical ortho-to-para ratio of water desorbed from ice at 10 kelvin

Water isomers hide their origin H2O exists in two spin isomers, ortho and para, in a ratio of 3:1 at room temperature. Some astronomical observations have found water with a ratio of less than 3, thought to be due to water being photodesorbed from ice that had been formed at very low temperatures (≾30 K). Hama et al. tested this idea in the laboratory, by forming water ice at low temperature and then photodesorbing it to measure the ortho:para ratio. They found a ratio of 3, even at 10 K. Thus, another explanation for the low ratios in some astronomical objects must be found. Science, this issue p. 65 The ortho-para ratio of water desorbed from ice does not reflect its formation temperature. The anomalously low ortho-to-para ratios (OPRs) exhibited by gaseous water in space have been used to determine the formation temperature (<50 kelvin) of ice on cold interstellar dust. This approach assumes that the OPR of water desorbed from ice is related to the ice formation temperature on the dust. However, we report that water desorbed from ice at 10 kelvin shows a statistical high-temperature OPR of 3, even when the ice is produced in situ by hydrogenation of O2, a known formation process of interstellar water. This invalidates the assumed relation between OPR and temperature. The necessary reinterpretation of the low OPRs will help elucidate the chemical history of interstellar water from molecular clouds and processes in the early solar system, including comet formation.

Signatures of Quantum-Tunneling Diffusion of Hydrogen Atoms on Water Ice at 10 K

Reported here is the first observation of the tunneling surface diffusion of a hydrogen (H) atom on water ice. Photostimulated desorption and resonance-enhanced multiphoton ionization methods were used to determine the diffusion rates at 10 K on amorphous solid water and polycrystalline ice. H-atom diffusion on polycrystalline ice was 2 orders of magnitude faster than that of deuterium atoms, indicating the occurrence of tunneling diffusion. Whether diffusion is by tunneling or thermal hopping also depends on the diffusion length of the atoms and the morphology of the surface. Our findings contribute to a better understanding of elementary physicochemical processes of hydrogen on cosmic ice dust.

Quantum Tunneling Hydrogenation of Solid Benzene and Its Control via Surface Structure.

Despite the rapid accumulation of structural information about organic materials, the correlation between the surface structure of these materials and their chemical properties, a potentially important aspect of their chemistry, is not fully understood. Here, we show that the amorphous or crystalline structure of a solid benzene surface controls its chemical reactivity toward hydrogen. In situ infrared spectroscopy revealed that cold hydrogen atoms can add to an amorphous benzene surface at 20 K to form cyclohexane by tunneling. However, hydrogenation is greatly reduced on crystalline benzene. We suggest that the origin of the high selectivity of this reaction is the large difference in geometric constraints between the amorphous and the crystalline surfaces. The present findings can lead us to a more complete understanding of heterogeneous reaction systems, especially those involving tunneling, as well as to the possibility of nonenergetic surface chemical modification without undesired side reactions or physical processes.

Nanodiamond Finding in the Hyblean Shallow Mantle Xenoliths

Most of Earth’s diamonds are connected with deep-seated mantle rocks; however, in recent years, μm-sized diamonds have been found in shallower metamorphic rocks, and the process of shallow-seated diamond formation has become a hotly debated topic. Nanodiamonds occur mainly in chondrite meteorites associated with organic matter and water. They can be synthesized in the stability field of graphite from organic compounds under hydrothermal conditions. Similar physicochemical conditions occur in serpentinite-hosted hydrothermal systems. Herein, we report the first finding of nanodiamonds, primarily of 6 and 10u2009nm, in Hyblean asphaltene-bearing serpentinite xenoliths (Sicily, Italy). The discovery was made by electron microscopy observations coupled with Raman spectroscopy analyses. The finding reveals new aspects of carbon speciation and diamond formation in shallow crustal settings. Nanodiamonds can grow during the hydrothermal alteration of ultramafic rocks, as well as during the lithogenesis of sediments bearing organic matter.

Quantum tunneling observed without its characteristic large kinetic isotope effects

Significance Quantum tunneling, an important phenomenon in many surface and interfacial chemical processes, is strongly dependent on the isotope of the tunneling atom. However, surface tunneling during the hydrogenation/deuteration of solid benzene at 15–25 K is accompanied by an almost semiclassical kinetic isotope effect (KIE) of 1–1.5, which is much lower than that intrinsic to tunneling (≳100), because isotopically insensitive surface diffusion of the adsorbed atoms controls the chemical kinetics. Our results suggest that tunneling has been unrecognized in studies of the chemistry of condensed phases, and small-KIE tunneling may account for the unexplained fast reactions of hydrogen and deuterium observed in surface/interface chemical systems such as aerosols, enzymes, and interstellar dust grains. Classical transition-state theory is fundamental to describing chemical kinetics; however, quantum tunneling is also important in explaining the unexpectedly large reaction efficiencies observed in many chemical systems. Tunneling is often indicated by anomalously large kinetic isotope effects (KIEs), because a particle’s ability to tunnel decreases significantly with its increasing mass. Here we experimentally demonstrate that cold hydrogen (H) and deuterium (D) atoms can add to solid benzene by tunneling; however, the observed H/D KIE was very small (1–1.5) despite the large intrinsic H/D KIE of tunneling (≳100). This strong reduction is due to the chemical kinetics being controlled not by tunneling but by the surface diffusion of the H/D atoms, a process not greatly affected by the isotope type. Because tunneling need not be accompanied by a large KIE in surface and interfacial chemical systems, it might be overlooked in other systems such as aerosols or enzymes. Our results suggest that surface tunneling reactions on interstellar dust may contribute to the deuteration of interstellar aromatic and aliphatic hydrocarbons, which could represent a major source of the deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. These findings could improve our understanding of interstellar physicochemical processes, including those during the formation of the solar system.

Surface Temperature Dependence of Hydrogen Ortho-Para Conversion on Amorphous Solid Water

The surface temperature dependence of the ortho-to-para conversion of H_{2} on amorphous solid water is first reported. A combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques allowed us to sensitively probe the conversion on the surface of amorphous solid water at temperatures of 9.2-16xa0K. Within a narrow temperature window of 8xa0K, the conversion time steeply varied from ∼4.1×10^{3} to ∼6.4×10^{2}u2009u2009s. The observed temperature dependence is discussed in the context of previously suggested models and the energy dissipation process. The two-phonon process most likely dominates the conversion rate at low temperatures.

Evolution of Morphological and Physical Properties of Laboratory Interstellar Organic Residues with Ultraviolet Irradiation

Refractory organic compounds formed in molecular clouds are among the building blocks of the solar system objects and could be the precursors of organic matter found in primitive meteorites and cometary materials. However, little is known about the evolutionary pathways of molecular cloud organics from dense molecular clouds to planetary systems. In this study, we focus on the evolution of the morphological and viscoelastic properties of molecular cloud refractory organic matter. We found that the organic residue, experimentally synthesized at ∼10 K from UV-irradiated H 2 O-CH 3 OH-NH 3 ice, changed significantly in terms of its nanometer-to micrometer-scale morphology and viscoelastic properties after UV irradiation at room temperature. The dose of this irradiation was equivalent to that experienced after short residence in diffuse clouds (10 4 years) or irradiation in outer protoplanetary disks. The irradiated organic residues became highly porous and more rigid and formed amorphous nanospherules. These nanospherules are morphologically similar to organic nanoglobules observed in the least-altered chondrites, chondritic porous interplanetary dust particles, and cometary samples, suggesting that irradiation of refractory organics could be a possible formation pathway for such nanoglobules. The storage modulus (elasticity) of photo-irradiated organic residues is ∼100 MPa irrespective of vibrational frequency, a value that is lower than the storage moduli of minerals and ice. Dust grains coated with such irradiated organics would therefore stick together efficiently, but growth to larger grains might be suppressed due to an increase in aggregate brittleness caused by the strong connections between grains.

Nuclear spin temperatures of hydrogen and water molecules on amorphous solid water

To clarify the meaning of the nuclear spin temperatures of H2 and H2O molecules associated with various astronomical targets, it is important to understand the mechanisms that could alter these temperatures; i.e., the molecules’ ortho/para nuclear-spin ratio (OPR). We have performed a series of experiments to investigate how the OPRs of H2 and H2O behave on the surface of amorphous solid water (ASW), which is analogous to cosmic ice dust. The OPR of H2 initially shows a high temperature limit of 3 upon its formation through H–H recombination at ∼10 K and gradually decreases toward lower temperatures on the surface. The spin temperatures of H2O molecules that are thermally desorbed from various types of ASW at ∼10 K always return the high-temperature limit.