Review pubs.acs.org/CR
Surface Processes on Interstellar Amorphous Solid Water: Adsorption, Diffusion, Tunneling Reactions, and Nuclear-Spin Conversion Tetsuya Hama and Naoki Watanabe* Institute of Low Temperature Science, Hokkaido University, N19W8 Kita-ku, Sapporo, Hokkaido 060-0819, Japan 5.2. Ortho and Para States of Other Abundant Molecules (H2O, etc.) 5.2.1. Nuclear-Spin Conversion of Polyatomic Molecules by Collisions 5.2.2. Nuclear-Spin Conversion of Polyatomic Molecules in Bonding Systems 6. Summary Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References
CONTENTS 1. Introduction 1.1. Importance of Surface Processes in Interstellar Chemistry 2. Physical Properties of Amorphous Solid Water (ASW): Ice-Mantle Analogues 3. Adsorption and Diffusion of Volatile Species 3.1. Sticking Coefficients 3.2. Adsorption (Desorption) Energies 3.3. Surface Diffusion 3.3.1. Surface Diffusion of H Atoms and H2 Formation through H-Atom Recombination 3.3.2. Energy Partitioning in H2 Formation through Recombination 4. Quantum-Tunneling Reactions on Surfaces 4.1. Water Formation 4.1.1. O2 + H (Sequential H-Atom Addition to O2) 4.1.2. OH + H2 4.1.3. Other Reactions 4.2. Formaldehyde and Methanol Formation and Deuteration 4.2.1. Hydrogen and Deuterium Addition to CO 4.2.2. Deuterium Enrichment of CH3OH and H2CO 4.3. Carbon Dioxide Formation 4.3.1. CO + O 4.3.2. CO + OH 5. Nuclear-Spin Modifications of Molecules 5.1. ortho- and para-H2 Molecules and Their Roles in Interstellar Chemistry 5.1.1. Nascent Ortho-to-Para Ratio of H 2 Molecules Formed on Ice Mantles 5.1.2. Nuclear-Spin Conversion of H2 Molecules
© XXXX American Chemical Society
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1. INTRODUCTION Since we intend to review experimental and theoretical studies of physicochemical processes occurring on cosmic dust grains in very cold regions in space, we start with a brief introduction of the properties of such regions; i.e., interstellar clouds and cosmic dust. Interstellar clouds, the birthplaces of stars and planets,1−4 consist of gases and submicrometer-sized dust grains formed from refractory compounds (e.g., silicates or carbonaceous material) assembled by gravity.5 Spectroscopic observations, such as infrared (IR) absorption measurements and (sub)millimeter-wave emission, have been successful in revealing in detail the chemical and physical conditions pertaining to such clouds. Interstellar clouds can be categorized into several types, depending on their size, density, temperature, and chemical composition.1,2,6 A diffuse cloud is a region where the temperature is ≤80 K, and the number density of gas particles represented by molecular hydrogen (H2) is approximately 102 molecules cm−3. Cold (10 K) and dense (typically containing 104 H2 molecules cm−3) regions are called “dense clouds” (see Figure 1). Compared with a gas density of 1019 molecules cm−3 under terrestrial atmospheric pressure, the pressure in such dense clouds is still extremely low. Low temperature is achieved by submicrometer-sized dust grains in clouds, which contribute ∼1% of matter by mass and ∼10−12 times as much as the gaseous H2 molecules by number.7 These dust grains shield the cloud’s inner region from radiation by external stars. In fact, dense clouds often look like dark silhouettes in visible light, and they are therefore also called
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Special Issue: 2013 Astrochemistry
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Received: February 13, 2013
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dx.doi.org/10.1021/cr4000978 | Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews
Review
Figure 1. (Left) Part of the Orion molecular cloud containing the Horse Head Nebula (also known as Barnard 33). The red glow behind the Horse Head is the emission nebula IC 434, which originates from hydrogen gas ionized by the bright star σ Orionis. Bright spots at the base of the Horse Head Nebula are young stars in the process of forming. (Credit and copyright: N. A. Sharp/NOAO/AURA/NSF.) (Right) Schematic description of the morphological and chemical structure of dust grains. After the temperature and photon field decrease when the density of dust particles increases in dense clouds, atoms (e.g., H, O, C, N) and molecules (e.g., CO) deposit onto the dust surfaces. Cold-surface reactions proceed on the grain surface, and an ice mantle is eventually formed.
“dark clouds”. In dense clouds, gas and dust collapse isothermally under the effects of gravity to form prestellar cores, where the density becomes higher (105−7 H2 molecules cm−3), while the temperature is still as low as 10 K. When they become optically thick, heating occurs, and IR radiation is emitted; the resulting objects are called young stellar objects (YSOs).3,8,9 Table 1 summarizes the cosmic abundances of some major elements, normalized to that of hydrogen.1,10−13 Among these,
Table 2. Gas-Phase Molecular Abundances in a Dense Cloud (Taurus Molecular Cloud-1, TMC-1), Normalized to H2a H2 CO O2b OH H2Oc C2 CN CH C4H NH3 H2CO CS SO CH3OH HCOOH
Table 1. Approximate Cosmic Abundances of Some Elements Normalized to Hydrogena H Heb Dc C N O Mg Si S Ca Ti Cr Fe Ni
1 1 2 2 7 5 3 2 1 2 7 3 3 1
× × × × × × × × × × × × ×
10−1 10−5 10−4 10−5 10−4 10−5 10−5 10−5 10−6 10−8 10−7 10−5 10−6
1 8 × 10−5
