Surface Processes on Interstellar Amorphous Solid Water: Adsorption, Diffusion, Tunneling Reactions, and Nuclear-Spin Conversion

In this paper, we have reviewed physicochemical processes of neutral species on the ASW surface, concentrating on surface dynamics, quantum-tunneling reactions, and NSC, which are all closely related to chemical evolution in dense interstellar clouds. Recent developments in experimental methods and advances in theory have answered a number of questions in astronomy, and at the same time a variety of further challenges have arisen alongside this progress. We first reviewed the present state of knowledge with respect to the structure and physical properties of ASW, as well as the surface physics of adsorbed species, focusing on H atoms and recombination dynamics yielding H2, in sections 2 and 3. There are multiple potential sites of different depths for adsorption and diffusion on the ASW surface. However, much less is known about the molecular-level structure of ASW at cryogenic temperatures, which is one of the important open issues in interstellar chemistry. Section 3 was devoted to describing adsorption and diffusion on ASW. Our understanding of the surface dynamics of adsorbed species on ASW is still rather qualitative, especially as regards diffusion of adsorbed species. In section 4 we highlighted chemical reactions on surfaces at low temperatures leading to the formation of astrochemically relevant molecules (H2O, H2CO, CH 3OH, and CO2) and discussed the role of quantum tunneling. In dense clouds, where temperatures are as low as 10 K, the long-time interaction between reactants on ice mantles enables reactions with small cross sections to occur. In addition, since the surface can act as absorber of the heat of the reaction, simple addition and recombination reactions without dissociation occur efficiently on dust grains. Adsorption, diffusion, and desorption of reactants significantly affect the overall rates of reactions and reaction dynamics on the surface. Moreover, interactions with environmental molecules (e.g., H2O) can modify the shape of the reaction potential and the energy levels of the initial, transition, and final states in the reaction system. These effects would be crucial for the efficiency of quantum tunneling. Recently catalytic effects for some reactions relevant to astrochemistry are studied theoretically, while an experimental approach is also necessary without doubt for investigation of the real effects of ASW surfaces. The challenges for surface chemistry are essential not only for astrochemistry, but also for a better understanding of heterogeneous chemistry. NSC on/in the condensed phase can be drastically accelerated by intermolecular interactions. Recent significant progress in both theory and experimentation reviewed in section 5 offers key insights to extract the physical and chemical processes behind astronomical observations. However, our understanding of NSC in molecules induced by intermolecular interactions is still at an early stage, and many questions have yet to be addressed, as summarized in section 5. In this review, we did not describe other important physical and chemical processes occurring in interstellar space, e.g., gas-phase reactions, radiation (photons, ions, and electrons) effects on the ice mantle, and surface processes on other astrochemically relevant materials, such as silicate, carbonaceous solids, and metals, where chemisorption can play a role. Nevertheless, as shown in this review, interstellar astrochemistry is truly an interdisciplinary science covering quantum chemistry, spectroscopy, chemical-reaction dynamics, and surface physics, as well as vacuum science and cryogenics. It is therefore fascinating to researchers in many fields. Approaches from each discipline are certainly needed to understand the meaning of astrochemical phenomena. © 2013 American Chemical Society.