A desorption mechanism of water following vacuum-ultraviolet irradiation on amorphous solid water at 90 K

Behavior of Hydroxyl Radicals on Water Ice at Low Temperatures

ConspectusBecause chemical reactions on/in cosmic ice dust grains covered by amorphous solid water (ASW) play important roles in generating a variety of molecules, many experimental and theoretical studies have focused on the chemical processes occurring on the ASW surface. In laboratory experiments, conventional spectroscopic and mass-spectrometric detection of stable products is generally employed to deduce reaction channels and mechanisms. However, despite their importance, the details of chemical reactions involving reactive species (i.e., free radicals) have not been clarified because of the absence of experimental methods for in situ detection of radicals. Because OH radicals can be easily produced in interstellar conditions by not only the photolysis and/or ion bombardments of H₂O but also the reaction of H and O atoms, they are thought to be one of the most abundant radicals on ice dust. In this context, the development of a close monitoring method of OH radicals on the ASW surface may help to elucidate the chemical reactions occurring on the ASW surface.Recently, to detect OH radicals adsorbed on the ASW surface, we applied our developed method to sensitively and selectively detect surface adsorbates with a combination of photostimulated desorption and resonance-enhanced multiphoton ionization techniques. Using this method, we showed that an OH radical on the ASW surface can be desorbed upon one-photon absorption at 532 nm, at which wavelength both the OH radical and H₂O molecule are transparent. Theoretical calculations addressing an OH radical adsorbed on water clusters indicated that the valence A-X transition of an OH radical significantly red-shifts by ∼2 eV when the OH radical is strongly adsorbed to ice through three hydrogen bonds. With this method, the number density of surface OH can be monitored as a snapshot so that the behaviors of OH radicals, such as surface diffusion, can be studied. Moreover, the development of a system for studying the wavelength dependence of photodesorption may establish a foundation for future research investigating the absorption spectra of surface adsorbed species.Owing to the large electron affinity of OH radicals on ice, they are expected to easily become OH^- by electron attachment on the ASW surface. We characterized the behavior of OH^- on ASW at low temperatures, which may be relevant not only to physicochemical processes on cosmic ice dust and planetary atmosphere but also to understanding the electrochemical properties of ice. A negative constant current was found when ASW at temperatures below 50 K was exposed to both UV photons and electrons. It was demonstrated that the negative current is initiated by the formation of OH^- ions on the ASW surface, and they are transported to the bulk via the proton-hole transfer mechanism, which was predicted 100 years ago as a mirror image of proton transfer known as the Grotthuss mechanism. These results indicate that OH^- ions are readily transported to the bulk ice and further induce reactions, even at low temperatures where thermal diffusion is negligible. Therefore, in-mantle chemical processes that have been considered inactive at low temperatures are worth reevaluating.

Hydrogen Dissociation Dynamics from Water Clusters on Triplet-State Energy Surfaces

The ice surface provides two-dimensional reaction fields for bimolecular collisions in interstellar space. As H₂O molecules on the surface are typically exposed to cosmic rays, H₂O in the excited state is easily dissociated into H + OH, where a H atom is released from the surface to the gas phase. In the present study, the reaction dynamics of small-sized water clusters on the triplet-state potential energy (T₁) surface, following the vertical electronic excitation from the ground state (S₀), were investigated using direct ab initio molecular dynamics to provide insights into the generation mechanism of H atoms from an irradiated ice surface. In all clusters, that is, (H₂O)_n (n = 2-6), the H atom was directly dissociated from one of the H₂O molecules in the clusters (direct dissociation), whereas the OH radical remained in the cluster. In the branched form of H₂O tetramer (n = 4) and the book form of H₂O hexamer (n = 6), the dissociated hydrogen atom (H') collided with the neighboring H₂O molecule, and the exchange of H atoms occurred as H' + H₂O → H'-H₂O (collision) → H'OH + H (hydrogen exchange). The translational energy of the exchanged H atom decreases significantly (by approximately 50%) because the kinetic energy of the H' atom is efficiently transferred to the vibrational modes of the cluster during the H-exchange reaction. The mechanism of H atom dissociation is discussed based on theoretical results.