Behavior of Hydroxyl Radicals on Water Ice at Low Temperatures

Abstract

Because 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 redshifts 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.

Proton Transport and Related Chemical Processes of Ice

“…Different proposals were also reported for the transport mechanism of negative charge carriers in ice. Recently, Watanabe and co-workers − studied the behavior of OH radicals produced by ultraviolet (UV) photolysis of ASW. When the ASW films grown on a metal substrate were exposed to both UV photons and low energy electrons at 10–50 K, negative current was measured at the metal substrate along with a decrease in OH radical population at the ASW surface.…”

Radical Addition and H Abstraction Reactions in C2H2, C2H4 and C2H6: A Gateway for Ethyl and Vinyl Bearing Molecules in the Interstellar Medium

“…However, since OH is less mobile, this reaction may compete with OH diffusion. Recent studies are highlighting the importance of the OH radical in surface astrochemistry (Tsuge & Watanabe 2021;Ishibashi et al 2021), and AD1.2 is a good candidate to study these new avenues for reactivity on interstellar ices, due to the small activation barrier. Figure 1 shows the importance of quantum tunneling at low temperatures, rendering the reaction rate constants with H at low temperature orders of magnitude higher than for the heavier radical counterparts in respect to their barriers.…”

“…All these motions favor hydrogen atoms as main chemistry initiators on grains, owing to a fast diffusion (Hama et al 2012;Senevirathne et al 2017;Ásgeirsson et al 2017;Nyman 2021) and the possibility of tunnel through potential energy barriers (Hama & Watanabe 2013). However, and thanks to non-diffusive mecha-nisms, appearing as a consequence of an excess of energy inoculated into a particle (i.e., after photodissociation, chemical reaction or cosmic-ray interaction Jin & Garrod (2020); Garrod et al (2022)), other radicals (OH, NH 2 , CH 3 ) may play an important role in the chemical evolution of the ISM (Fedoseev et al 2017;Chuang et al 2020;Ioppolo et al 2021;Tsuge & Watanabe 2021;Ishibashi et al 2021).…”

Methanediol CH2(OH)2 and hydroxymethyl CH2OH+: key organic intermediates on the path to complex organic molecules

“…This possibility is yet to be investigated. A recent experimental and theoretical quantum investigation points out that simulations with high accuracy using a large ice cluster are very demanding (Miyazaki et al 2020;Tsuge & Watanabe 2021). Thus, an alternative approach for those calculations was to use small clusters as a realistic amorphous model to interpret the redshift observed under their experiments (Miyazaki et al 2020).…”