Surface reactions with OH radical on interstellar ice grains would contribute to the formation of complex molecules or those precursors in molecular clouds. Therefore, diffusion process of OH radicals on dust grain surface should be clarified experimentally. However, there have been no experimental reports on the diffusion of OH because of experimental difficulty for detecting OH on ice. In an infrared absorption spectroscopy, an OH radical spectrum can be hardly separated from H2O band in ice. Temperature-programmed desorption little provides the information about behavior of OH radical on water ice surface. Using the combination of photostimulated desorption (PSD) and resonance-enhanced multiphoton ionization (REMPI), we succeeded in detecting OH radicals on the ice surface, which were produced by UV exposure. The intensity of OH radical was found to depend on the substrate temperature and decreased rapidly from 60 K to 80 K. Assuming that the decrease results from OH-OH recombination: the formation of H2O2. The activation energy of recombination was determined by Arrhenius equation. Because the recombination would be a radical-radical barrierless reaction, it would be limited by the surface diffusion of OH radical. Diffusion activation energy of OH radical on the ice surface is derived to be 0.13 ± 0.01 eV.

Activation energy of OH-radical diffusion on water ice surface

2019 Fall National Meeting

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