  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.
