  Using a combination of photostimulated desorption and resonance-enhanced
multiphoton ionization methods, the behaviors of OH radicals on the surface of
interstellar ice analog was monitored at temperatures between 54 and 80 K. The
OH number density on the surface of ultraviolet (UV)-irradiated compact
amorphous solid water gradually decreased at temperatures above 60 K. Analyzing
the temperature dependence of OH intensities with the Arrhenius equation, the
decrease can be explained by recombination of two OH radicals, which is
rate-limited by thermal diffusion of OH. The activation energy for surface
diffusion was experimentally determined for the first time to be 0.14 +/- 0.01
eV, which is larger than or equivalent to those assumed in theoretical models.
This value implies that the diffusive reaction of OH radicals starts to be
activated at approximately 36 K on interstellar ice.
