  The photodesorption efficiencies of the hydroxyl (OH) radical from the water
ice surface were measured in the range of 310-700 nm for the first time.
Although isolated H$_2$O molecules and OH radicals do not absorb visible
photons, the photodesorption of OH interacting with the ice surface was found
to occur by a one-photon process in the entire visible range. The
photodesorption efficiency strongly depended on the wavelength, and the
analyses of photodesorption cross sections indicated that only strongly bound
OH radicals were desorbed at longer wavelengths, whereas both weakly and
strongly bound OH radicals were desorbed at shorter wavelengths.
