A rotatable quadrupole mass spectrometer was used for thermal desorption spectroscopy and time-of-flight studies of mols. desorbing from Pd(111) covered by <1 monolayer H₂O during and after excimer light irradn. (at 5.0, 6.4 eV). A bilayer structure is formed even at submonolayer coverage. The UV irradn. shifts the H2O desorption peak to lower temps. (approaching the temp. for multilayer ice) and gives rise to a new feature at 185 K. The 185 K peak is attributed to recombination of photodissocd. H₂O.