Translational and rotational energy measurements of desorbed water molecules in their vibrational ground state following 157 nm irradiation of amorphous solid water

Water ice is the major solid component in a variety of astrophysical environments, e.g., cold and dense molecular clouds. Photodesorption plays a dominant role in consuming ice in such cold regions. In this study, photodesorption of vibrationally ground-state H₂O(v = 0) from amorphous solid water has been investigated at 157 nm. Using a resonance-enhanced multiphoton ionization technique, the translational and rotational energy distributions of photodesorbed H₂O(v = 0) were measured, i.e., Boltzmann distributions at 1800 and 300 K, respectively. These energies are in good accordance with those predicted by classical molecular calculations for water photodesorption due to a kick-out mechanism following absorption of a single photon; hot H atom released by photodissociation of H₂O in ice transfers enough momentum to another H₂O molecule to kick it off the surface. Desorption of D₂O(v = 0) following 193 nm photoirradiation of a D₂O/H₂S mixed ice was investigated to provide further direct evidence for the operation of a kick-out mechanism. The other desorption mechanisms were also discussed in the context of possible photodesorption of vibrationally excited H₂O.