Mechanisms and cross sections for water desorption from a lunar impact melt breccia

Desorption of H 2 O (ν = 0) following 157‐nm irradiation of amorphous solid water on a lunar impact melt breccia was measured with resonance‐enhanced multiphoton ionization. Photofragments of vibrationally excited water were detected with nonresonant ionization. The average cross section for H 2 O (ν = 0) removal and destruction at 0.1 Langmuir (1 L = 10 −6 Torr∙s) H 2 O exposure was measured to be (7.1 ± 1.9) × 10 −19 cm 2 . Cross sections were also measured at 0.3, 1, 5, and 10 L exposures. Because these cross sections increase drastically with decreasing water coverage, water is not expected to remain intact as H 2 O on the sunlit lunar surface. Instead, photons are likely to cause H 2 O to desorb or dissociate. The OH + fragment of H 2 O (ν*) increased in intensity with increasing irradiation as hydroxyl groups built up on the surface and then recombined. The OH + signal eventually began to decrease after a dose of 5 × 10 18 photons cm −2 . Under these conditions, the cross section for H 2 O (ν*) photodesorption was measured to be 6.4 × 10 −20 cm 2 for an initial exposure of 5 L H 2 O.