Photodissociation of water and O( 3 P J ) formation on a lunar impact melt breccia

Abstract Photodissociation of water deposited on an impact melt breccia collected during Apollo 16 was studied by measuring O( 3 P J = 2,1,0 ) photoproducts detected with resonance‐enhanced multiphoton ionization. For each spin‐orbit state, the oxygen atom time‐of‐flight (TOF) spectrum was measured as a function of H 2 O exposure and 157 nm irradiation time. Four Maxwell‐Boltzmann distributions with translational temperatures of 10,000 K, 1800 K, 400 K, and 102 K were required to fit the data. The most likely formation mechanisms are molecular hydrogen elimination following ion‐electron recombination, secondary recombination of hydroxyl radicals, and photodissociation of adsorbed hydroxyls. The irradiation time required to reach maximum oxygen signal suggests that water clusters into islands when adsorbing on the lunar impact melt breccia. After enough irradiation for the oxygen atom yield to reach its maximum, the slowly decreasing signal was fit with an exponential curve to obtain a cross section that represents the rate of surface hydroxyl depletion. For 0.1, 1, and 5 Langmuir (1 L = 10 −6  Torr s) H 2 O exposure, respectively, the measured O( 3 P) depletion cross sections were 4.9 × 10 −20 , 6.6 × 10 −20 , and 4.6 × 10 −20  cm 2 . These results imply that photodissociation of water on the lunar surface cannot account for the large mass‐16 (±1 amu) signal observed in the lunar atmosphere. Unless another significant source of oxygen atoms is present, this unexpectedly large signal is likely due to CH 4 or OH.