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Departmental Colloquium

Non-thermal Processes on Lunar Surfaces and at Buried Ice: Graphite Grain Interfaces  
Guest Speaker
Prof. Thomas Orlando  
Guest Affiliation
School of Chemistry and Biochemistry and School of Physics Georgia Institute of Technology  
Thursday, November 7, 2013 4:00 pm - 5:00 pm  
Physics Auditorium Rm. 202  

Photodesorption and photodissociation of H2O following 157-nm irradiation of amorphous solid water deposited on an impact melt breccia from the Apollo 16 mission has been studied. The desorbing H2O and O(3PJ=2,1,0) products were detected with resonance-enhanced multiphoton ionization (REMPI). Vibrationally excited water was also detected with non-resonant ionization. Direct desorption involves exciton decay which is in competition with molecular dissociation events on the surface. Cross sections for H2O (v = 0) removal were found to increase with decreasing coverage and are considerably higher than photodesorption from common metal oxides. This work represents the first measurement of an absolute photodesorption cross section from an actual lunar sample. The vacuum ultraviolet (121.6 nm) synthesis of carbon dioxide on ice-coated graphite and isotopic labeled 13C graphite has also been examined. The results show that CO2 can be formed at the buried ice:graphite interface with Lyman-α photon irradiation via reaction of radicals (O and OH) produced by direct photodissociation and dissociative electron attachment (DEA) of the interfacial water molecules. The synthesized CO2 molecules can desorb in hot photon dominated regions (PDRs) and lost to space when ice coated carbonaceous dust grains cycle within the protoplanetary disks. Thus, non-thermal formation of CO2 at the buried ice:grain interface by VUV photons may regulate the carbon inventory during the early stage of planet formation. This may help explain the carbon deficits in our solar system, and suggests that a universal carbon deficit gradient may be expected within astrophysical bodies surrounding center stars.