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Browsing by Author "DeSimone, Alice J."

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    Mechanisms and Cross Sections for Water Desorption from a Lunar Impact Melt Breccia
    (2014-04-30) DeSimone, Alice J.; Orlando, Thomas M.; University of Alabama Tuscaloosa
    Desorption of H2O (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 H2O (ν = 0) removal and destruction at 0.1 Langmuir (1 L = 10−6 Torr∙s) H2O exposure was measured to be (7.1 ± 1.9) × 10−19 cm2. 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 H2O on the sunlit lunar surface. Instead, photons are likely to cause H2O to desorb or dissociate. The OH+ fragment of H2O (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 × 1018 photons cm−2. Under these conditions, the cross section for H2O (ν*) photodesorption was measured to be 6.4 × 10−20 cm2 for an initial exposure of 5 L H2O.
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    Mechanisms of H2O Desorption from Amorphous Solid Water by 157-nm Irradiation: An Experimental and Theoretical Study
    (2013-10-23) DeSimone, Alice J.; Crowell, Vernon D.; Sherrill, C. David; Orlando, Thomas M.; University of Alabama Tuscaloosa
    The photodesorption of water molecules from amorphous solid water (ASW) by 157-nm irradiation has been examined using resonance-enhanced multiphoton ionization. The rotational temperature has been determined, by comparison with simulations, to be 425 ± 75 K. The time-of-flight spectrum of H2O (v = 0) has been fit with a Maxwell-Boltzmann distribution with a translational temperature of 700 ± 200 K (0.12 ± 0.03 eV). H+ and OH+ fragment ions have been detected with non-resonant multiphoton ionization, indicating vibrationally excited parent water molecules with translational energies of 0.24 ± 0.08 eV. The cross section for water removal from ASW by 7.9-eV photons near 100 K is (6.9 ± 1.8) × 10−20 cm2 for >10 L H2O exposure. Electronic structure computations have also probed the excited states of water and the mechanisms of desorption. Calculated electron attachment and detachment densities show that exciton delocalization leads to a dipole reversal state in the first singlet excited state of a model system of hexagonal water ice. Ab Initio Molecular Dynamics simulations show possible desorption of a photo-excited water molecule from this cluster, though the non-hydrogen bonded OH bond is stretched significantly before desorption. Potential energy curves of this OH stretch in the electronic excited state show a barrier to dissociation, lending credence to the dipole reversal mechanism.
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    O(P-3(J)) formation and desorption by 157-nm photoirradiation of amorphous solid water
    (American Institute of Physics, 2014-03-06) DeSimone, Alice J.; Orlando, Thomas M.; University System of Georgia; Georgia Institute of Technology; University of Alabama Tuscaloosa
    Photodissociation of amorphous solid water (ASW) deposited on a thinly oxidized copper substrate at 82 K was studied by measuring O(P-3(J=2,1,0)) photoproducts detected with resonance-enhanced multiphoton ionization. For each spin-orbit state, the oxygen atom time-of-flight spectrum was measured as a function of H2O exposure, which is related to ice thickness, and 157-nm irradiation time. Four Maxwell-Boltzmann distributions with translational temperatures of 10 000 K, 1800 K, 400 K, and 82 K were found to fit the data. The most likely formation mechanisms are molecular elimination following ionization of water and ion-electron recombination, secondary recombination of hydroxyl radicals, and photodissociation of adsorbed hydroxyl radicals. Evidence for O-atom diffusion through bulk ASW was found for H2O exposures of at least 5 Langmuir (1 L = 10(-6) Torr s). The cross sections for O(P-3(2)) depletion were 1.3 x 10(-19) and 6.5 x 10(-20) cm(2) for 1 and 5 L, respectively. (C) 2014 AIP Publishing LLC.
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    Photodissociation of methyl iodide adsorbed on low-temperature amorphous ice surfaces
    (American Institute of Physics, 2013-02-22) DeSimone, Alice J.; Olanrewaju, Babajide O.; Grieves, Gregory A.; Orlando, Thomas M.; University System of Georgia; Georgia Institute of Technology; University of Alabama Tuscaloosa
    Photodissociation dynamics of methyl iodide (CH3I) adsorbed on both amorphous solid water (ASW) and porous amorphous solid water (PASW) has been investigated. The ejected ground-state I(P-2(3/2)) and excited-state I(P-2(1/2)) photofragments produced by 260- and 290-nm photons were detected using laser resonance-enhanced multiphoton ionization. In contrast to gas-phase photodissociation, (i) the I(P-2(3/2)) photofragment is favored compared to I(P-2(1/2)) at both wavelengths, (ii) I(P-2(3/2)) and I(P-2(1/2)) have velocity distributions that depend upon ice morphology, and (iii) I-2 is produced on ASW. The total iodine [I(P-2(3/2))+ I(P-2(1/2))+ I-2] yield varies with substrate morphology, with greater yield from ASW than PASW using both 260- and 290-nm photons. Temperature-programmed desorption studies demonstrate that ice porosity enhances the trapping of adsorbed CH3I, while pore-free ice likely allows monomer adsorption and the formation of two-dimensional CH3I clusters. Reactions or collisions involving these clusters, I atomic fragments, or I-containing molecular fragments at the vacuum-surface interface can result in I-2 formation. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790585]
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    Photodissociation of water and O(P-3(J)) formation on a lunar impact melt breccia
    (American Geophysical Union, 2014-04-30) DeSimone, Alice J.; Orlando, Thomas M.; University System of Georgia; Georgia Institute of Technology; University of Alabama Tuscaloosa
    Photodissociation of water deposited on an impact melt breccia collected during Apollo 16 was studied by measuring O(P-3(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 H2O exposure and 157nm irradiation time. Four Maxwell-Boltzmann distributions with translational temperatures of 10,000K, 1800K, 400K, and 102K 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 (1L=10(-6)Torrs) H2O exposure, respectively, the measured O(P-3) depletion cross sections were 4.9 x 10(-20), 6.6 x 10(-20), and 4.6 x 10(-20)cm(2). These results imply that photodissociation of water on the lunar surface cannot account for the large mass-16 (1amu) signal observed in the lunar atmosphere. Unless another significant source of oxygen atoms is present, this unexpectedly large signal is likely due to CH4 or OH.