AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA6
Effects of Water Ice Films on Thermal Stability and Electron-activated Decomposition of CF@sub 2@Cl@sub 2@ on Metal Surfaces

Monday, November 3, 2003, 3:40 pm, Room 326

Session: Stimulated Processes at Surfaces
Presenter: N.S. Faradzhev, Rutgers University
Authors: N.S. Faradzhev, Rutgers University
C.C. Perry, Johns Hopkins University
D.O. Kusmierek, Rutgers University
D.H. Fairbrother, Johns Hopkins University
T.E. Madey, Rutgers University
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We report the kinetics of processes, activated by low energy electrons, for an environmentally important molecule CF@sub 2@Cl@sub 2@ co-adsorbed with water ice on metal surfaces (Ru; Au) at low temperature <100K. The results are compared with similar data obtained for CCl@sub 4@. TPD, RAIRS and XPS has been utilized. TPD reveals an increase of thermal stability of molecular CF@sub 2@Cl@sub 2@ on Ru(0001) when deposited under an amorphous solid water (ASW) overlayer at 25K or adsorbed inside ASW ice matrix; dissociation is completely inhibited. Weakly-bound CF@sub 2@Cl@sub 2@ desorbs from ASW surface at ~110K, but desorption of CF@sub 2@Cl@sub 2@ trapped in the ASW matrix is impeded until the onset of ice crystallization at ~155K. All techniques indicate that ionizing radiation (electrons or X-rays) incident on CF@sub 2@Cl@sub 2@/H@sub 2@O layer leads to rapid decomposition of halocarbon, which proceeds via dissociative attachment (DA) of low energy secondary electrons. The rates of decomposition for 1ML of CF@sub 2@Cl@sub 2@ and CCl@sub 4@ are similar on the metal surface and increase in the ASW ice environment indicating a possibly important role of a polar medium in halocarbon dissociation. The highest decomposition cross-sections are observed for submonolayer halocarbon coverage on a water ice surface: 1.0x10@super -15@ cm@super 2@ for CF@sub 2@Cl@sub 2@ and 2.5x10@super -15@ cm@super 2@ for CCl@sub 4@. The cross-sections are identical for halocarbon adsorbed on an ice surface and caged in a water matrix. The initial dominant radiation-induced process in adsorbed CF@sub 2@Cl@sub 2@ is C-Cl bond cleavage. For CF@sub 2@Cl@sub 2@ in ASW, fragments and reaction products include Cl@super -@ and F@super -@, H@sub 3@O@super +@, COF@sub 2@, and CO@sub 2@; for CCl@sub 4@ in ASW, COCl@sub 2@ and C@sub 2@Cl@sub 4@ are also seen. Product distributions, the kinetics of halocarbon damage, and subsequent reaction pathways are qualitatively independent of the radiation source.