XUV photodesorption of carbon cluster ions and ionic photofragments from a mixed methane–water ice

Abstract : The photochemical processing of a CH 4 : D 2 O 1 : 3.3 ice mixture adsorbed on an HOPG surface in the XUV regime was investigated using pulses obtained from the Free-electron LASer in Hamburg (FLASH) facility. Ice films were exposed to femtosecond pulses with a photon energy of hn = 40.8 eV, consistent with the HeII resonance line. Cationic species desorbing directly from the ice films were detected using time-of-flight (ToF) mass spectrometry. Simple ions formed through the fragmentation of the parent molecules and subsequent recombination reactions were detected and are consistent with efficient D + and H + ejection from the parent species, similar to the case for low energy electron irradiation. The FEL fluence dependencies of these ions are linear or exhibit a non-linear order of up to 3. In addition, a series of C n + cluster ions (with n up to 12) were also identified. These ions display a highly non-linear desorption yield with respect to the FEL fluence, having an order of 6-10, suggesting a complex multi-step process involving the primary products of CH 4 fragmentation. Two-pulse correlation measurements were performed to gain further insight into the underlying reaction dynamics of the photo-chemical reactions. The yield of the D 2 O derived products displayed a different temporal behaviour with respect to the C n + ions, indicating the presence of very different reaction pathways to the two families of ionic products.
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T. Suhasaria, J. Thrower, R. Frigge, S. Roling, Mathieu Bertin, et al.. XUV photodesorption of carbon cluster ions and ionic photofragments from a mixed methane–water ice. Physical Chemistry Chemical Physics, Royal Society of Chemistry, 2018, 20 (11), pp.7457-7469. ⟨10.1039/c8cp00171e⟩. ⟨hal-02349594⟩

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