Radiation Processing of Formamide and Formamide:Water Ices on Silicate Grain Analogue

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• 作者：M. Michele Dawley ; Claire Pirim ; Thomas M. Orlando
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：February 20, 2014
• 年：2014
• 卷：118
• 期：7
• 页码：1228-1236
• 全文大小：407K
• ISSN：1520-5215

文摘

Lyman-伪 (121.6 nm) photon and 1 keV electron-beam irradiation of pure HCONH₂ (FA) ice and H₂O:HCONH₂ ice mixtures on high-surface-area SiO₂ nanoparticles have been investigated with FT-IR spectroscopy and temperature programmed desorption (TPD). Lyman-伪 photolysis of pure amorphous FA ice grown at 70 K and crystalline FA ice produced by annealing to 165 K gives spectral signatures between 2120 and 2195 cm^鈥? that we assign primarily to OCN^{鈥?/sup> and CO. The OCN鈥?/sup> and CO yields are 25% less abundant for crystalline FA ice. Photon and electron processing also produces H₂ that is released from the ice between 90 and 140 K. A decrease in the H₂ TPD peak is seen for irradiated crystalline HCONH₂ ice. Lyman-伪 photolysis of H₂O:HCONH₂ mixed ices increases OCN鈥?/sup> and CO production, suggesting a catalytic role of H₂O. Also, for pure FA, 1 keV electron irradiation slightly increases the yield of OCN鈥?/sup>, while CO decarboxylation is selectively prevented. CO is also not produced in H₂O:HCONH₂ ices upon electron irradiation. Dissociative ionization, direct dissociative excitation, and dissociative electron attachment (DEA) channels are accessible in the Lyman-伪 (121.6 nm) photon and 1 keV electron-beam energy range. DEA energetically favors OCN鈥?/sup> and H鈥?/sup> formation, with the latter leading to H₂ formation. The FA fragment product identities, yields, and branching ratios are considerably different relative to the gas phase and depend upon the radiation type, ice structure, and the presence of SiO₂ nanoparticles. The latter may increase ion鈥揺lectron recombination and radical recombination rates. The main products observed suggest very different condensed-phase dissociation channels from those reported for gas-phase dissociation. Formation of ions/products from FA is not negligible upon Lyman-伪 photolysis or electron irradiation, both of which could process ices in interstellar regions as well as in Titan鈥檚 atmosphere.}