Article pubs.acs.org/JPCA
Radiation Processing of Formamide and Formamide:Water Ices on Silicate Grain Analogue M. Michele Dawley, Claire Pirim, and Thomas M. Orlando* School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, Georgia 30332-0400, United States S Supporting Information *
ABSTRACT: Lyman-α (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-α 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−1 that we assign primarily to OCN− and CO. The OCN− 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-α photolysis of H2O:HCONH2 mixed ices increases OCN− and CO production, suggesting a catalytic role of H2O. Also, for pure FA, 1 keV electron irradiation slightly increases the yield of OCN−, 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-α (121.6 nm) photon and 1 keV electron-beam energy range. DEA energetically favors OCN− and H− 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−electron 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-α photolysis or electron irradiation, both of which could process ices in interstellar regions as well as in Titan’s atmosphere. has been classified as a hot first-generation molecule (>100 K)8 that likely forms in the ices from processing of smaller interstellar molecules (H2O, NH3, HCN, and CO). FA could then undergo radiation processing to form larger biomolecules. Although not yet directly identified in Titan’s atmosphere (Saturn’s largest moon), FA may form from radiation processing of HCN + H2O or NH3 + CO mixtures on aerosols in the atmosphere of Titan.9−11 Though NH3 and CO are not very abundant on Titan, they have been found in trace amounts in the stratosphere12,13 and as trace components of haze aerosols and surface ices by Cassini.11 Titan has been considered by some to be a present day prebiotic planetary body that could yield clues regarding the thermal or nonthermal formation of molecules of biological relevance. Indeed, the organic chemistry of the atmosphere and surface can be quite complex due to the radiation sources.14,15 Solar extreme ultraviolet (∼10−121 nm) and Lyman-α radiation (121.6 nm) penetrate into Titan’s haze
1. INTRODUCTION Icy surfaces are prevalent in cold regions of space, including satellites and planets, interstellar dust grains, ring particle grains, and comets. These ices are exposed to stellar winds, galactic cosmic rays, magnetospheric electrons and ions, and, in some locations, ultraviolet (UV) irradiation.1 The bombardment of ices by energetic particles and photons can lead to fragmentation or sputtering of molecules residing on or within the ices. Formation of new molecules can also occur from recombination of the radicals or ions produced by the radiation. Generally, the dominant energy-loss processes associated with photon and electron excitation are ionization and direct electronic excitation. In the case of inelastic electron scattering, dissociative singlet and triplet states are produced. In addition, transient negative ions can also form, and these can decay via a process typically referred to as dissociative electron attachment (DEA). As discussed in the companion article,2 complex molecules including formamide (HCONH2, hereafter denoted FA) have been observed toward the galactic centers, Sgr A and Sgr B,3−5 where temperatures can range from 40 to 300 K.6 FA has also been identified in high-mass young stellar objects (YSOs,7 and © 2014 American Chemical Society
Received: April 30, 2013 Revised: January 23, 2014 Published: January 24, 2014 1228
dx.doi.org/10.1021/jp4042815 | J. Phys. Chem. A 2014, 118, 1228−1236
The Journal of Physical Chemistry A
Article
down to 700 km15 where cold organic-laden aerosols also exist.12 Titan’s surface temperature is ∼94 K; however, temperatures can reach as high as 175 K in the upper atmosphere,14 and exogenous and endogenous energy sources can initiate reactions of small organics. These can then precipitate onto the surface for further processing.16 FA’s decomposition channels have been studied theoretically17 and experimentally.10,18−22 A recent theoretical study of FA decomposition in the gas-phase17 reported that H2O loss (dehydration of HCONH2 to HCN + H2O) was the most favorable channel and occurred through a multistep pathway involving a formimidic acid (H−NC(H)−OH) intermediate. In addition, CO elimination (decarboxylation to CO + NH3) is the second most kinetically favored pathway. Finally, H2 loss (dehydrogenation to H2 + HNCO) was found to occur primarily through a one-step process, although a two-step mechanism is competitive. Recent FA gas-phase laser spark experiments indicate the formation of HCN, CO, NH3, CO2, N2O, HONH2, and CH3OH from FA.18 Several studies have also focused on the photodecomposition of FA in matrices, leading to HNCO + H2 and NH3 + CO or dehydration products (HCN + H2O and HNC + H2O).10,19,20 Other studies include liquid FA irradiation by 200 nm femtosecond pulses21 and irradiation of ∼300 K FA by synchrotron radiation at 10−20 eV.22 The latter study demonstrated formation of the HNCO+ ion from two pathways, one involving H2 formation and the other involving H + H loss.22 That work also reports the gas-phase ionization energy (IE or IP) of FA as 10.220 ± 0.005 eV, which may result in a negligible ionization of FA in interstellar regions where hydrogen Lyman-α emission is prevalent.22 However, recent ab initio calculations report the ionization potentials to be ∼10 eV for gas-phase and ∼8 eV for aqueous solution.23 This suggests that the IP is
