Formamide has been detected in many stellar and interstellar objects like the comet C/1995 O1 (Hale-Bopp),1 in the solid phase of dust grains around the young stellar object W33A2 and in the interstellar medium in general3. Formamide is proposed to be a key-molecule in the abiotic formation of important biomolecules4 and may be brought to planet Earth by the impact of dust particles or comets. However, there is still a debate on the formation mechanism of formamide. Observations with radio telescopes revealed that formamide and isocyanic acid have similar isotopic ratios5 and that their amounts correlate quiet well over a wide range of abundances6 suggesting that both species are chemically related to each other.
This contribution reports the electron-induced formation of formamide7 and isocyanic acid in condensed mixtures of CO and NH3. The results can help to unravel the reaction mechanism leading to their formation in space since high-energy radiation of all kinds produce copious numbers of low-energy secondary electrons when interacting with condensed matter.8 The secondary electrons can trigger the formation of reactive species like radicals and ions by neutral dissociation (ND), dissociative electron attachment (DEA) or electron impact ionization (EI) which subsequently can react with adjacent molecules to form more complex molecules.8 To provide evidence for the formation of formamide and isocyanic acid upon electron exposure, we prepared multilayer films containing equal amounts of CO and NH3 on a tantalum (Ta) substrate at cryogenic temperatures of ~30 K under UHV conditions and subsequently irradiate these films with low-energy electrons (E0<20 eV). Formamide and isocyanic acid were identified from characteristic mass-over-charge ratios using post-irradiation thermal desorption spectrometry (TDS). Product yields of isocyanic acid and formamide show the same dependence on incident electron-energy and the amount of isocyanic acid correlates linear to the amount of formamide suggesting a common intermediate in the formation process. For the formation of formamide we recently proposed two reaction mechanisms.7 At energies above the ionization threshold, we proposed a mechanism triggered by EI whereas at energies below the ionization threshold we proposed a mechanism triggered by DEA.7 However, our current models for the formation of formamide initiated by EI or DEA do not provide a straightforward explanation for the formation of isocyanic acid. Our hypothesis is that ND to NH3 produces a copious number of free hydrogen radicals which may react with an intermediate of the formamide production. This side reaction is favoured by the production of molecular nitrogen and may thus be very efficient. Future investigations may aim to determine ND cross sections for NH3 which could help to verify the proposed mechanism.
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(6) López-Sepulcre, A.; Jaber, A.A.; Mendoza, E.; Lefloch, B.; Ceccarelli, C.; Vastel, C.; Bachiller, R.; Cernicharo, J.; Codella, C.; Kahane, C.; Kama, M.; Tafalla, M. Shedding light on the formation of the pre-biotic molecule formamide with ASAI. Mon. Not. R. Astron. Soc. 2015, 449 (3), 2438–2458.
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