Radical-induced chemistry from VUV photolysis of interstellar ice analogues containing formaldehyde

Abstract

Surface processes and radical chemistry within interstellar ices are increasingly suspected to play an important role in the formation of complex organic molecules (COMs) observed in several astrophysical regions and cometary environments. We present new laboratory experiments on the low-temperature solid state formation of complex organic molecules -glycolaldehyde, ethylene glycol, and polyoxymethylene -through radical-induced reactivity from VUV photolysis of formaldehyde in water-free and water-dominated ices. Radical reactivity and endogenous formation of COMs were monitored in situ via infrared spectroscopy in the solid state and post photolysis with temperature programmed desorption (TPD) using a quadripole mass spectrometer. We show the ability of free radicals to be stored when formed at low temperature in water-dominated ices, and to react with other radicals or on double bonds of unsaturated molecules when the temperature increases. It experimentally confirms the role of thermal diffusion in radical reactivity. We propose a new pathway for formaldehyde polymerisation induced by HCO radicals that might explain some observations made by the Ptolemy instrument on board the Rosetta lander Philae. In addition, our results seem to indicate that H-atom additions on H 2 CO proceed preferentially through CH 2 OH intermediate radicals rather than the CH 3 O radical.

Radical-assisted polymerization in interstellar ice analogues: formyl radical and polyoxymethylene

“…It is also supported by laboratory experiments. POM and POM-like polymers can be easily formed from the heating of interstellar/cometary ice analogs containing H 2 CO, H 2 O, and a nucleophile (NH 3 , CN − ) (Schutte et al 1993;Danger et al 2014;Noble et al 2012;Vinogradoff et al 2011;Duvernay et al 2014) or from UV irradiation of interstellar/cometary ice analogues containing formaldehyde (Butscher et al 2016).…”

“…Thus, by exposing energetic and non-energetic processing on ice analogues, COMs formation is indeed observed but almost no reaction intermediates are identified this way (Schutte et al 1993;Butscher et al 2016;Bennett & Kaiser 2007). To solve this problem, we resort to the cryogenic matrix technique.…”

Vacuum Ultraviolet Photodesorption and Photofragmentation of Formaldehyde-Containing Ices

“…The formation of these CO and H 2 photoproducts was also detected in the H 2 -lamp irradiation of H 2 CO ices. 43,44 In addition, more complex molecules (glycolaldehyde, ethylene glycol and the formaldehyde polymer, polyoxymethylene (POM)) were also observed in Butscher et al 44 's experiments where a higher fluence than ours was used.…”

“…45). Understanding the formation of COMs is the objective of several experiments 44,46,47 and of several models, including grain surface reactions or possibly gas-phase reactions, [48][49][50][51][52] but the recent observation of COMs in the gas phase in pre-stellar cores, which are cold environments, was quite puzzling. [53][54][55] Their presence could be explained by non-thermal desorption, however the nature of the desorption mechanisms still has to be unravelled, with a joint experimental and modelling effort.…”

Efficient formation route of the prebiotic molecule formamide on interstellar dust grains

“…One experimental study also involves simultaneous UV irradiation and addition of H atoms onto mixtures of NO:CO, NO:H 2 CO or NO:CH 3 OH ices, which leads to a relatively high efficiency of NH 2 CHO production (Fedoseev et al 2016). However, the initial ice composition used in those experiments may be quite differ-ent from the observed water-dominated interstellar icy dust grains and water can change dramatically the final product of UV irradiation or H exposure (Butscher et al 2016;Watanabe et al 2004). The third and last scenario of formamide formation implies its direct synthesis as soon as the molecular mantle covering dust grains is being formed.…”