Thermal desorption of formamide and methylamine from graphite and amorphous water ice surfaces

Abstract

Context. Formamide (NH 2 CHO) and methylamine (CH 3 NH 2 ) are known to be the most abundant amine-containing molecules in many astrophysical environments. The presence of these molecules in the gas phase may result from thermal desorption of interstellar ices. Aims. The aim of this work is to determine the values of the desorption energies of formamide and methylamine from analogues of interstellar dust grain surfaces and to understand their interaction with water ice. Methods. Temperature programmed desorption (TPD) experiments of formamide and methylamine ices were performed in the submonolayer and monolayer regimes on graphite (HOPG) and non-porous amorphous solid water (np-ASW) ice surfaces at temperatures 40-240 K. The desorption energy distributions of these two molecules were calculated from TPD measurements using a set of independent Polanyi-Wigner equations.Results. The maximum of the desorption of formamide from both graphite and ASW ice surfaces occurs at 176 K after the desorption of H 2 O molecules, whereas the desorption profile of methylamine depends strongly on the substrate. Solid methylamine starts to desorb below 100 K from the graphite surface. Its desorption from the water ice surface occurs after 120 K and stops during the water ice sublimation around 150 K. It continues to desorb from the graphite surface at temperatures higher than160 K. Conclusions. More than 95 % of solid NH 2 CHO diffuses through the np-ASW ice surface towards the graphitic substrate and is released into the gas phase with a desorption energy distribution E des = 7460 − 9380 K, which is measured with the best-fit preexponential factor A=10 18 s −1 . However, the desorption energy distribution of methylamine from the np-ASW ice surface (E des =3850-8420 K) is measured with the best-fit pre-exponential factor A=10 12 s −1 . A fraction of solid methylamine monolayer of roughly 0.15 diffuses through the water ice surface towards the HOPG substrate. This small amount of methylamine desorbs later with higher binding energies (5050-8420 K) that exceed that of the crystalline water ice (E des = 4930 K), which is calculated with the same preexponential factor A=10 12 s −1 . The best wetting ability of methylamine compared to H 2 O molecules makes CH 3 NH 2 molecules a refractory species for low coverage. Other binding energies of astrophysical relevant molecules are gathered and compared, but we could not link the chemical functional groups (amino, methyl, hydroxyl, and carbonyl) with the binding energy properties. Implications of these high binding energies are discussed.

Reactivity of formic acid (HCOOH) with H atoms on cold surfaces of interstellar interest

“…Using the TPD data of HCOOH on gold surface for m/z 46 and the set of independant Polanyi-Wigner equations described in the paper (Chaabouni et al 2018), we estimated the desorption energy distribution of HCOOH on gold surface for HCOOH exposure dose N = 1.3 ML. Figure.…”

The formation of peptide-like molecules on interstellar dust grains

“…In pure form CH 3 NH 2 thermally desorbs at relatively low temperatures of 100-120 K (e.g. Chaabouni et al 2018), while in these experiments it desorbs at ∼220 K. Due to bulk ice release with HNCO it is unclear in the TPD traces if some CH 3 NH 2 releases around 110 K. The release of CH 3 NH 2 around 220 K can be explained by the formation of an OCN − CH 3 NH + 3 salt complex, which has a higher desorption temperature, similar to the OCN − NH + 4 complex. Reference data on the thermal decomposition of the OCN − CH 3 NH + 3 complex is needed to confirm this.…”

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

“…Taking into account the very low volatility of formamide (Chaabouni et al 2018) and the efficient mechanism to form this pre-biotic molecule in the solid state as presented in this work, allows us to reconcile observations of a high amount of formamide in material sampled by COSAC at the comet 67P surface and its low gas phase abundance. In addition, the relative low abundance of HNCO measured in cometary soil corroborates a solid state formation route to formamide decoupled from HNCO formation.…”

“…Recently, it has been presented as the link in the chemical chain able to reconcile the "genetic first" and "metabolism first" paradigms (Saladino et al 2012). Of all iCOMs, formamide has a very high binding energy, is known to survive on dust grains beyond sublimation of water (Urso et al 2017;Chaabouni et al 2018)â Aȃ, and is therefore a very important molecule being also a source of interstellar nitrogen locked into a refractory animated species. Formamide has been detected in comet coma with a relatively low abundance (some 10 −4 , with respect to water) (Biver et al 2014;Le Roy et al 2015), while it is the second most abundant cometary soil compound (1.8% relative to water ice) as measured by the COSAC mass spectrometer on board the Philae lander that touched down on comet 67P/C-G (Goesmann et al 2015)â Aȃ.…”