Segregation effect and N₂ binding energy reduction in CO-N₂ systems adsorbed on water ice substrates

Abstract

Context. CO and N2 are two abundant species in molecular clouds. CO molecules are heavily depleted from the gas phase towards the centre of pre-stellar cores, whereas N2 maintains a high gas phase abundance. For example, in the molecular cloud L183, CO is depleted by a factor of ≈400 in its centre with respect to the outer regions of the cloud, whereas N2 is only depleted by a factor of ≈20. The reason for this difference is not yet clear, since CO and N2 have identical masses, similar sticking properties, and a relatively close energy of adsorption. Aims. We present a study of the CO-N2 system in sub-monolayer regimes, with the aim to measure, analyse and elucidate how the adsorption energy of the two species varies with coverage, with much attention to the case where CO is more abundant than N2. Methods. Experiments were carried out using the ultra-high vacuum (UHV) set-up called VENUS. Sub-monolayers of either pure 13CO or pure 15N2 and 13CO:15N2 mixtures were deposited on compact amorphous solid water ice, and crystalline water ice. Temperature-programmed desorption experiments, monitored by mass spectrometry, are used to analyse the distributions of binding energies of 13CO and 15N2 when adsorbed together in different proportions. Results. The distribution of binding energies of pure species varies from 990 K to 1630 K for 13CO, and from 890 K to 1430 K for 15N2. When a CO:N2 mixture is deposited, the 15N2 binding energy distribution is strongly affected by the presence of 13CO, whereas the adsorption energy of CO is unaltered. Conclusions. Whatever types of water ice substrate we used, the N2 effective binding energy was significantly lowered by the presence of CO molecules. We discuss the possible impact of this finding in the context of pre-stellar cores.

High-sensitivity maps of molecular ions in L1544

“…We find that N 2 H + is more deuterated than HCO + , as expected since N 2 (and N 2 H + in turn) is a late-type molecule and is less affected by depletion (Nguyen et al 2018). Therefore, N 2 H + appears in conditions where CO is already depleted and H 2 D + is more abundant.…”

Structure and water attachment rates of ice in the atmosphere: role of nitrogen

“…[37][38][39]42] As a final comment, notice that the conclusions drawn here stem mainly from the low adsorption energies of nitrogen on ice. Since our own results provide an adsorption energy barely 2.5 kJ/mol smaller than the upper bound of 9.6 kJ/mol reported in Temperature Programmed Desorption experiments, [34][35][36] we expect that improvements on the force field employed here will not change our conclusions qualitatively. into the surface.…”

“…Our results provide ∆H ads = 7.4 kJ/mol for the basal face and ∆H ads = 6.5 kJ/mol for the prismatic face. Bearing in mind that these results are measured from adsorption data in the range between 210 and 270 K, i.e., 150 K above the BET experiments, and enthalpies of adsorption are expected to decrease with temperature, [34,36] our results seem very reasonable. For each isotherm, the pressure as a function of density was fitted to:…”

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

“…During the warm-up phase of the sample from 10 to 200 K, the species desorbing from the surface into the gas phase are detected with the QMS by their mass-to-charge ratio (m/z). The VENUS setup described in recent papers (Minissale et al 2019;Nguyen et al 2018) is composed of a main UHV chamber with a low pressure of 10 −10 mbar, where a gold-coated copper sample holder is placed in the central position of the main chamber and thermally connected to a Helium liquid closed cycle cryocooler. This main chamber itself is linked to an intermediate UHV chamber through five diaphragms of 3 mm in diameter materializing the direction of five beam-lines aimed to the center of the gold surface.…”