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

¹ Université de Cergy-Pontoise, Sorbonne Université, Observatoire de Paris, PSL University, CNRS, LERMA, 95000 Cergy-Pontoise, France
² Observatoire de Paris, Sorbonne Université, PSL University, CNRS, LERMA, 75014 Paris, France

Received: 5 February 2018
Accepted: 29 July 2018

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.