Sticking and desorption of small physisorbed molecules: laboratory experiments and astrophysical applications

Small molecules like O2, N2 and CO are key species in the chemical evolution of preand protostellar cores. Since O2 and N2 do not have an electric dipole moment, direct observations of these spieces in cold environments are difficult. Gaseous N2 is known to be abundant in dense cores from indirect observations of the chemically related N2H + ion, but gas-phase O2 appears to be largely absent, with upper limits from space observations giving N(O2)/N(H2) ≤ 5·10−8 [1]. At sufficiently high densities (n(H2)> 10 5 cm−3) and low temperatures (<20 K), even volatile molecules like CO, N2 and O2 freeze-out from the gas-phase onto grains, forming a water-poor icy mantle layer. This freeze-out significantly influences the gas-phase chemistry of many species including N2H , since CO is one of its main destroyers. It also affects the abundance of H+3 and its level of deuterium fractionation [2]. Solid CO is readily observed through its stretching mode at 4.67 μm with an abundance comparable to, or larger than, that in the gas in the coldest regions [3]. Again, O2 and N2 cannot be detected directly in the ice but their presence may be inferred by their effect on the solid CO line profile. It is clear that in order to properly model the chemistry in cold dense cores, a good understanding of the freeze-out and desorption of these species is needed. In this contribution laboratory experiments will be presented using our new ultra-high vacuum experiment CRYOPAD to determine fundamental properties of ices, in particular sticking probabilities at low temperatures and molecular desorption energies. CO, N2 and O2 have been deposited on a poly-crystalline gold surface at 14 K in pure, mixed and layered structures [4,5,6]. For astrophysical relevance, ices with different thicknesses between 10 and 160 mono-layers have been investigated using Temperature Programmed Desorption (TPD) and Reflection Absorption InfraRed Spectra (RAIRS) techniques. The TPD spectra are used to determine the binding energies and desorption kinetics of these species under various conditions. It is concluded that N2 and CO molecules interact significantly with each other whereas O2 and CO molecules do not. The sticking coefficients of CO, N2 and O2 are all close to unity at low temperatures. The RAIR spectra of mixed and layered N2 -CO and O2 -CO ices are discussed