Isotopic segregation of molecular hydrogen on water ice surface at low temperature: Importance for interstellar grain chemistry

A first step, before studying the formation of molecules on surfaces at low temperature by recombination of atoms, is a good understanding of the adsorption/desorption mechanisms of the molecules themselves and a correct interpretation of the results of TPD (Thermally Programmed Desorption) experiments. Experimental studies (performed on our newly built experimental setup "FORMOLISM") of adsorption and desorption of molecular hydrogen and deuterium on an amorphous porous solid water (ASW) ice surface between 10 and 35 K reveal a very efficient isotopic segregation process. The slight difference in the maximum desorption temperature between the two isotopes leads to a preferential coverage of the sites binding more tightly of the ice surface by D-2, independent of the gas deposition process history. A statistical model, which takes into account thermodynamic aspects of adsorption sites as well as isotopic competition, is proposed to understand the enhancement of deuterium fractionation. Model results are in very good agreement with the experimental ones. This mechanism could play a key role in chemistry at the surface of interstellar dust grains. It could in particular explain the isotopic enrichment observed in some dark clouds. The next step will be to measure the rovibrational energy of the molecular hydrogen formed on such surfaces using a REMPI-TOF detection which allows for selective quantum state identification.