STAGE Nuclear spin Conversion of molecular hydrogen trapped on icy surfaces

Due to Pauli’s exclusion principle, molecules like H2, H2O or CH4 having protons in exchangeable positions exist in several nuclear spin configurations. In case of water, they are called ortho and para depending if the spins of the protons are parallel (total nuclear spin I=1) or anti-parallel (I=0). In gaseous phase, each rotational state is associated with only one of the nuclear magnetic species and in the high temperature limit (above 50 K), it is known that 1/4 of the molecules are para while 3/4 are ortho. Below 50 K, the Ortho-to-Para Ratio (OPR) at equilibrium becomes strongly temperature-dependent. Under stellar radiations, comets and interstellar dust release in space a great variety of hydrogenated molecules detected from space or ground-based telescopes. The abundances of ortho- and para- H2 are known to play a strong role in the chemistry of hydrogenated molecules like H2O, NH3, or c-C3H2. It is then crucial to understand and quantify the physical processes that may influence these abundances in the interstellar medium, especially on the surface of the icy mantles of interstellar grains. Performed in physical conditions close to the ones found in space, the measurement will help to answer to the following question: is the disequilibrium observed between the relative abundance of nuclear spin families a clue of the thermal history of the hydrogenated molecules in space?