Nuclear spin conversion in hydrogenated molecules at gas-solid interface – GASOSPIN

Molecules with identical nuclei can exist in different spin modification (or 'nuclear spin isomer'). The best known example is molecular hydrogen H2 (H atom having a spin ½) that exists as ortho and para isomers, depending whether the spins of the protons are parallel (total nuclear spin I=1) or anti-parallel (total nuclear spin I=0), respectively. Numerous hydrogenated molecules that are observed in comets and interstellar medium do present such isomers. In accordance with Pauli's principle and symmetry properties the molecular wave functions, spin isomer states can be identified through rotation-vibration spectroscopy. From the relative intensities of spectral lines recorded by spatial and earth-based telescopes, it is possible to derive the population ratios of different nuclear spin isomers. With thermodynamic equilibrium, an ortho :para ratio of 3 :1 is for instance expected for H2O or H2CO at temperatures higher than 50 K. Below this threshold, the ratios become strongly temperature dependent. In many astrophysical media however, the ratios are not consistent with the local temperature of the gas. Assuming a negligible probability for Nuclear Spin Conversion (NSC) in low densities gas phase media, astrophysicists have suggested that ortho:para ratio might be a signature of physical conditions prevailing in the past, such as initial molecular formation temperature or interactions with dust grains (that might strongly affect those ratio). Our understanding of the nuclear spin conversion dynamics is not currently sufficient to validate such hypothesis. Within scientific programs named "Planetary science" and "Physics and Chemistry of the Interstellar Medium" driven by the French National Institute for Science of the Universe (INSU-CNRS), two research teams of PhLAM (Lille) and LPMAA (Paris) laboratories collaborate since 2005, with the aim to characterise conversion probabilities for various species. A recent work performed by the PhLAM group has confirmed that H2CO can not be converted in a gaseous interstellar environment. In parallel, studies in condensed phase performed by the LPMAA group have demonstrated the ability of relatively fast interconversion for water at very low temperature. New investigations at gas-solid interfaces and after adsorption-desorption processes on cold ices or grains are now highly required in order to interpret astrophysical data. In this project and with the participation of a third partner (PIIM, Marseille), we propose to provide new quantitative experimental data and models for NSC of hydrogenated molecules of astrophysical interest in various physical conditions. Our research program addresses in particular the role of interactions and interplay between gas and solid matter. Our strategy is based on our long standing expertise in theoretical end experimental investigations of the NSC process, and consists in priority of performing a set of complementary well-controlled experiments. A part of our program is devoted the development of new techniques for the separation of spin isomers together with the development sensitive diagnostics schemes for the determination of ortho-para ratio in pressure and temperatures ranges that are very relevant for astrophysics.