Experimental investigation of the ortho/para ratio of newly formed molecular hydrogen on amorphous solid water

Several astronomical observations have shown that the ortho/para ratio (OPR) of H₂ can differ from the expected statistical value of 3 or the local thermodynamic equilibrium (LTE) value at the gas or dust temperature. It is thus important to know the OPR of H₂ newly formed on dust grain surfaces, in order to clarify the dependence of the observed OPR in space on the formation process. Using an experimental setup designed to mimic interstellar medium environments, we measured the OPR of H₂ and D₂ formed on the surface of porous amorphous water ice held at 10K. We report for the first time the OPR value for newly formed D₂, consistent with the expected LTE value at the high-temperature limit found by previous theoretical and experimental works on the determination of the OPR upon H₂ formation on surfaces at low temperature.