Are molecule-covered dust grains efficient catalysts of H ₂ formation in the cold ISM?

The formation of H ₂ in the interstellar medium (ISM) involves complex processes, some of which are still not understood. In cold regions, it is assumed that H ₂ formation follows Langmuir kinetics, i.e. the immediate desorption of incoming atoms or molecules on a surface already saturated with molecules. Our experiments address this issue by studying the formation of H ₂ on a dust surface dosed with molecules prior to atomic exposure. We simulate ISM conditions at T _dust < 10K and T _gas ~ 90K and use a synthesized amorphous silicate. By coupling laser detection to thermal desorption spectroscopy, we confirm that hydrogen recombination is promptly enhanced. We interpret this as a result of enhanced atomic diffusion (both hopping thermal and quantum mechanical tunnelling). Moreover, since H ₂ formation is the most exothermic chemical reaction per unit mass, we elucidate its importance as a non-thermal desorption mechanism. We apply these results to dense ISM regions where H ₂ formation and its induced desorption are curbed by a declining atomic gas-phase abundance. We further propose this as a pathway to deuterium fractionation in pre-stellar cores. More importantly, we show that dust remains an active catalyst even in the coldest ISM.