The formation of molecular hydrogen on silicate dust analogs: The rotational distribution

Our laboratory experiments continue to explore how the formation of molecular hydrogen is influenced by dust and how dust thereby affects hydrogen molecules adsorbed on its surface. In Sabri et al., we present the preparation of nanometer-sized silicate grain analogs via laser ablation. These analogs illustrate extremes in structure (fully crystalline or fully amorphous grains), and stoichiometry (the forsterite and fayalite end-members of the olivine family). These were inserted in FORMOLISM, an ultra-high vacuum setup where they can be cooled down to ∼5 K. Atomic beams are directed at these surfaces and the formation of new molecules is studied via REMPI(2+1) spectroscopy. We explored the rotational distribution (0 ≤ J″ ≤ 5) of v″ = 0 of the ground electronic state of H₂. The results of these measurements are reported here. Surprisingly, molecules formed and ejected from crystalline silicates have a cold (T _rot ∼ 120 K) rotational energy distribution, while for molecules formed on and ejected from amorphous silicate films, the rotational temperature is ∼310 K. These results are compared to previous experiments on metallic surfaces and theoretical simulations. Solid-state surface analysis suggests that flatter grains could hinder the cartwheel rotation mode. A search for hot hydrogen, predicted as a result of H₂ formation, hints at its production. For the first time, the rotational distribution of hydrogen molecules formed on silicate dust is reported. These results are essential to understanding the chemistry of astrophysical media containing bare dust grains.