Diffusion mechanism of hydrogen and deuterium atoms and the spin temperature of molecules on the surface of water ice at 8-15 K

Details Diffusion mechanism of hydrogen and deuterium atoms and the spin temperature of molecules on the surface of water ice at 8-15 K Diffusion mechanism of hydrogen and deuterium atoms and the spin temperature of molecules on the surface of water ice at 8-15 K

May 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.190h&link_type=abstract

The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole

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Scientific paper

The formation of H2 is the most fundamental chemical process in interstellar clouds, since it initiates the chemical evolution there. It has been widely accepted that the H2 is formed on interstellar dust by recombination subsequent to elementary processes of H-atoms, i.e., adsorption, diffusion, and encounter with another adsorbed H-atom. Recently, several groups have experimentally approached the issue of H-atom diffusion on amorphous solid water (ASW) at around 10 K. However, the activation energy of H-atom-diffusion on ASW has been still under debate. In the present study, we investigated the behavior of H-atoms on the surface of ASW using photo-stimulated desorption and resonance enhanced multiphoton ionization methods. The diffusion rate of H-atoms was directly measured after H atom deposition on ASW at 8 K, and is found to consist of the fast and slow components due to the presence of at least two types of potential sites on ASW with the energy depths of ˜ 20 and >50 meV, respectively. Efficient H2 formation was observed on ASW during the H-atom deposition on ASW at 8 -15 K via the fast diffusion at the shallow sites, while H-atoms trapped in the deeper sites hardly migrate at 8 K. When H-atoms were deposited on polycrystalline ice (PCI) at 8 K, the number of H-atoms on PCI was found to diminish very rapidly, implying that the surface of PCI consists of only shallow potential sites on which H-atoms diffuse very rapidly, resulting in H2 formation. The diffusion rates were found to be very similar between H- and D-atoms, suggesting that the thermal hopping mechanism better explains the diffusion rather than tunneling diffusion. In addition, we determined the ortho/para nuclear spin ratio (OPR) of nascent H2 formed via recombination and also the spin conversion on ASW. The OPR of nascent H2 formed from H-atoms on ASW at 8 K is very close to that of H2 adsorbed on ASW. However, when we left the H2 molecules on ASW, the population of the para state increases slightly while that of the ortho state decreases. This result indicates that the nuclear spin temperature, which is defined by OPR of H2, decreases on ASW by the spin conversion. We further tried to measure the nuclear spin temperature of another molecule in the condensed phase. This result will be also reported.

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