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#274 new enhancement

H2 formation pumping

Reported by: Gary J. Ferland Owned by: nobody
Priority: good to do Milestone:
Component: atomic/molecular data base Version: trunk
Keywords: Cc:


The Formation of Molecular Hydrogen on Silicate Dust Analogs: The Rotational 
Gavilan, L.; Lemaire, J. L.; Vidali, G.; Sabri, T.; Jæger, C.
AA(;, AB(LERMA, UMR 8112 du CNRS, de l'Observatoire de Paris et de l'Université de Cergy Pontoise, 5 mail Gay Lussac, F-95000 Cergy 
Pontoise Cedex, France), AC(Visiting Professor. Permanent address: Syracuse 
University, Physics Department, Syracuse, NY 13244-1320, USA.), AD(Laboratory 
Astrophysics and Cluster Physics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena.), AE(Laboratory Astrophysics and Cluster Physics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena.)
The Astrophysical Journal, Volume 781, Issue 2, article id. 79, 13 pp. (2014). (ApJ Homepage)
Publication Date:	
Astronomy Keywords:	
astrochemistry, dust, extinction, ISM: molecules, molecular processes
Bibliographic Code:	

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.

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