Formation Pumping of Molecular Hydrogen in Dark Clouds

Abstract

Many theoretical and laboratory studies predict H 2 to be formed in highly excited ro-vibrational states. The consequent relaxation of excited levels via a cascade of infrared transitions might be observable in emission from suitable interstellar regions. In this work, we model H 2 formation pumping in standard dense clouds, taking into account the H/H 2 transition zone, through an accurate description of chemistry and radiative transfer. The model includes recent laboratory data on H 2 formation, as well as the effects of the interstellar UV field, predicting the populations of gas-phase H 2 molecules and their IR emission spectra. Calculations suggest that some vibrationally excited states of H 2 might be detectable towards lines of sight where significant destruction of H 2 occurs, such as X-ray sources, and provide a possible explanation as to why observational attempts resulted in no detections reported to date.Subject headings: interstellar: molecules -molecular: processes -infrared: ISM: lines and bands HD formed on HOPG held at 15 K has been found in ro-vibrational states (v, J) = (1 −

Surface chemistry in the interstellar medium

“…Le Bourlot et al (1995a) showed that the excitation due to the secondary electrons generated by cosmic rays was much more efficient, even for the standard value of the cosmic ionization rate. Similar conclusions were reached by Islam et al (2010) in a detailed modeling study including the most recent experimental information on the excitation of H 2 and assuming however the "standard" formation rate of 3 ×10 −17 n H n(H) cm 3 s −1 . Congiu et al (2009) also found that internal excitation is quenched before desorption from experiments involving amorphous water ice.…”

“…Models labelled "A" correspond to a fixed formation rate of molecular hydrogen of 3 × 10 −17 cm 3 s −1 , following the prescriptions of Hollenbach et al (2009) and Islam et al (2010); in models "B", H 2 is formed only by LH mechanism, and in models "C" both the LH and ER mechanisms are activated to form H 2 . The model geometry is a plane parallel infinite slab with an isotropic radiation field impinging on both sides.…”

“…In summary, despite the large number of laboratory and theoretical studies that have been completed on this subject, a detailed description of H 2 formation is still missing. As a consequence, A76, page 2 of 17 several authors (Hollenbach et al 2009;Islam et al 2010;Kaufman et al 2006;Sheffer et al 2011) prefer to use an ad hoc "standard" value of 3 × 10 −17 n H n(H) cm 3 s −1 , determined by Copernicus and FUSE observations for diffuse clouds, and/or introduce an additional multiplication factor of two for dense and bright PDRs.…”

“…Alternative scenarios were proposed by Black & van Dishoeck (1987). Recent experimental results obtained for cold graphitic surfaces (Islam et al 2010) and silicates (Lemaire et al 2010) show that high vibrational levels are preferentially populated in the formation process.…”

Trends in the adsorption and reactivity of hydrogen on magnesium silicate nanoclusters

“…9a'' signifies that the starting 2H chem configuration is that of Fig. We also include data calculated for C2/C1 adsorption of two H atoms on the (MgO) 6 cluster, the nano-olivine_b and nano-olivine_c clusters based on the initial C2-type H adsorptions shown in Fig. 5e.…”