Speaker
Description
In the field of astrochemistry and surface science a primary objective is to identify and characterize effective catalysts which have a plausible existence in the interstellar medium (ISM). Molecular hydrogen ($H_2$) is the most abundant molecule in the ISM with well-established and efficient catalytic formation routes in many regions of interstellar space. However, in certain regions of the ISM, discrepancies between formation rates and dissociations rates have been found [1]. Specifically, in Photodissociation regions (PDRs), the dissociation rate is so high that prevailing formation routes may not be efficient enough to explain the observed abundancies of $H_2$ and needed formation rates. Here we examine a group of planar nanosized molecules called polycyclic aromatic hydrocarbons (PAHs) as possible catalysts of $H_2$ formation. Significant abundancies of PAHs have been observed to spatially overlap with regions of high $H_2$ formation rates [2].
We have used temperature programmed desorption (TPD) and scanning tunneling microscopy (STM) to examine a template PAH, coronene ($C_{24}H_{12}$). Density functional theory (DFT) reveals that coronene and possibly other PAHs will have 0eV energy barriers for both Hydrogen (H) addition and $H_2$ abstraction even at low super-hydrogenation degrees (4 adatoms) [3]. The state of hydrogenation will therefore depend on these competing processes and their relative cross sections for addition, $\sigma_{add}$, and abstraction, $\sigma_{abs}$.
Monolayers of neutral PAH molecules are deposited on a highly oriented pyrolytic graphite (HOPG) surface and exposed to different fluences of D or H atoms. The atomic beam is estimated to have a temperature of approximately 1000K. Fully deuterated coronene ($C_{24}D_{36}$) is observed, indicating formation of HD via an exchange process between H and D [4].
First addition cross sections, $\sigma_{add}$(0) for D/H addition can be found experimentally from the exponential decay in pristine coronene as a function of D/H fluence. Cross sections of $\sigma_{add,H}$(0) = $0.25 \pm {0.14\atop 0.05} $Å$^2$ for H addition and $\sigma_{add,D}$(0) = $0.065 \pm {0.10\atop 0.05} $Å$ ^2$ for D addition are best fits to data. Determination of sequential addition and abstraction cross sections, however, require further modelling and will here be determined through comparisons to Kinetic Monte-Carlo (KMC) simulations.
Also observed from TPD is indications of preferred D/H-PAH configurations with high stability and increased barriers against further D/H addition. Experimental data points towards a barrier preventing addition to the center ring. These barriers are not detected when a high temperature atomic beam is used (T≈2000K) [5].
References:
[1] Tielens, A., Reviews of Modern Physics, 85 (2013) 1021-1081.
[2] Habart, E.; et. al., Astronomy and Astrophysics 397 (2003) 623-634.
[3] Rauls, E.; and Hornekær L., The Astrophysical Journal 679 (2008) 531.
[4] Mennella, Vito, et alThe Astrophysical Journal Letters 745.1 (2011): L2.
[5] Thrower, J. D., et al The Astrophysical Journal 752.1 (2012): 3.
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