Photoinduced polycyclic aromatic hydrocarbon dehydrogenation The competition between H- and H2-loss

Polycyclic aromatic hydrocarbons (PAHs) constitute a major component of the interstellar medium carbon budget, locking up to 10–20% of the elemental carbon. Sequential fragmentation induced by energetic photons leads to the formation of new species, including fullerenes. However, the exact chemical routes involved in this process remain largely unexplored. In this work, we focus on the first photofragmentation steps, which involve the dehydrogenation of these molecules. For this, we consider a multidisciplinary approach, taking into account the results from experiments, density functional theory (DFT) calculations, and modeling using dedicated Monte-Carlo simulations. By considering the simplest isomerization pathways — i.e., hydrogen roaming along the edges of the molecule — we are able to characterize the most likely photodissociation pathways for the molecules studied here. These comprise nine PAHs with clearly different structural properties. The formation of aliphatic-like side groups is found to be critical in the first fragmentation step and, furthermore, sets the balance of the competition between H- and H2-loss. We show that the presence of trio hydrogens, especially in combination with bay regions in small PAHs plays an important part in the experimentally established variations in the odd-to-even H-atom loss ratios. In addition, we find that, as PAH size increases, H2 formation becomes dominant, and sequential hydrogen loss only plays a marginal role. We also find disagreements between experiments and calculations for large, solo containing PAHs, which need to be accounted for. In order to match theoretical and experimental results, we have modified the energy barriers and restricted the H-hopping to tertiary atoms. The formation of H2 in large PAHs upon irradiation appears to be the dominant fragmentation channel, suggesting an efficient formation path for molecular hydrogen in photodissociation regions (PDRs).

Incorporación del modelo de Landau al modelo fractal de una erupción

El modelo Sequential Fragmentation/Transport (SFT, por su sigla en Inglés) se basa en una fragmentación secuencial a nivel macroscópico. En Brenes & Alvarado (2013) dicho modelo sirvió de base para proponer un modelo fractal de una erupción, al sistematizar todo el proceso con la inclusión del coeficiente H de Hurst. Esta sistematización fue posteriormente ampliada en Brenes (2014), con ayuda del modelo fractal de fractura propuesto por Balankin, para incluir procesos de fractura a nivel microscópico. En el presente trabajo, la inclusión del modelo de Landau para transformaciones de segundo orden permite deducir el modelo de fractura de Giffith y la regla de Eötvös, y obtener como sub-producto expresiones que sugieren la existencia de procesos de fractura tales como la corrosión por esfuerzos, endurecimiento por dilatación y la vitrificación. Finalmente, se presentan resultados sugerentes de una transición al vidrio en el magma.