Chemistry and photophysics of polycyclic aromatic hydrocarbons in the interstellar medium

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium, and it is thought that they are a key factor in the formation of molecular hydrogen at high gas and dust grain temperatures. We have explored how PAHs can contribute to the formation of H2 by taking a small PAH molecule, coronene, and expose it to a beam of hydrogen atoms. We found that this leads to the addition of H atoms to the coronene molecules. The addition process is impeded by a barrier for the addition of every second H atom. Moreover, we find that the hydrogenation states with 5, 11, and 17 additional H atoms are extra stable. To study how these molecules can withstand the harsh conditions of the interstellar medium, we have exposed regular and superhydrogenated coronene to UV radiation from the BESSY II synchrotron in Berlin. These experiments show that superhydrogenated coronene is prone to losing its extra H atom upon absorption of a photon, whereas regular coronene will be mainly ionized. Lastly, the experimental results are used as input for a numerical model of a radiation-rich interstellar gas cloud, to understand how this affects the formation of H2. The results of this simulation show that PAHs are the dominant source of H2 formation in these clouds, and that PAHs are necessary to explain the observed rates of H2 formation

The sequence to hydrogenate coronene cations:A journey guided by magic numbers

The understanding of hydrogen attachment to carbonaceous surfaces is essential to a wide variety of research fields and technologies such as hydrogen storage for transportation, precise localization of hydrogen in electronic devices and the formation of cosmic H2. For coronene cations as prototypical Polycyclic Aromatic Hydrocarbon (PAH) molecules, the existence of magic numbers upon hydrogenation was uncovered experimentally. Quantum chemistry calculations show that hydrogenation follows a site-specific sequence leading to the appearance of cations having 5, 11, or 17 hydrogen atoms attached, exactly the magic numbers found in the experiments. For these closed-shell cations, further hydrogenation requires appreciable structural changes associated with a high transition barrier. Controlling specific hydrogenation pathways would provide the possibility to tune the location of hydrogen attachment and the stability of the system. The sequence to hydrogenate PAHs, leading to PAHs with magic numbers of H atoms attached, provides clues to understand that carbon in space is mostly aromatic and partially aliphatic in PAHs. PAH hydrogenation is fundamental to assess the contribution of PAHs to the formation of cosmic H2.</p

Deexcitation Dynamics of Superhydrogenated Polycyclic Aromatic Hydrocarbon Cations after Soft-x-Ray Absorption

We have investigated the response of superhydrogenated gas-phase coronene cations upon soft x-ray absorption. Carbon (1s)⟶π⋆ transitions were resonantly excited at hν =285 eV. The resulting core hole is then filled in an Auger decay process, with the excess energy being released in the form of an Auger electron. Predominantly highly excited dications are thus formed, which cool down by hydrogen emission. In superhydrogenated systems, the additional H atoms act as a buffer, quenching loss of native H atoms and molecular fragmentation. Dissociation and transition state energies for several H loss channels were computed by means of density functional theory. Using these energies as input into an Arrhenius-type cascade model, very good agreement with the experimental data is found. The results have important implications for the survival of polyaromatic hydrocarbons in the interstellar medium and reflect key aspects of graphene hydrogenation

The sequence to hydrogenate coronene cations: A journey guided by magic numbers

The understanding of hydrogen attachment to carbonaceous surfaces is essential to a wide variety of research fields and technologies such as hydrogen storage for transportation, precise localization of hydrogen in electronic devices and the formation of cosmic H2. For coronene cations as prototypical Polycyclic Aromatic Hydrocarbon (PAH) molecules, the existence of magic numbers upon hydrogenation was uncovered experimentally. Quantum chemistry calculations show that hydrogenation follows a site-specific sequence leading to the appearance of cations having 5, 11, or 17 hydrogen atoms attached, exactly the magic numbers found in the experiments. For these closed-shell cations, further hydrogenation requires appreciable structural changes associated with a high transition barrier. Controlling specific hydrogenation pathways would provide the possibility to tune the location of hydrogen attachment and the stability of the system. The sequence to hydrogenate PAHs, leading to PAHs with magic numbers of H atoms attached, provides clues to understand that carbon in space is mostly aromatic and partially aliphatic in PAHs. PAH hydrogenation is fundamental to assess the contribution of PAHs to the formation of cosmic H2

Mid-Eocene thrusting in the Northern Lesser Antilles:Unraveling the eastern imprint of the Bahamas Bank collision

International audienceUpper plates in subduction zones are prone to record slab dynamics as their strain pattern, uplift-subsidence records and volcanic arc activity accommodate variations of slab parameters in terms of dip, density and rheology. The ANR GAARAnti aims at tracking the timing of land emersions and submersions along the Lesser Antilles subduction zone, which is key to understand the long-term mechanical behavior of this subduction zone. Here we focus on the subduction of a buoyant anomaly that flattens the slab, slows down subduction rate and can even lead to collision and crustal thickening if the anomaly is large enough. Although the accommodation by the upper plate of such episodes is rather well described across colliding zones (Cyclades, Cuba, Tibet), the along strike evolution of the trench from collision to flat slab remains poorly studied. In this frame, the junction between the Greater and Lesser Antilles appears to be an ideal target to track the lateral impact of such collision within the upper plate: indeed, at 56 Ma, the buoyant Bahamas Bank entered the Greater Caribbean subduction zone at its eastern tip and collided with the volcanic arc from West to East diachronically. To the West, along Cuba the collision led to the suture of the subduction zone. Along Hispaniola and Puerto Rico, the upper plate shortening, which is limited to the southeast by the Anegada Trough, accommodated the oblique collision. Combining onshore (structural and geological mapping, absolute dating and biostratigraphy) and offshore investigations (seismic profiling from the 2017 GARANTI Cruise), we evidence an unprecedently described episode of Mid-Eocene shortening, south of the Anegada Trough. After a restoration of the Cayman Trough to the Mid Eocene, the regional compressive structures are interpreted to be the eastward propagation of the compressional domain that accommodated the N-S shortening triggered by the collision of the Bahamas Bank. A crustal-scale cross section drawn from the forearc to the backarc across the thrusts allows us to discuss the origin of crustal thickening, magmatism and tectonics, in the study area