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