Quantifying the balance between dissociation and radiative cooling in interstellar PAHs

by Arun Subramani (Stockholm University)

Friday 27 Mar 2026, 13:00 → 14:30 Europe/Stockholm

FB55 (AlbaNova Main Building)

Abstract :
Polycyclic aromatic hydrocarbons (PAH) constitute an ubiquitous class of organic molecules in the interstellar medium. In recent years, they have also been detected in cold molecular clouds through radio astronomical surveys, while observations from the James Webb Space Telescope have enabled their spatial distributions and abundances to be characterized in unprecedented detail. Despite this, the stability of PAHs under astrophysical conditions remains poorly constrained. For instance, astrochemical models that simulate the abundances of aliphatic hydrocarbons in molecular clouds underestimate PAH abundances by several orders of magnitude. The main limitation in these models is that they are built on theoretical assumptions that lack experimental verification.
This licentiate thesis presents laboratory studies of indene, indenyl, and 2-cyanoindene monocations, PAHs whose neutral counterparts have been detected or are suspected to be present in the molecular cloud, TMC-1. The balance between dissociation and radiative cooling was investigated through absolute dissociation rate measurements performed using a cryogenic ion beam storage ring at the DESIREE facility. For cyanoindene, complementary dissociative photoionisation experiments were carried out at the DESIRS beamline of the SOLEIL synchrotron. The measured absolute dissociation rates and breakdown curves were interpreted using a statistical framework based on microcanonical rate coefficients for all relevant unimolecular processes. The results provide quantitative constraints on PAH stability and are discussed in the context of their implications for astrochemical modelling. In particular, the results demonstrate that radiative cooling plays a decisive role under astrophysically relevant excitation conditions, significantly reshaping the balance between energy loss and fragmentation, challenging the current simplified treatments of PAH stability.