Polycyclic Aromatic Hydrocarbons (PAHs) are believed to be widespread in the Interstellar Medium (ISM), indicated by infrared (IR) emission bands matching their vibrational transition energies. However, these bands are common to many PAHs, hindering the identification of specific molecules. The prevailing view was that interstellar PAHs must be large (more than 50 carbon atoms) to resist fragmentation after collisions or photon absorption. This study challenges this view by presenting experimental evidence that a small, vibrationally hot PAH cation is stabilized much faster than previously thought, primarily through Recurrent Fluorescence (RF). The recent identification of two cyanonaphthalene (CNN, C₁₀H₇CN) isomers in TMC-1 by McGuire et al. using radio telescope observations marks the first definitive identification of a specific PAH molecule in space. Remarkably, the observed CNN abundance is six orders of magnitude higher than predicted by astrochemical models, which accurately model the abundance of linear and monocyclic nitriles but underpredicts that of bicyclic indene. This discrepancy highlights the need for a more comprehensive understanding of PAH stability and the processes governing their abundance in interstellar environments. The study aims to investigate the mechanisms behind the unexpectedly high abundance of CNN in TMC-1, focusing on the role of rapid radiative cooling through RF in stabilizing small PAH cations.

Previous research has established the presence of PAHs in the ISM based on observed IR emission bands. However, resolving specific PAH species from these bands has proven difficult. The prevailing model suggested that only large PAHs (over 50 carbon atoms) could survive the harsh interstellar environment due to their resistance to fragmentation following collisions or photon absorption. McGuire et al.'s groundbreaking work provided the first definitive identification of a specific PAH—cyanonaphthalene (CNN)—in space. This discovery was accompanied by a significant discrepancy: the observed abundance of CNN in TMC-1 was six orders of magnitude greater than predicted by astrochemical models. These models have been successful in predicting the abundances of other nitrogen-bearing molecules, such as linear and monocyclic nitriles, but underpredict the abundance of bicyclic indene by four orders of magnitude. This highlights limitations in the understanding of the formation and destruction mechanisms of PAHs in interstellar clouds. Existing studies have investigated PAH formation pathways at low temperatures, emphasizing the importance of the initial reactants and conditions. However, the stability and survival mechanisms of small PAHs in the interstellar environment have not been fully elucidated, especially the role of radiative cooling mechanisms in counteracting fragmentation.

Experiments were conducted at the DESIREE (Double ElectroStatic Ion Ring Experiment) facility at Stockholm University. The cryogenically cooled storage ring (≈13 K) provided an environment with low residual gas density (≈10⁶ cm⁻³). 1-CNN cations were generated using an electron cyclotron resonance (ECR) ion source, mass-selected, and stored in the DESIREE ring at a kinetic energy of 34 keV. Two types of experiments were performed: a single-pass measurement with a continuous beam and a stored beam circulating for 200 ms. Neutral fragments emitted from dissociating ions were detected using a position-sensitive imaging detector, enabling measurement of kinetic energy release (KER) distributions and dissociation rates. Three-dimensional Newton spheres were reconstructed from the imaging data using an inverse Abel transform to determine KER distributions. The analysis focused on the HCN-loss channel, with H-loss considered as a secondary, less efficiently detected channel. The unimolecular dissociation rate coefficient, k_diss(E), was modeled using the inverse Laplace transform and the Beyer-Swinehart algorithm for vibrational level density calculations. Infrared radiative cooling (k_IR) was calculated using the Simple Harmonic Cascade (SHC) approximation, and the recurrent fluorescence (RF) rate coefficient (k_RF) was calculated using an expression incorporating the electronic transition energy, Einstein coefficient, and oscillator strength. The oscillator strength was modeled using Franck-Condon-Herzberg-Teller simulations, considering Herzberg-Teller vibronic coupling. A master equation approach was used to model the dissociation and radiative cooling processes. Calculations incorporated the dissociation rate coefficient, IR cooling rate coefficient, and RF cooling rate coefficient to simulate the time evolution of the vibrational energy distribution and the dissociation rate. The initial vibrational energy distribution was modeled using Boltzmann statistics, with the initial temperature refined by comparing simulated and experimental dissociation rates.

The experimental results demonstrate that the dissociation rate of 1-CNN⁺ is significantly quenched by radiative cooling. The measured dissociation rate coefficients show a strong dependence on vibrational energy. Recurrent Fluorescence (RF) is identified as the dominant radiative cooling mechanism, with Herzberg-Teller vibronic coupling playing a crucial role in enhancing the electronic transition probability. Including Herzberg-Teller coupling in the calculations leads to an oscillator strength (f = 0.011) that results in RF rate coefficients two orders of magnitude higher compared to calculations neglecting vibronic coupling. Master equation simulations show that RF is much more efficient than IR cooling in stabilizing 1-CNN⁺ for a critical range of internal excitation energies. A single RF photon is sufficient to stabilize 1-CNN⁺, while multiple IR photons are required. The activation energy for the HCN-loss dissociation pathway of 1-CNN+ is determined to be 3.16(4) eV. The simulation of the experimental dissociation rate using the master equation approach is in good agreement with the experimental data, with deviations at longer times potentially attributed to sequential fragmentation processes. The results suggest that 1-CNN⁺ can survive in interstellar environments with high internal energies, well above the dissociation threshold, due to the efficient stabilization by RF.

The findings directly address the anomalous abundance of CNN observed in TMC-1. The unexpectedly high abundance is likely due to a combination of underestimated formation rates and overestimated destruction rates in existing astrochemical models. The study's results suggest that the destruction rates are overestimated because the models might overemphasize fragmentation reactions while underestimating the importance of charge transfer reactions, which would not lead to fragmentation and loss of CNN. Furthermore, the efficient radiative stabilization of 1-CNN⁺ by RF challenges the long-held assumption that small PAHs cannot survive in space due to rapid UV photodestruction. The current results suggest that RF, enhanced by Herzberg-Teller vibronic coupling, is a crucial stabilizing mechanism for small PAHs, counteracting photodissociation. This calls for a reevaluation of the assumptions of previous astrochemical models that predict PAH abundances in interstellar clouds. Calculations of both dissociation rate coefficients and optical transition probabilities, including Herzberg-Teller coupling, are crucial for accurate modeling of PAH stability in space. The study emphasizes the importance of incorporating vibronic coupling into astrochemical models to accurately reflect the stability and abundance of small PAHs in interstellar environments.

This study demonstrates that recurrent fluorescence, significantly enhanced by Herzberg-Teller vibronic coupling, is a highly efficient stabilization mechanism for small PAH cations such as 1-CNN⁺. This efficient radiative cooling allows small PAHs to survive in harsh interstellar conditions, offering a plausible explanation for the unexpectedly high abundance of CNN observed in TMC-1. The findings challenge the long-held assumption that small PAHs are rapidly depleted in space, highlighting the need to incorporate these radiative cooling processes into astrochemical models. Future studies should focus on expanding these investigations to a broader range of PAHs and exploring the combined effects of various processes, including photodissociation, charge transfer reactions, and ion-neutral reactions, under astrophysically relevant conditions.

The analysis focuses primarily on the HCN-loss dissociation pathway, while H-loss is considered a secondary channel. This simplification could potentially impact the accuracy of the determined dissociation rates. The master equation simulations assume a Boltzmann distribution for the initial vibrational energy distribution, which may not perfectly represent the actual distribution. Further studies are necessary to verify the accuracy of this assumption and explore potential deviations from the Boltzmann distribution. The experimental setup and data analysis could only reveal what processes occur with significant enough products that are detectable. These limitations could affect the interpretation and generalization of the results.