https://doi.org/10.3847/1538-4357/ae1581

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The Astrophysical Journal, 997:49 (I7pp), 2026 January 20
© 2026. The Author(s). Published by the American Astronomical Society.
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Dissociation and Cooling Dynamics of 2-cyanoindene Monocations: Survival of Small
PAHs in Harsh Interstellar Radiation Fields
Arun Subramani 1	, James Flotte de Pouzols2 , Paul Martini 1 , José E. Navarro Navarrete 1 , Eleanor K. Ashworth3 ,
James N. Bull3 . Gustavo A. Garcia4 , Laurent Nahon4 . Bérenger Gans5 , Ugo Jacovella5 , Corentin Rossi5 ,
Henrik Cederquist1 , Henning T. Schmidt1 , Henning Zettergren 1 , and Maik H. Stockett1
1		Department of Physics, Stockholm University, Stockholm, 114 19, Sweden; mark.stockett@fysik.su.se
2		Department of Chemistry, Universite Paris-Saclay, Gif-sur-Yvette, France
3	Department of Chemistry, Faculty of Science, University of East Anglia, Norwich NR4 7TJ, UK
Synchrotron SOLEIL, L’Orme des Merisiers, Departementale 128, Saint Aubin F-91190, France
-	Universite Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91405 Orsay, France
Received 2025 May 19; revised 2025 October 17; accepted 2025 October 19; published 2026 January 16
Abstract
The molecule, 2-cyanoindene, C|0H7N (2CNI) is the only cyanosubstituted polycyclic aromatic hydrocarbon (PAH) detected in space, for which the hydrocarbon counterpart, indene, has also been observed in the same astrochemical environment—the molecular cloud TMC-I. In this study, based on experiments in two different laboratories, the collision and radiation-driven dissociation and cooling dynamics of the 2-cyanoindene monocations are investigated using one of the electrostatic ion-beam storage rings of the DESIREE facility, and the DESIRS beamline at the SOLEIL synchrotron radiation facility. The storage ring experiments quantify the balance between fragmentation and radiative cooling of the stored cations, while the synchrotron experiments characterize dissociation channels from the vacuum ultraviolet-induced dissociative photoionization of the neutrals. Recurrent fluorescence is shown to play an important role in the radiative stabilization of 2CNI+. The results from both sets of experiments are combined to obtain a self-consistent set of microcanonical rate coefficients for dissociation and radiative cooling that completely describe the near-dissociation threshold dynamics of 2CNI+ across the microseconds-seconds time range. This timescale is suitable for incorporation into astrochemical models of PAH growth and destruction lifecycles. This study extends its findings to different astrochemical environments by simulating the extent of fragmentation and the cascade emission spectra of 2CNI+ under varying interstellar radiation fields. These results indicate that radiative cooling enhances the resilience of 2-cyanoindene to harsh radiation conditions, suggesting that small cyano-PAHs may survive longer than previously assumed in a wider range of astrochemical environments, extending beyond cold, dark molecular clouds.
Unified Astronomy Thesaurus concepts: Laboratory astrophysics (2004); Polycyclic aromatic hydrocarbons (1280)

1.		Introduction
Polycyclic aromatic hydrocarbons (PAHs) are organic molecules with two or more fused aromatic rings. The physical and chemical properties of PAHs are fundamental to astrochemistry (A. Tielens 2008; A. G. G. M. Tielens 2013). Astronomical models estimate that interstellar PAHs account for 10%-25% of the cosmic carbon inventory (A. Li 2020). PAHs are widespread in space, as evidenced by the aromatic infrared bands (AIBs)—emission features in the 3-20 cm spectral range—attributed to these molecules (A. Leger & J. Puget 1984; L. Allumandola et al. 1985). Contributing to up to 20 % of the total infrared (IR) power in the Milky Way and other star-forming galaxies (A. Li & B. Draine 2001), these AIBs have also been detected in protoplanetary disks (G. Meeus et al. 2001; K. Maaskant et al. 2014; J. Y. Seok & A. Li 2017), reflection nebulae (K. Sellgren et al. 2007), planetary nebulae (D. Beintema et al. 1996), even in harsher environments such as HII regions (D. Stock & E. Peeters 2017) and near active galactic nuclei (P. Esquej et al. 2013; J. Jensen et al. 2017).
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Further down the electromagnetic spectrum, radioastronomy observations of rotational transitions have led to the first detections of individual PAH species in space. To date, two isomers of cyanonaphthalene, C11H-N (B. A. McGuire et al. 2021), indene, C9Hs (A. M. Burkhardt et al. 2021; J. Cernicharo et al. 2021), 2-cyanoindene, CiOH7N (M. L. Sita et al. 2022), three isomers of cyanopyrene, C1-H4N (G. Wenzel et al. 2024, 2025b), two isomers of cyanoacenaphthylene, C13H-N (J. Cernicharo et al. 2024) and cyanocoronene, C25H||N (G. Wenzel et al. 2025a) have been detected in the cold dark molecular cloud, TMC-1. Recent studies have observed these PAHs in molecular clouds other than TMC-1 (M. Agùndez et al. 2023). Notably, apart from indene, all of these detections involve cyano-substituted PAHs. likely due to the strong dipole moment imparted by the CN group, making them detectable via radioastronomy. Given the abundance of CN radicals in interstellar environments (S. Federman et al. 1984; H. Liszt & R. Lucas 2001) and the efficiency of barrierless CN addition to benzene rings (H. Liszt & R. Lucas 2001; K. L. K. Lee et al. 2019), it is expected that unsubstituted PAHs should also be present in comparable or even greater abundances (M. L. Sita et al. 2022).
PAH chemistry actively shapes the physical conditions of astrochemical environments. For instance, PAHs play a crucial role in heating neutral gas in galaxies and the upper layers of protoplanetary disks via the photoelectric effect (I. Kamp & C. Dullemond 2004; O. Berné et al. 2022; H. R. Hrodmarsson et al. 2025). Likewise, being important sinks for electrons, PAHs significantly influence the ionization balance in molecular clouds (V. Wakelam & E. Herbst 2008; C. Joblin et al. 2011). Despite their significance, key aspects of PAH astrochemistry remain poorly understood. For instance, recent observations of bright emission features in the IR, linked to smaller PAH molecules have been found close to star-forming regions with high star formation surface density, contrary to theoretical predictions (Κ. Ujjwal et al. 2024). Likewise, astrochemical models that accurately model the abundances of linear carbon chains in the molecular clouds systematically underpredict the column densities of PAHs by several orders of magnitude (M. L. Sita et al. 2022).
Understanding the processes that govern PAH excitation and dissociation is critical for resolving these discrepancies. For instance, the emission of IR photons during radiative cooling (RC) of energized PAHs gives rise to the AIBs. Yet, RC processes are entirely neglected in astrochemical models, which assume that PAHs fragment into linear chains following ionizing collisions with small cations (B. A. McGuire et al. 2021). A further RC mechanism that plays an important role in PAHs is recurrent fluorescence (RF), corresponding to the emission of optical photons from thermally excited electronic states (A. Léger et al. 1988; P. Boissel et al. 1997). Several studies have demonstrated that RF is the primary mechanism leading to radiative stabilization in several small to mediumsized PAH cations (S. Martin et al. 2013; Μ. Η. Stockett et al. 2020; J. Bernard et al. 2023; J. W. Lee et al. 2023; M. Η. Stockett et al. 2023, 2025b; J. Ν. Bull et al. 2025). The role of RF is often overlooked in photoionization and dissociation modeling studies, which only take into account IR cooling when setting lower limits of expected sizes of PAHs in harsh ultraviolet (UV) radiation environments (H. Jochims et al. 1994). However, RF in PAHs excited by vacuum ultraviolet (VUV) photons, has been proposed as the source of the extended red emission observed in the Red Rectangle and other reflection nebulae (T. S. Lai et al. 2020; A. N. Witt & T. S.-Y. Lai 2020), although this interpretation remains a subject of debate (A. Li & B. Draine 2002).
Laboratory experiments provide crucial insights to refine PAH-abundance and PAH-stabilization models. For instance, a cryogenic ion-beam storage ring study by M. H. Stockett et al. (2023) showed that the rapid cooling provided by RF in 1-cyanonaphthalene (1CNN) closes some of the key destruction pathways in the astrochemical models that underpredicted 1CNN abundance by six orders of magnitude in TMC-1 (B. A. McGuire et al. 2021). Likewise, another storage ring study by J. N. Bull et al. (2025) demonstrated that indenyl cation cools efficiently by RF. Owing to its closed-shell electronic structure, the indenyl cation may serve as an appropriate proxy for neutral PAHs, such as those recently identified in TMC-1. From a synchrotron light-based study into the photoelectron spectroscopy of 1CNN, J. N. Bull et al. (2023) identified the key ionization mechanisms and suggested that it is more likely present as a cation than as a neutral in UV-dominated regions of space. Similarly, from another synchrotron study on 1- and 2CNN, M. Roy Chowdhury et al. (2024) characterized the kinetic energy distributions of photoelectrons emitted during photoionization, refining the

Figure 1. Structure of 2-cyanoindene+:Target of current study with carbon sites numbered.

contribution from cyanonaphtalene to the photoelectric heating effect.
In this work, we present a multi-laboratory-based investigation into the energetic processing of 2-cyanoindene monocations, C1OH7N (2CNI+) under different excitation conditions. The target, 2CNI+ (shown in Figure 1), represents a unique opportunity because its neutral form is the only cyano-PAH detected whose pure hydrocarbon counterpart— indene—has also been observed in the same region of space. This makes 2CNI an important test case for understanding the impact of cyanosubstitution on the observed PAH abundances. The present work, combined with a previous report on indene (M. H. Stockett et al. 2025b), is expected to contribute to this understanding by determining destruction and radiative stabilization rates of the cations formed as the first step of the assumed destruction channels of PAHs in TMC-1 (A. M. Burkhardt et al. 2021; Β. A. McGuire et al. 2021). Despite this, there are very few experimental studies on 2CNI so far. Recently, T. E. Douglas-Walker et al. (2025) carried out vibrational and electronic spectroscopy studies of 2CNI+, characterizing its mid-IR and visible absorption spectra. M. H. Stockett et al. (2025a) reported mid-IR absorption spectra of neutral 2CNI and simulated an IR emission spectrum based on an anharmonic cascade model with inputs from their experimental results. Here, we study the collision and radiation-driven dissociation and cooling dynamics of the 2-cyanoindene monocation using two complementary experimental approaches based on the storage of ions in an electrostatic ion-beam storage ring and the production of ions using synchrotron radiation coupled to a photoelectron/pho- toion coincidence (PEPICO) detection scheme that allows for state selection. The synchrotron light source provides light in the astronomically relevant VUV energy window and is thus well suited for studies of the dissociation dynamics with photoexcited molecules. However, these experiments are limited in that the time-of-flight mass spectrometry method employed here samples only dissociation occurring within a few microseconds after ionization. Storage ring experiments probe the cooling dynamics of molecular ions over the tens of microseconds to seconds time windows and far beyond, which is ideal for studies of slow RC processes. By combining results from both sets of experiments and from theoretical calculations of potential energy surfaces and electronic states, we arrived at a consistent set of rate coefficients for the cooling and dissociation of 2CNI+ on microsecond to second timescales. While astrochemical processes may in general occur on much longer timescales, unimolecular reactions such as dissociative photoionization are constrained by the interaction with radiation (B. Draine & A. Li 2001), with the ensuing molecular dynamics limited to the microsecond-second regime because slower processes are outcompeted by RC.

2.		
Figure 2. (a) DELICOUS3 spectrometer at the SAPHIRS endstation on the DESIRS beamline at the SOLEIL synchrotron facility (e and it are electrons and ions, respectively) and (b) one of the cryogenic ion-beam storage rings at the DESIREE facility. PSD is a position sensitive detector.

Methods
2.1.		Experiments
Experiments have been carried out at the VUV undulatorbased DESIRS beamline (L. Nahon et al. 2012) of the French synchrotron radiation facility SOLEIL, and at the Double Electrostatic Ion Ring Experiment (DESIREE) cryogenic ionbeam storage ring facility (R. D. Thomas et al. 2011; Η. Τ. Schmidt et al. 2013a) located at Stockholm University. The respective experimental procedures used for the present experiments at both facilities have previously been described (M. H. Stockett et al. 2023; Μ. Roy Chowdhury et al. 2024), and only specific details will be given here. The synthesis of the samples used in this work has also been described previously (T. E. Douglas-Walker et al. 2025).
2.1.1.		DESIRS
Powdered 2CNI was placed directly in an in-vacuum stainless-steel oven without further purification and was heated to 50°C. The sublimed vapor was then seeded with an argon gas jet, which was subsequently expanded supersonically through a 100 cm diameter nozzle inside the SAPHIRS molecular beam expansion chamber (X. Tang et al. 2015). The resulting free molecular jet was doubly skimmed and intersected the photon beam from the synchrotron perpendicularly in the center of the ionization chamber. The jet was irradiated with mono- chromatized synchrotron light (with a maximum photon energy spread of 70 meV depending on the photon energy) fed to the SAPHIRS endstation at DESIRS. The ejected photoelectrons and photoions were extracted and accelerated in opposite directions in the DELICIOUS3 (G. A. Garcia et al. 2013) double imaging electron/ion coincidence (i2PEPICO) spectrometer (see Figure 2(a)). Two photon-energy scans were performed with the photon energy ranging from 8.4 to 15.5 eV and from 15 to 22 eV in steps of 0.1 eV. For the low-energy scan, an argon gas filter was used for the suppression of contributions from higher harmonics of the monochromator energy. Breakdown curves were obtained from these scans, using a method similar to the slow photoelectron spectrum technique (J. C. Poully et al. 2010). The breakdown curves are the relative product branching ratios as a function of the parent ion’s internal energy and are often used as an experimental standard to test statistical theories (Τ. Baer & W. L. Hase 1996). The internal energy of the parent ion can be determined as,
hv = eKE + IE + E,	(1)
where hv is the energy of the incoming photon, eKE the energy of the outgoing electron, IE is the ionization energy and E, the internal energy of the parent ion. A third scan with photon energies ranging from hv = 13 to 15.5 eV in 0.1 eV steps was performed with the “ion 3D momentum imaging” mode of the spectrometer and with longer acquisition times, from which photon energy-dependent kinetic energy release distributions (KERDs) of the ionic fragments were obtained (G. A. Garcia et al. 2013).
2.1.2.		DESIREE
Powdered 2CNI was sublimed in a resistively heated oven coupled to an electron cyclotron resonance (ECR) ion-source (Pantechnik Monogan), generating ions with high internal energies. Ions were produced by collisions with electrons in the plasma source, accelerated to 30 keV, mass-selected with a bending magnet (m/z = 141), and injected into the storage ring (see Figure 2(b)). A fraction of the ions stored would still possess enough internal energy to “spontaneously” dissociate, emitting neutrals in the process. This signal is termed the spontaneous dissociation signal. The neutrals emitted during passage through the lower straight section of the ring impinged on a 75 mm diameter Z-stack microchannel plate detector (Photonis) with a phosphor screen anode (phosphor type P24). The position and arrival time of each detected neutral was recorded using a TimePix3 (A. Nomerotski et al. 2023) hybrid pixel detector (Amsterdam Scientific) located outside the DESIREE vacuum chamber. At the end of each storage cycle, the beam was dumped into a Faraday cup coupled to a fast amplifier for current normalization.
The signal due to spontaneous dissociation disappears after a critical “quenching” time, beyond which RC dominates over dissociation. The majority of ions remain stored after the disappearance of the spontaneous dissociation signal. Once the dissociation has been quenched, the stored ions were overlapped collinearly with light from an optical parametric oscillator pulsed laser system (EKSPLA NT242). Laser pulses at wavelengths 330 nm and 560 nm at a repetition rate of 62.5 Hz with a pulse duration of ^5 ns and pulse energy of >1.44 mJ were used for the experiments. The neutrals emitted after each laser shot fired constitute the “laser-induced dissociation” signal.
The position information from the camera was also time- stamped and synchronized to the storage cycle. This data was then binned with bin widths equal to the revolution period (43.328 ps) for a turn by turn analysis and averaged over numerous storage cycles. The total number of detections per bin was used to obtain the total spontaneous dissociation yield. The position information was used to construct threedimensional Newton spheres, obtained by performing an inverse Abel transform using the “Hansen-Law” algorithm (E. W. Hansen & P.-L. Law 1985) as implemented in PyAbel package (S. Gibson et al. 2022). Assuming all neutrals are
emitted from the midpoint of the straight section, the resulting density distribution is related to the KERD by (M. H. Stockett et al. 2025b)
—, (2) L mid /
where e(p) is the KER associated with the radius p of the Newton sphere, mneut and mi-cat are the masses of the neutral and cationic reaction products, Eacc is the beam energy, and Lmid = 1.68 m is the distance from the detector to the midpoint of the straight section. A more rigorous treatment, allowing neutral emission to occur throughout the straight section, typically yields an insignificant correction and was therefore not considered in this work.
2.2.		Theory
Unimolecular dissociation mechanisms are the pathways an energized molecule follows to access specific product channels, as dictated by its internal energy and the potential energy surface. Dissociation mechanism calculations rely on the computations of the different intermediates and the transition states (TSs) connecting these intermediates. Here, dissociation mechanism calculations for HCN loss and Η loss for 2CNI+ were calculated out at the B3LYP/cc-pVTZ (T. H. Dunning 1989; A. D. Becke 1993; P. J. Stephens et al. 1994) level of theory in Gaussian (M. J. Frisch et al. 2016). B3LYP/cc-pVTZ was chosen as it could reproduce the experimentally measured IR spectrum of the bare 2CNI+ cation reasonably well (T. E. Douglas-Walker et al. 2025). Initial guess structures were taken from constrained coordinate scans. The final geometries of the TS were optimized using the Berny algorithm (H. B. Schlegel 1982) at the same level of theory. The vibrational frequencies were used to characterize the optimized geometries, confirming intermediates as minima on the potential energy surface and TS as first-order saddle points.
Certain steps in these mechanisms would possess no reverse barrier and are referred to as loose TSs (Τ. Baer & W. L. Hase 1996). For mechanisms involving loose TSs, a form of variational transition state theory (VTST) was employed (W.L. Hase 1983). In the case of 2CNI, the relevant pathways are the hydrogen loss channels and the final dissociation step in the HCN channel. In the calculation, the C(1)-H bond length was first scanned in small steps until the dissociating plateau was reached. Constrained geometry optimizations were then performed along this path and vibrational analysis was carried out for structures from these optimizations. Each of these structures had one imaginary frequency corresponding to the C(1)-H stretch. With the remaining real frequencies, the sum of states for these structures was calculated. The structure with the smallest sum of states was assumed to be a close approximation to the true TS for the loss of Η from the C(1) site. The C(1)-H bond length of this structure was assumed to be the crossover point at which the TS occurs for all hydrogen loss channels. The approximations for the other hydrogen loss TS were then found by freezing the C(x)-H (where X = 1-10) and allowing the other coordinates to relax during the optimization process. This procedure has been employed successfully to model the kinetics of hydrogen loss (A.-M. Boulanger et al. 2007) and other bond cleavage reactions (B. Sztàray & Τ. Baer 2000; Β. Sztàray et al. 2010). Anharmonic frequencies for the IR RC rate are also computed
Table 1
Transition Energies and Oscillator Strengths for the D1, D2, D3 —> D0 Transitions, Respectively
Transition	Ej	fj	
D1 - > Do	1.027 eV	0.004	
D2 —» D0	2.068 eV	0.014	
Ds -> Do	3.086 eV	0.316	

at the same level of theory. In the absence of a complete anharmonic treatment (M. H. Stockett et al. 2025a), which is beyond the scope of the current work, the usage of anharmonic frequencies has previously been shown to better reproduce experimental results (I. N. Bull et al. 2019).
Temperature-dependent ab initio molecular dynamics simulations (R. Iftimie et al. 2005) at wB97X-D/cc-pVTZ (J.-D. Chai & M. Head-Gordon 2008) level of theory at 100, 300, 500, 1000, 1250, 1500, 1625, 2000, 2500, and 3000 Κ with canonical sampling through velocity rescaling (G. Bussi et al. 2007) were also performed as part of the RF modeling. Trajectories at a series of initial temperatures, converted to internal energies according to the caloric curve based on the state density, were run for 1 ps in 0.5 fs steps, with the vertical transition energies and oscillator strengths for the lowest three electronic states computed at each step. These were used to find RF rate coefficients for each temperature, from which the parameters for the microcanonical RF rate coefficients were determined (J. N. Bull et al. 2025; Μ. Η. Stockett et al. 2025b).
Master equation simulations were performed to model the breakdown curves from DESIRS and the spontaneous dissociation signal of DESIREE. The former involves only isomerization and dissociative processes because RC does not play a role on the short timescales of the DESIRS experiments (less than 2 ps). Cooling through IR photon emission and RF are included in the DESIREE simulations. The IR rate coefficients were computed using the simple harmonic cascade (SHC) approximation (V. Chandrasekaran et al. 2014),
τζΕ/Λν,
IIR - ΛIR V
v = 1

p(E — vhv4)
p(E)

(3)

where v, hvz, p(E), AIR are the vibrational quantum number, the IR transition energy, the density of states, and the Einstein coefficient of the mode s, respectively. The SHC approximation assumes that vibrational levels are equally spaced with Av, = —I transitions being the only allowed transitions, and the Einstein coefficient for the v —> v — 1 transition is v times the coefficient for the 1 —> 0 transition, which is strictly true for simple harmonic oscillators. Likewise, the RF emission rate coefficients were parameterized as (P. Boissel et al. 1997)

kf(E) = A™Ρ(Ε
J p(E)

(4)

where E; and ARK are the transition energy and Einstein coefficient which depend on the oscillator strength, f of the Dj —> D0 electronic transition. Ej and f were taken from fits to the MD simulations (J. N. Bull et al. 2025; Μ. Η. Stockett et al. 2025b) and are reported in Table 1. The rate coefficients
for isomerization and dissociation are calculated using the Rice-Ramsperger-Kassel—Marcus (RRKM) theory as (T. Baer & W. L. Hase 1996):

kdiss(E) =
hp(E)

(5)

where E is the internal energy in the cation, σ is the reaction path degeneracy, and N is the sum of vibrational states of the system in the TS geometry up to an energy E — E with E being the activation energy and p(E) is the density of vibrational states of the reactant. Using the B3LYP/cc-pVTZ vibrational frequencies of the system at the various intermediate and TS geometries as input, the density and sum of vibrational states were evaluated in energy bins of 1 cm-1, by using direct count algorithms under the harmonic approximation (Τ. Beyer & D. F. Swinehart 1973). The vibrational frequencies of the parent, 2CNI+ are reported in Appendix A. Both density and sum of states have to be counted from the zero-point energy onward. For reactions without a well-defined TS, we use rigid activated complex RRKM theory (RAC-RRKM) (B. Sztàray et al. 2010) based on VTST. Here, the lowest six frequencies (excluding the reaction coordinate) and the activation energy are scaled until agreement with experimental data was achieved. In our case, we simultaneously fit the DESIREE and the DESIRS data to find a single set of rate coefficients to explain the dynamics of the system over the ps-s timescale of both experiments.
2.3.		Simulations
The procedure employed to model the spontaneous dissociation signal measured in DESIREE has been described elsewhere (M. H. Stockett et al. 2020). The internal energy distribution g(E. t) of the ensemble after excitation was initialized as a normalized Boltzmann distribution at 1 = 0. The master equation propagates the distribution according to:
K. Ramanathan et al. 2022; J. W. Lee et al. 2023). In the current study, this strategy was employed for the HCN pathway.
For the modeling of the DESIRS breakdown curves, we considered rate laws for all relevant dissociation pathways. These first-order differential equations were then solved up to the time relevant to the experiment to determine the relative yields of the different channels. This was repeated for the photon energies used in the experiment. The scaling factors for the activation energies and the transitional frequencies for the loose TSs for the dissociation rate coefficients were the only fit parameters used in both DESIREE and DESIRS experiments. In addition, the initial effective temperature of the Boltzmann distribution and the efficiency of the detector are additional fit parameters used in the analysis of the DESIREE data; however, these parameters have known estimates and thus can be used as a consistency check to other similar experiments.
3.		Results and Discussion
3.1.		Mass Spectra
Figure 3(a) depicts the mass spectrum of 2CNI recorded from a photon-energy scan of hv = 15-22 eV at DESIRS. The peak at m/z. = 141 corresponds to the parent with the 13C isotope peaks at 142 and 143. H loss from the 2CNI with one 13C also contributes to the peak at m/z = 141; however, this is estimated to be less than 3% of the total peak. The dominant dissociation channel seen in the mass spectra is the loss of hydrogen, Η, evidenced by the peak at m/z = 140. The second major channel corresponds to loss of mass 27 amu, resulting in the peak at m/z= 114, as seen from inset (i) in Figure 3(a). This was attributed to the loss of hydrogen cyanide, HCN, or its isomer hydrogen isocyanide, HNC, commonly seen in the dissociation of cyano-PAHs (B. J. West et al. 2019; Μ. Roy Chowdhury et al. 2024). Peaks at m/z = 131, 132 are also seen in the mass spectra. These peaks marked by * in Figure 3(a) are assigned to be impurities from the oven and are not relevant to the current study given that the loss

(6)

4R(E, t) = - k^(E)g(E, t) + _[K,JR (E + hv,)g(E + Iivs, t) - k,JB(E)g(E, 0]
Σ^Γ ^ + EWE + Avy, a - kJF(E)g(E. n].
The first term gives the depletion of the population by unim- olecular dissociation. The next term represents v + 1 —> v vibrational emission from levels above E, while the second corresponds to v —> v — 1 emission to levels below E. Similarly, the second sum accounts for RF emission from the electronically excited states D(j = 1, 2, 3) to the ground state D0. In this case, only the D1 state contributes significantly to RF. The dissociation rate is given by T(t) = fkletS(E)g(E, t)d E. All dissociation processes contribute to the depletion of the ensemble; however, only those whose neutral fragments can be efficiently detected are included in the calculation of the dissociation rate, keiss. Note that some dissociative processes involve several intermediate steps. In such cases, the total dissociation rate coefficient is calculated using a series of steady-state approximations to reduce computational complexity. This has been successfully employed in the past to model kinetics of multistep reactions (E. Solano & P. Mayer 2015; of mass 9 and 10 amu from the parent is not possible considering the atomic constituents of 2CNI (see Figure 1). Figure 3(b) depicts the mass spectrum of 2CNI recorded at DESIREE. This trace was obtained by using the ring as an electrostatic energy analyzer, i.e., by systematically scaling down the voltages on the electrodes after mass selection, such that they pass ions corresponding to lower masses, with the same beam velocity and are detected using the Faraday cup after one turn, as shown in Figure 2(b). This procedure samples the warmest ions produced from the source and was then assumed to be representative of the decaying ensemble being stored. The only peak of note apart from the parent is at m/z = 114, which is also seen in the DESIRS experiments. The hydrogen loss peak at m/z = 140 could not be resolved from the parent due to relatively poorer resolution at DESIREE, but was still assumed to play a role, considering its dominance in DESIRS. Other minor peaks in DESIRS’s mass spectrum include the peak at m/z = 139 attributed to the loss of molecular hydrogen, as seen in inset (ii) in

(a)

Figure 3. (a) Mass spectrum of 2CNI recorded at DESIRS with the peak assignments; insets show zoomed in traces at two mass ranges, and (b) mass spectrum recorded at DESIREE, obtained by scanning the electrodes after mass selection. Both spectra are normalized to the parent such that the parent peak is at unity.

Figure 4. Summary of the potential energy surfaces for the dissociation of 2CNI+ used in the simulations. Energies in eV (referenced to the parent cation inside the rectangular frame) of the TSs (except the final dissociation step which was taken from the simulations) are reported in gray while the energies of the intermediates and products are in black. Migrating hydrogen is highlighted in pink. This figure is reproduced with the units in kJ/mol”1 in Figure 13.

Figure 3(a), H2 or two consecutive atomic hydrogens, 2H, and m/z = 115 attributed to loss of acetylene, C2H2 or the cyanide functional group CN, as seen in inset (i) in Figure 3(a). The 13C isotope of the m/z = 114 ions are also expected to contribute to a portion of the peak at m/z = 115. These are minor dissociation channels that open at higher energies, which are not seen in DESIREE and are also not considered in the present analysis.
3.2.		Dissociation Pathways
Figure 4 summarizes the potential energy surface of the dissociation channels used in the simulations. We focused on low-energy threshold processes involving minimal structural deformation because these are most pertinent to both experimental setups. As shown later in the simulations, these pathways effectively capture the system’s dynamics. The molecule 2CNI possesses an sp3-hybridized carbon with a labile hydrogen. The HCN elimination process begins with the migration of one of these hydrogens to the carbon of the CN functional group. Hydrogen migration is important to all dissociation pathways in the current study. Thus, the migrating hydrogen is highlighted in pink in Figure 4. Following the formation of the HCN moiety, internal rotation occurs before HCN is eliminated. A TS is identified for this final dissociation step, though it lies only 85 meV above the products. Additionally, vibrational analysis reveals three modes below 100 cm-1, indicating a flat potential energy surface in this region. Due to this flatness, we apply the same variational treatment as for the H-loss TS. This is further justified by its low KERD, which suggests a barrierless process (J. Laskin & C. Lifshitz 2001) requiring variational treatment. The proposed pathway is similar to the direct HCN/HNC loss pathway suggested in D. B. Rap et al. (2023). Pathways to the loss of the higher energy isomer HNC are not expected to open near the dissociation threshold. Hence, while they might occur at higher energies in DESIRS, they are not expected to occur in DESIREE and are not further explored here. For hydrogen loss channels, we also considered hydrogen migration before elimination. Hydrogen migration plays a critical role in PAH- dissociation (G. Trinquier et al. 2017) and is believed to influence dissociation in various PAHs (B. West et al. 2014; P. Castellanos et al. 2018; Μ. Vinitha et al. 2022; Μ. Selvaraj et al. 2023; Α. Subramani et al. 2023). Therefore, we account for migration throughout the molecular skeleton prior to dissociation. Although such extensive migration is unlikely near the dissociation threshold relevant in the DESIREE measurements, it is expected at higher energies relevant to DESIRS. Hydrogen loss from aromatic carbon sites was also excluded because it requires 4.7-5.3 eV, whereas loss from sp3 carbons requires only ^2.9 eV. As mentioned earlier, CN or C2H2 loss could contribute to the minor peak at m/z = 115. The direct loss of CN from 2CNI requires at least 5.7 eV, assuming barrierless dissociation. If CN loss occurs after hydrogen migration to C(2) (referring to the labeling in Figure 1), the resulting ion would be the indenyl ion, with an activation energy of at least 3.5 eV under the same assumption. The most stable product after C2H2 elimination is indole; however, forming it requires extensive rearrangement to incorporate nitrogen into the main carbon-skeleton, which is assumed to be energetically costly. Another possible pathway involves C2H2 loss from the six-membered ring, but this first requires loss of aromaticity, which is also energetically unfavorable. For these reasons, these pathways are not explored in this study.
3.3.		Rate Coefficients
The frequencies computed as part of the potential energy surface explorations form the basis on which the rate coefficients were computed based on Equations (3)-(5). The microcanonical rate coefficients used in the simulations are depicted in Figure 5. Overlaid on top of the figures are the timescales and the energy scales probed by the two experiments. The first ions in DESIREE are detected after the first turn in the ring, at a time, t1 = 12.5 ps, setting the upper limit to the sampled rate coefficients to be k1 = 1/t1. The lower limit to the experimentally sampled rate coefficients was determined by the background rate. At DESIRS, the first dissociation channel opens at hv z 13 eV, thus the corresponding rate coefficient at that cation internal energy, which is 13 eV minus the ionization energy of 2CNI, would be the lower limit of the rate coefficient that DESIRS samples. The energy scales are depicted by the simulated energy distribution of the ion ensemble as they enter the ring in DESIREE and the breakdown curves for the Η and HCN channels in DESIRS. As can be seen, both experiments constrain the rate coefficients in complementary ways. DESIREE probes an energy regime

Figure 5. Rate coefficients for RC through RF, (kRF), and through vibrational relaxation giving rise to IR emission (fIR) used in the DESIREE simulations and dissociation through Η (kgliss) and HCN loss (k(NCN) used in DESIREE and DESIRS simulations (left axis). Overlaid on top are the simulated energy distribution (g(t1)) of ion ensemble as they enter the ring in DESIREE, the measured breakdown curves for the Η and HCN channels (BDH,BDHCN) in DESIRS (right axis), and the rate coefficient range (depicted as the blue and red windows for DESIREE and DESIRS, respectively) the two experiments probe.

where RC and dissociation are comparable, while DESIRS probes a regime in which dissociation (and isomerisation) are the only processes at play. The rate coefficient at which RC and dissociation are equal lies around 200 s 1. This value, termed k, defines the critical rate coefficient, such that ions with rate coefficients (and by extension, internal energies) higher than kc dissociate, while those with lower rate coefficients are stored. The value of kc can also be determined experimentally from the neutralization rates (as explained later) and in the current experiment was found to be 173.8 ± 1.7 s 1, indicating a fair agreement with the simulated rate coefficients. Note that only the rate coefficient for hydrogen loss prior to any migration and the effective rate coefficient for HCN loss is depicted in Figure 5. At this stage, the role of RF in ion cooling becomes evident, with a small energy regime above 3.5 eV where RF-driven cooling is dominant. In this context, cyanoindene cations sit between indene cations (M. H. Stockett et al. 2025b), where RF has minimal influence, and indenyl cations (J. N. Bull et al. 2025), where RF overwhelmingly governs the cooling dynamics.
3.4.		Kinetic Energy Release Distributions
Another key comparison between the experiments is the KERD. By design, the neutrals are detected in DESIREE, while the corresponding ions are detected in DESIRS, making the two setups complementary. Both distributions can be related to the total KERD through simple kinematics (J. Laskin & C. Lifshitz 2001). In this study, we compare the KERDs of HCN loss because hydrogen loss imparts minimal energy to the ion and, with our current setup, cannot be distinguished from the molecular beam’s intrinsic thermal spread at DESIRS. The KERDs from DESIRS were taken from a scan of photon energy hv = 13-15.5 eV, thus probing the threshold region where the HCN-loss channel first appears (^13.6 eV). Figure 6 presents the KERD of the decaying ensemble during the first turn in DESIREE alongside the KERD recorded at three photon energies in DESIRS. The strong agreement between the curves indicates that the underlying processes are

Figure 6. KERD of HCN loss from 2CNI+ recorded at DESIRS and DESIREE. The close agreement is consistent with a common dissociation mechanism.

the same. The minor difference in the signal at zero arises from the distinct procedures used to obtain the KERD at the two facilities. We then concluded that the threshold channel, i.e., the loss of HCN that first opens in DESIRS, is the same as the one through which the ion ensemble decays in DESIREE. Throughout the scanned photon-energy range, the KERD remains largely unchanged. Similarly, in DESIREE, the KERD shows minimal variation during the first 80 turns of the ion ensemble, beyond which poor statistics prevent reliable Abel inversion. Furthermore, the magnitude of the KERD is very small, indicative of a barrierless dissociation process (J. Laskin & C. Lifshitz 2001), in line with the proposed dissociation pathway from the current density functional theory (DFT) calculations.
3.5.		Breakdown Curves
The breakdown curves for the two major channels along with the parent are depicted in Figure 7. The entire energy range was covered in two photon-energy scans, denoted by the open and filled data points in the figure. As can be seen from Figure 3(a), the peak at m/z = 115 sits on the tail of the major peak at m/z = 114 and thus could not be resolved. Hence, the entire region is taken to construct the breakdown with the implicit assumption that contributions from C2H2/CN would be minor and open at higher energies (because this peak is not seen in the lower energy scan). The appearance energy (ΑΕ) of the channels (obtained using the procedure outlined in Appendix B) is reported in Table 2. The ionization energy of 2CNI is found to be 8.66 ± 0.01 eV (see Appendix C). From the IE and the observed ΑΕ, an estimate to the experimental activation energies (Δ£) could be found which can be compared to the theoretical values shown in Table 2; however, such comparisons would be difficult due to kinetic shifts of the measured distributions. A fair comparison between experiment and theory can, however, be made via simulations based on theory results, taking experimental boundary conditions (e.g., the time windows for the measurements etc.) into account.
The disappearance of the parent ion and the onset of the first hydrogen loss channel align well with the simulation. While the simulation overestimates hydrogen loss at higher energies, this is primarily a consequence of normalization and stems from the underestimation of HCN loss. Good agreement with

Figure 7. Breakdown curve of the parent (2CNI+), H loss, and HCN loss (witli possible contributions from HNC. CN, C2H2 loss represented as the * in the legend) obtained from the corresponding measurements in DESIRS. Open and filled circles denote two photon-energy scans. Solid lines are from simulations. The bottom axis shows the difference between photon energy and the electron energy (see Equation (l)). and the top axis the internal energy of the ion.

Table 2
Experimental ΑΕ for 2CNI + AE —> [2CNI -H]+ + Η + e and 2CN1 + ΑΕ — [2CNI -HCN]+ + HCN + e~ Characterized as the Energy Onset of the Dissociation Channels (See Appendix B) Measured in DESIRS, and Theoretical Activation Energies. Defined as the Minimal Internal Energies Needed for the Corresponding Fragmentation of 2CNI+, to be Realized According to DFT and the VTST Based Simulations (VTST-Sim) for the Two Channels
Channel	ΑΕ (Exp)	AE (DFT)	ΔΕ (VTST-Sim)	
Η loss	13.16 ± 0.08 eV	3.06 eV	2.91 ± 0.02 eV	
HCN loss	13.82 ± 0.1 eV	3.16 eV	3.23 ± 0.07 eV	
Note. See Appendix D for uncertainties in the VTST-simulations.

the experiment was achieved only after including hydrogen migration across the skeleton, supporting that the labile hydrogen indeed undergoes hopping, as previously expected. Regarding the HCN channel, the simulation reproduces its onset well but predicts a slightly steeper increase. The comparatively gradual increase around 14 eV is what leads to a bump in the experimental hydrogen loss channel, which is not reproduced by the simulations. At higher energies, the model underestimates the HCN yield, which is expected since only one HCN-loss pathway is included, while additional dissociation channels may open at higher energies. Possible HNC. CN, and C2H2 loss channels are also not accounted for. However, this discrepancy remains within 5% of the relative yield, indicating that the simulated HCN loss is indeed the dominant channel. Additionally, given the agreement in onset behavior, this channel is likely the first to open and corresponds to the threshold channel observed in the KERDs.
3.6.		Absolute Dissociation Rates
The total neutral product count rate R(t) for 2CNI+ ions stored in DESIREE is depicted in the inset of Figure 8. Typically, decay rate curves of ion ensembles with broad internal energy distributions measured in electrostatic storage devices are fitted with an analytical expression such as (J. U. Andersen et al. 2002: Κ. Hansen et al. 2017:

Figure 8. Absolute dissociation rate of spontaneous dissociation, T(1), of 2CNI+ from DESIREE with the inset depicting the measured count rate, R(t), with PL denoting power law (see Equation (7)). Also included are master equation simulations (Sim) and excluding contributions from RF (No RF). DTC stands for dead-time corrected data (see Appendix Ε).

E. K. Anderson et al. 2018),
R(t) = Γ^-^' (7) where the scaling coefficient ro contains relevant experimental parameters, kg is the critical rate coefficient where radiative stabilization rates and dissociation rates are comparable, and t, the time after ionization. Deviations from this inverse powerlaw behavior can arise from various factors, such as the ion’s finite heat capacity (Κ. Hansen et al. 2001), the shape of the internal energy distribution of the stored ion ensemble (C. Breitenfeldt et al. 2016), competition between different channels (J. U. Andersen et al. 2003) or due to experimental limitations such as the non-zero dead time of the detector. A fit with a k = 173.8 ± 1.7 s 1 to the raw data reveals deviations from the Equation (7) at earlier times (for the first six turns) as seen in the inset of Figure 8. This is an instrumental effect, which we attribute to the finite dead time of the detector (see Appendix Ε) and is corrected for, resulting in curves labeled “DTC” in Figure 8.
The absolute per-particle dissociation rate T(f) is related to the measured count rate R(t) by (H. Τ. Schmidt et al. 2013a; M. H. Stockett et al. 2025b),
Γ(0 = Λ^ W 'Idei G^avg
where frev = 23.080 kHz is the revolution frequency of ions circulating in the ring, qe is the elementary charge, G = 0.11 is the fraction of the circumference of the storage ring visible to the detector, 7/det is the detector efficiency, and lavg = 14 ρΑ is the average ion-beam current measured at the end of each storage cycle. The results of the master equation simulation using the rate coefficient plotted in Figure 5 are plotted in Figure 8. The simulation assumes a Boltzmann distribution of internal energies at an initial effective temperature T0 for the ion ensemble. Additionally, "/del is used as a fitting parameter to account for the absolute scaling between the simulation and experiment. The best-fit parameters, depicted by the purple trace in Figure 8, were det = 6.66% and T0 = 1605 K. The temperature is consistent with previous temperatures used for modeling PAH ions from the ECR source (M. H. Stockett et al. 2020; Β. Zhu et al. 2022), while the detector’s efficiency is lower than in other experiments using the same detector system (M. H. Stockett et al. 2023), although it remains within previously reported values (M. H. Stockett et al. 2022). The agreement between simulation and experiment, while reasonable. is not perfect. Several factors could contribute to this discrepancy. For example, in reactions involving light hydrogen atoms, tunneling effects can alter the rate coefficients (J. I. Steinfeld et al. 1999). Additionally, while anharmonicity was partially accounted for by the use of anharmonic fundamental frequencies, overtones and 1 + 1 combination bands were not included in the calculation of the IR rate. While incorporating these corrections may improve the fit, the overall magnitude and trend of the rate coefficients remain consistent with experimental data, supporting the validity of the current model within the expected uncertainties. For instance, we ran the simulation without any RF cooling with the same parameters. The resulting green trace in Figure 8 diverges significantly at later times from the experiments, reinforcing the role RF cooling plays in the simulated cooling dynamics.
3.7.		Cooling Rates
The spontaneous dissociation signal tracks the ions up to a few hundred milliseconds, beyond which RC has cooled the remaining ions such that they are no longer dissociating. To probe the ions beyond this limit, we switch to the laser-induced dissociation signal. Due to the relatively small fraction of the ions being excited (depending on the ion-beam densities, and the overlap between the laser pulse and the ion beam), the data is free from dead-time issues. We assume that the portion of the ions excited by the laser pulse is reheated such that its internal energy distribution now resembles the energy distribution that it had at an earlier time after ionization, before any laser excitation. Likewise, two laser pulses with different energies shot at two different times would reheat a small portion of the ion ensemble back to the same earlier time. If the time(s) at which the /th laser pulse of wavelength(s) λ is fired is denoted as t, it is estimated that the active part of the ion ensemble loses an average energy equal to the difference in the photon energies in this time interval. This procedure is implemented by calculating the x2 test statistic between the laser-induced dissociation signal from the two wavelengths, i.e., for a given 1560mm, the 1,330nm with the lowest x2 test statistic is presumed to have heated the ion ensemble back to the same energy distribution and an "experimental” cooling rate is calculated as /R(/330nm I/560nm) where v is the frequency
,_ ,560 nm ' X of the laser at λ = 560, 330 nm and h is Plank’s constant. The physics of the procedure employed here is the same as in A. Sundén et al. (2009) and J. E. N. Navarrete et al. (2023); however, explicit functional forms for the laser-induced signal are not used here, providing for a model-free treatment of the raw experimental data. In order to track the energy distribution of the ion ensemble, the simulation used to model the spontaneous dissociation data is extended until the last of the laser firing times. At each time step, the average energy of the distribution is computed, whose derivative with time would be the “simulated” cooling rate.

Figure 9. Comparison of the experimental cooling rates of 2CNT+ derived from the laser-induced dissociation signal measured in DESIREE, with the simulation cooling rate.

Molecule	20,000 K	Habing	Mathis	10,000 K	4000 K	Lya	
	RC	No RC	RC	No RC	RC	No RC	RC	No RC	RC	No RC	RC	No RC	
2CNI+	71.88	49.12	61.9	38.9	72.31	49.84	91.62	80.25	99.97	99.76	100	100	
2CNI1- Η	22.76	49.06	30.25	58.33	22.14	48.2	6.86	19.23	0.026	0.24	0	0	
2CXI -HCN	5.37	1.82	7.84	2.77	5.56	1.95	1.51	0.5	0.004	0.001	0	0	

Table 3
Branching Fractions of Intact 2CNI1 Ions, Ions Having Lost a Hydrogen, and Ions Having Lost HCN, Starting from an Ion Ensemble Irradiated by Different ISRFs, Taking Into Account RC and No RC

Both these rates are depicted in Figure 9, along with t. While energy loss occurs throughout the interval, it is assumed to be concentrated near the beginning; thus, the calculated cooling rate is plotted at the geometric mean, V1330nm,560nm This approach yields good agreement in both trend and magnitude between the simulated and experimental curves. Closer agreement could be achieved if the exact timing of the cooling was known, but it could not be determined from the current experiment.
3.8.		Astrochemical Implications
The DESIRS and DESIREE results presented here correspond to different ways of preparing vibrationally excited cations. In DESIRS, neutral 2CNI undergo VUV-induced dissociative photoionization. In DESIREE, spontaneous dissociation occurs in internally hot 2CNI+ excited by electron collisions inside the magnetic field of the ECR source, while laser-induced dissociation involves cold 2CNI+ reheated by optical photons. Despite these differences in generation, a single set of rate coefficients accurately describes both cooling and dissociation dynamics, confirming the statistical nature of the decay processes. While factors such as the initial internal distribution of the cations in the two experiments differ due to different Franck-Condon factors, the outputs of the statistical models, within their approximations, do not depend on the preparation method. This enables the results of the study to be extended to excitation mechanisms directly relevant to astronomical environments. To achieve this, an estimate of the molecule's internal energy is required. This is approximated as the product of the interstellar radiation field (ISRF) and the total ion yield as a function of photon energy, which is indicative of the photoionization cross section (A. Heays et al. 2017). The radiation fields used here are taken from A. Heays et al. (2017) and H. Hrodmarsson & E. Van Dishoeck (2023), and have been used to calculate photodissociation and photoionization cross sections of molecules of astronomical interest. Radiation from a 20,000 K blackbody is reminiscent of hot stars such as 0 type stars. The so-called Mathis ISRF is fitted to the observed radiation field in the solar vicinity (J. Mathis et al. 1983). The Habing ISRF is another estimate of the ISRF in the H-II region around 0 and B type stars (H. Habing 1968). Blackbodies ranging 4000-10,000 K are representative of conditions around cooler stars. The Ly„ ISRF is a pure Lya line with a 200 km s 1 Doppler broadening added to its Lor- entzian natural linewidth. This is representative of environments around Τ Tauri stars, where almost 90 % of its total far- UV flux is emitted in this band (Ε. Bergin et al. 2003). Flux at wavelengths shorter than 91.2 nm is intentionally suppressed to account for interstellar extinction by atomic hydrogen in all ISRFs.
To illustrate the adaptability of the simulations, we estimated the extent of fragmentation of 2CNI+ under different photoexcitation conditions. For this, we use the kinetic simulation employed to model DESIRS’s breakdown curve. We run the simulation for 5.8 ms—the timescale, tc= 1/k beyond which RC should take over according to our measurement of the critical rate coefficient, k.c, at which radiative quenching becomes significant. The resulting breakdown curve is then weighted according to the expected internal energy distribution of the molecule in each environment. The computed branching ratios under these different conditions are presented in the “RC” columns of Table 3. Astrochemical models often neglect the effects of RC. To assess its influence, we estimate the branching ratios without including RC. Since running the simulation to the theoretical limit of t —> 00 is not feasible, we approximate the breakdown curve as the ratio of the dissociation rate coefficient for a given channel to the sum of all dissociation rate coefficients (M. H. Stockett et al. 2023). For hydrogen loss, we account for hydrogen migration by assuming an effective rate coefficient 10 times the computed hydrogen loss rate coefficient, reflecting hydrogen loss from each of the 10 carbon sites. The results are shown in the “No RC” columns of Table 3. Note that the percentages in Table 3 coiTespond to the branching ratios of the different channels, i.e., the distribution between the intact parent ion and the various dissociative channels. Absolute ionization yields, and thus the balance between ionized and neutral molecules, are not quantified here.
The importance of including RC is evident from the difference between the “RC” and “no RC” simulation cases summarized in Table 3, particularly in harsher environments.

Wavelength : ^m
Figure 10. IR Emission spectra of 2CNI+ ions excited by 20,000 K and 4000 K blackbody radiation field. The simulation considers only fundamental frequencies.

Without taking into account radiative cooling, the parent abundance is reduced by nearly 30%. This effect arises because RC plays a crucial role near the dissociation threshold, which is the most relevant energy regime in most astronomical environments. Excitations at photon energies greater than near the dissociation threshold are rare, at least in the ISRFs considered here. After accounting for RC, we see that almost 60%-70% of the parents survive in the harsher environments and even more so in the cooler environments, suggesting that small PA(N)Hs and cyano-PAHs could survive to a larger extent than what has commonly been assumed up to now (H. Jochims et al. 1994, 1999). This is highly relevant in view of the recent observations from the James Webb Space Telescope (T. S.-Y. Lai et al. 2023; J. Chastenet et al. 2023; Κ. Ujjwal et al. 2024). J. Chastenet et al. (2023) observe enhanced 3.3 pm emission features in the H II regions of NGC 628, NGC 1365, and NGC 7496. Τ. S.-Y. Lai et al. (2023) observe a similar enhancement of the same feature in NGC 7469. The 3.3 cm feature is accepted to be tracer of the smaller PAH populations, previously thought to be most vulnerable to photodestruction (J. Zhen et al. 2016; Β. Draine et al. 2021). The results here demonstrate that the RC increases the probability of survival, which would affect the lifetime of these molecules in different ISRFs. However, it is to be noted that the ISRFs considered here have an intentional suppression of flux for wavelengths lower than 91.2 nm, where photon energies greater than this cutoff are seen in the H II regions where the JWST observations were recorded. A direct comparison to these regions would require knowledge of the radiation fields more appropriate to these environments. Using a procedure similar to that applied to other PAHs (J. W. Lee et al. 2023; Μ. Η. Stockett et al. 2023, 2025b; J. N. Bull et al. 2025), we obtain a value for Ε,., the critical cation internal energy up to which 2CNI+ can be stabilized by RC, to be 4.05 eV.
To further investigate RC and its role in these environments, we turn to simulations designed to model the DESIREE data. These simulations track the evolution of the ion ensemble’s internal energy distribution over time. By averaging the IR rate coefficients of different fundamental vibrational modes over this energy distribution, we determine each mode’s contribution to the ensemble's cooling. A time-averaged representation of these contributions results in a stick spectrum proportional to the molecule’s IR emission spectrum as it cools. Figure 10 presents two such spectra of ions excited by blackbody radiation fields at 20,000 and 4000 K. The most prominent vibrational modes involved in ion cooling are the CN stretch, ring stretching and breathing modes, and bending modes. These modes were also the most prominent features in the messenger tagging action spectra recorded by T. E. Douglas-Walker et al. (2025). Further laboratory investigations into this spectral region would help in characterization of the possible aromatic IR emission from small cyano- PAHs. The spectral region corresponding to the CN stretch is of particular interest because it may serve as a tracer for cyano- PAHs (V. J. Esposito et al. 2024; Y. Xu & M. Biczysko 2024;
V. J. Esposito et al. 2025). Observations from the Infrared Space Observatory and the AKARI IR satellite have revealed emission bands at 4.4 and 4.65 pm across various astrophysical environments (Ε. Peeters et al. 2004; Τ. Onaka et al. 2013; K. D. Doney et al. 2016). Although these features lie close to the expected position of the CN stretch, they have been attributed to CD stretching modes in deuterated PAHs (B. Draine et al. 2025; X. Yang & A. Li 2025). The Spitzer Space Telescope, despite its superior sensitivity, lacked coverage below 5 um, and was therefore unable to probe this spectral region. With the deployment of JWST and its enhanced sensitivity and broader spectral range, new opportunities arise to detect the CN stretch band (if it exists)—and by extension, to assess the abundance and distribution of cyano-PAHs in space. Furthermore, the overall spectral shape is similar across the two excitation conditions, with minor variations in the relative band intensity. Although the current model is limited in scope and aims only to identify the dominant vibrational modes involved in RC, detailed studies into the emission spectra of neutral indene (Κ. Li et al. 2024a) and 1- and 2-cyanonaphthalene (Κ. Li et al. 2024b) also report little variation in the overall spectral shape across different excitation conditions.
4.		Conclusion
To summarize, the collision and the photodriven dissociation and cooling dynamics of the 2-cyanoindene monocation were explored using a joint storage ring-synchrotron lightbased investigation. The VUV photodissociation of 2CNI+ leads to atomic hydrogen and hydrogen cyanide loss. Mole- cular/consecutive hydrogen and acetylene/cyanide loss were minor channels that open at higher energies. The experimental AEs of these channels were also reported. HCN loss was also seen in the collision-induced dissociation of 2CNI+ at DESIREE. The threshold KERD from DESIRS was reported and compared with DESIREE, showing identical distributions, indicating that the same process occurs in both experiments. Furthennore, the very small magnitude of the KERD hints that the threshold HCN-loss channel was barrierless. This was further supported by DFT explorations, wherein a candidate for this threshold dissociation mechanism was proposed. Additionally, pathways for hydrogen migration and subsequent hydrogen loss were also reported. The vibrational frequencies from these calculations were used as inputs for calculating rate coefficients for dissociation and RC through IR emission and RF. The experimental breakdown curve, including the parent, Η, and HCN loss, was obtained from the slow photoelectron spectra. Simulations were used to fit the breakdown curve and a set of rate coefficients for dissociation were obtained. The same rate coefficients were also used to simulate the spontaneous and laser-induced dissociation results from DESIREE. We found that, irrespective of the excitation condition, the rate coefficients were consistent witli both experiments. In accord with a growing set of studies on the cooling dynamics of PAHs (L. Chen et al. 2014; Μ. Η. Stockett et al. 2020; J. Bernard et al. 2023; J. W. Lee et al. 2023; Μ. Η. Stockett et al. 2023; J. Ν. Bull et al. 2025), RF was shown to play an important role in 2CNI+ in the internal energy range corresponding to the onset of dissociation.
The simulations were then further adapted for astrophysical conditions by using different ISRFs. RC processes were shown to significantly enhance the survival probability of small PAHs under various radiation fields, allowing them to be radiatively stabilized with up to 4.05 eV of internal energy. The IR emission spectra relevant to the current experimental conditions were also modeled under different astrophysical excitation conditions. It was found that the CN stretching, the ring stretching and breathing, and the out-ofplane bending modes were the most active vibrational modes participating in the cooling of the ion via IR emission. The conclusions presented here are relevant to a wide variety of astrophysical environments, from cold molecular clouds such as TMC-1, where cyanoindene was first identified, to photodissociation regions where new insights into PAH chemistry are continuously being discovered with JWST observations.
Acknowledgments
This work was supported by Swedish Research Council grants No. 2018-04092 (H.T.S.), No. 2023-03833 (H.C.), No. 2020-03437 (H.Z.), Knut and Alice Wallenberg Foundation Grant No. 2018.0028 (H.C., H.T.S., and H.Z.), EPSRC grant No. EP/W018691 (J.N.B.), Olle Engkvist Foundation grant No. 200-575 (M.H.S.), and Swedish Foundation for International Collaboration in Research and Higher Education (STINT) grant No. ΡΤ2017-7328 (J.N.B. and M.H.S.). We acknowledge the DESIREE infrastructure for the provision of facilities and experimental support, and we thank the operators and technical staff for their invaluable assistance. The DESIREE infrastructure receives funding from the Swedish Research Council under grants No. 2021-00155 and No. 2023-00170. We thank the SOLEIL staff for running the facility and providing beamtime under project #20231655. This article is based upon work from COST Action CA21126-Carbon molecular nanostructures in space (NanoSpace), supported by COST (European Cooperation in Science and Technology).
Appendix A
Frequencies
The calculated frequencies of the 2-cyanoindene monocation at the B3LYP/cc-pVTZ level of theory are reported in Table 4.
Calculated Anharmonic Fundamental Frequencies of 2CNI at B3LYP/ cc-pVTZ	
Frequency (cm-1)	Intensity (km mol-1)	
83.72	1.9453	
134.88	4.911	
180.36	0.0296	
225.51	9.159	
341.74	0.9337	
372.86	3.2513	
410.94	12.5783	
420.24	0.537	
461.43	1.2676	
538.94	1.0401	
573.29	0.1685	
574.59	0.3601	
615.2	7.518	
692.04	27.1278	
765.73	1.6789	
792.09	17.3342	
859.14	1.436	
865.55	4.8252	
877.9	5.737	
920.22	13.6983	
949.36	5.5145	
988.41	5.8956	
995.11	11.0826	
1015.49	0.5732	
1109.86	0.2672	
1134.41	2.0328	
1136.93	0.2692	
1164.79	23.1537	
1170.27	12.5054	
1204.88	1.2261	
1237.98	4.5036	
1305.04	2.6148	
1346.54	15.2436	
1365.34	50.2108	
1389.31	25.9049	
1417.89	50.5093	
1435.83	3.2701	
1482.62	0.7368	
Table 4
Table 4
(Continued)
Frequency (cm-1)	Intensity (km mol-1)	
1517.89	13.9375	
1606.83	38.9369	
2212.44	73.3264	
2907.57	11.1063	
2913.89	3.2586	
3060.71	1.1926	
3080.64	1.4935	
3094.36	0.9566	
3084.72	0.0728	
3100.01	5.344	
Appendix B
Appearance Energies
To obtain the AEs of the dissociation channels, we fit a straight line to the onset of the unnormalized breakdown curves of each channel (M. H. Stockett et al. 2021) as shown in Figure 11. The X-intercept of the fitted straight line is reported as the AE. The accompanying 95 % prediction bounds of the fit is used to obtain the error estimate. It also follows that such an error estimate reflects only the statistical error in the fit and not the systematic errors, such as the photon-energy step sizes, etc. For the HCN-loss channel at m/z = 114, contributions from the neighboring peak m/z = 115 could not be removed, so the entire mass range is considered. Under the assumption that the m/ z = 115 channel opens at higher energies, the ΑΕ reported here should not be affected by the contamination. As explained in the main text, the AEs reported here are affected by kinetic shifts. Thus, while the AEs are related to the intrinsic properties of the molecule, they are also instrument-dependent. A more accurate way of measuring AEs is from photoionization efficiency measurements, which is beyond the scope of the current work.
500
450
400
8350
5 300
< 250
ω 200
Η 150
100
50
0
8	9	10	11	12	13	14	15	16
Photon energy(h») - eKE: eV
Figure 11. Extrapolation procedure used to determine the AEs for m/z fragments from the dissociative ionization of 2CN1 using synchrotron radiation.

(a)

Figure 12. (a) TPEPICO spectrum of the parent, 2CNI+, and (b) procedure to determine the ionization energy from the TPEPICO trace.

(b)

Appendix C
Ionization Energy
The ionization energy reported in this work was derived from the Threshold Photo-Electron Photo-Ion Coincidence (TPEPICO) spectrum of the parent, 2CNI+, shown in Figure 12(a). The high-resolution TPEPICO trace (after normalizing to the photon flux delivered by the synchrotron source), depicted in Figure 12(a), was obtained from a photonenergy scan from hv = 8.4 to 14.5 eV in 7 meV steps with a
threshold electron resolution of 21 meV and using the methods described in B. Sztàray & Τ. Baer (2003) and G. A. Garcia et al. (2009). To determine the adiabatic ionization energy, we fit a Gaussian to the lowest discernible band (M. Roy Chowdhury et al. 2024) as shown in Figure 12(b). The mean of the Gaussian fit would then be the IE and in this case is 8.66 ± 0.01 eV. A detailed analysis, assigning the transitions and performing Franck-Condon simulations, is beyond the scope of the current work and is not carried out here.

Appendix D
Breakdown Curve Model
The rate laws used to model the breakdown curve measured in SOLEIL are as follows:
= k4[x9] + k1[x2] - (k2 + k? + kn)[n] dt	21121	1 I a
d[x2]2	. ,2Γ Ί, ,2Γ Ί
— : — - ^ [X1 + k_tel + k|o[xio] dt	-
— (kg + k§ + Ζ:20 + kne)[xal
die = klxal + k/ixal - (k3 + k§ + kis) Del
411 = k [] +4[.ϊ5] - ^ + k] + WM dLtsl = 4w+ k/lxal - ^+ k$+ ks)[xsl d[xel = 4m + k4[x7] - (k2 + k§ + K16)[X6]
del = 4M + k[xs] - (k.§ + k# + W[x7] d[xs] = k [] + 4m - (k& + k§ + kng)[Xs]
— = ^[-vs] + k,P[x] - (k§ + k] + Wl^] dt	9
- — K2 Lx2 T Krrot Xrot
dt
- (k1) + krot + feioltaio]
	,	— Krot 1*101 “ Krrot T Khcn) rotl
dt
d[H]
— -— = ^ih] + fete] + kA3[X3] dt
+ fetal + feste]
+ festas! + ferte] + festa»]
+ festas] + feiotaio]
a[HCN]
;	= khcn Xrot.	(Dl)
dt where [x] are the hydrogen-shifted isomers of 2CNI+ with the hydrogen at the ί carbon site, [xrot] is the final intermediate before HCN elimination, fe is the rate coefficient for hydrogen migration from carbon site x to y, khi is the rate coefficient for hydrogen loss from carbon site i, khcn is the rate coefficient for the final HCN elimination, krot (and krrot) are the rate coefficients for the penultimate internal rotation of the HCN moiety (and the corresponding reverse step), and [Η] and [HCN] are the hydrogen and hydrogen cyanide loss channels, respectively. The dependence of the rate coefficients on the internal energy is not shown explicitly for brevity. This system of first- order differential equations is solved using a home written code for the photon energies of relevance, and the relative yields are plotted as a function of the photon energy, constituting the modeled breakdown curve.
All rate coefficients are calculated from Equation (5) with the frequencies of the reactants taken as inputs from the DFT calculations. The final elimination steps are barrierless, and therefore their corresponding rate coefficients, khcn and fe,· are treated variationally. The scaling factors for the lowest six frequencies of the final TS in the HCN (WHCN) and the Η (αΗ) elimination, and the activation energies (Table 2) are the fit parameters that went into the variational treatment. The uncertainties in the parameters were found by varying one parameter, while keeping the others fixed, until the reduced χ2 statistic increased by one unit from the best fit. WHCN and CH were found to be 0.78 ± 0.08 and 1.3 ± 0.06, respectively.
Appendix Ε
Dead-time Correction
In the DESIREE experiments, we observed a low-energy KERD, which would imply the same region in the detector would be triggered repeatedly. This is particularly problematic in the first few turns, when the ion signal is maximum, potentially preventing the detector from fully recovering between events, leading to an underestimation of the neutral count rate. To correct for this finite detector dead time, we apply (G. F. Knoll 2010)
RDTC(f) = R(D) (E1) 1 - R(t)T
where τ is the dead time of the detector and RDTC(f) would be the measured count rate if there was no dead time (i.e., all neutrals were detected). The dead-time-corrected (DTC) data, shown in red in the inset of Figure 8, is computed assuming τ = 700 ns consistent with the expected dead time of our detector system (T. Poikela et al. 2014). The DTC curve follows exactly the expected power-law behavior, reinforcing the validity of our correction.
Appendix F
Potential Energy Surface
The potential energy surface depicted in Figure 4 is reproduced with the relative energies in kJ mol in Figure 13.

ORCID iDs
Arun Subramani © https:/orcid.org/0000-0002-9489-4580
James Flotte de Pouzols © https://orcid.org/0000-0001-9364-1031 Paul Martini © https:/orcid.org/0000-0002-8627-1009
José E. Navarro Navarrete © https:/orcid.org/0000-0003- 4392-9867
Eleanor K. Ashworth © https:/orcid.org/0000-0003-4805-4860
James N. Bull © https://orcid.org/0000-0003-0953-1716
Gustavo A. Garcia © https://orcid.org/0000-0003-2915-2553
Laurent Nahon © https://orcid.org/0000-0001-9898-5693
Bérenger Gans © https://orcid.org/0000-0001-9658-2436
Ugo Jacovella © https://orcid.org/0000-0003-2152-261X
Corentin Rossi © https://orcid.org/0000-0002-5903-9933
Henrik Cederquist © https://orcid.org/0000-0002-0815-0658
Henning T. Schmidt © https:/orcid.org/0000-0002-8209-5095
Henning Zettergren ) https://orcid.org/0000-0002-2493-4161
Mark H. Stockett © https:/orcid.org/0000-0003-4603-5172
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