Indene (C9H8) is the only polycyclic pure hydrocarbon identified in the interstellar medium to date, with an observed abundance orders of magnitude higher than predicted by astrochemical models. The dissociation and radiative stabilization of vibrationally hot indene cations are investigated by measuring the time-dependent neutral particle emission rate from ions in a cryogenic ion-beam storage ring for up to 100 ms. Time-resolved measurements of the kinetic energy released upon hydrogen atom loss from C 9 H 8 + , analyzed in view of a model of tunneling through a potential energy barrier, provide the dissociation rate coefficient. Master equation simulations of the dissociation in competition with vibrational and electronic radiative cooling reproduce the measured dissociation rate. We find that radiative stabilization arrests one of the main C9H8 destruction channels included in astrochemical models, helping to rationalize its high observed abundance.

We measured the thresholds for photodetachment from the first and second excited rotational levels of CH− to the lowest vibrational, rotational, and fine-structure level of CH to be E1 = 1.213 ± 0.002 eV and E2 = 1.206 ± 0.002 eV, respectively. Based on these measurements and the rigid rotor approximation, we arrive at an electron affinity of EA = E1 + 1/2 (E1 − E2 ) = 1.217 ± 0.002 eV. This value deviates from earlier experimental results but agrees with the calculation by Feller [J. Chem. Phys. 144, 014105 (2016)]. In the present experiment, we stored ensembles of initially hot CH− in the cryogenic ion-beam storage ring Double ElectroStatic Ion-Ring ExpEriment (DESIREE) for tens of seconds such that the vast majority of the ions were in the few lowest excited rotational levels of the electronic and vibrational ground state.We identified the initial states for photodetachment channels with threshold energies E1 and E2 by comparing the time dependences of measured photodetachment signals with radiative rotational-cooling rates calculated using the literature values of the dipole moment of CH−. The conditions of a few occupied rotational levels are superior to those of previous studies of this system and an important step toward future studies with an all-rotational-ground-state ion beam.

Context. Mutual neutralization (MN) between cations and anions plays an important role in determining the charge balance in certain astrophysical environments. However, empirical data for such reactions involving complex molecular species have been lacking due to challenges in performing experimental studies, leaving the astronomical community to rely on decades-old models with large uncertainties for describing these processes in the interstellar medium.Aims. Our aim is to investigate the MN reaction C60+ + C60− → C60* + C60 for collisions at interstellar-like conditions.Methods. We studied the MN reaction between C60+ and C60− at collision energies of 100 meV using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) and its merged beam capabilities. To aid in the interpretation of the experimental results, semiclassical modeling based on the Landau-Zener approach was performed for the studied reaction.Results. We experimentally identified a narrow range of kinetic energies for the neutral reaction products. Modeling was used to calculate the quantum state-selective reaction probabilities, absolute cross sections, and rate coefficients of these MN reactions, using the experimental results as a benchmark. We compared the MN cross sections with model results for electron attachment to C60 and electron recombination with C60+.Conclusions. Our results show that it is crucial to take mutual polarization effects, the finite sizes, and the final quantum states of both molecular ions into account in order to obtain reliable predictions of MN rates expected to strongly influence the charge balance and chemistry in environments such as dense molecular clouds.

An electrospray ion source has been coupled to a cryogenic electrostatic ion-beam storage ring to enable experimental studies of the fundamental properties of biomolecular ions and their reactions in the gas phase on longer timescales than with previous instruments. Using this equipment, we have measured the vibrational radiative cooling rate of the deprotonated anion of the chromophore of the cyan fluorescent protein, a color-shifted mutant of the iconic green fluorescent protein. Time-resolved dissociation rates of collisionally activated ions are first measured to benchmark a model of the dissociation rate coefficient. Storage time-dependent laser-induced dissociation rates are then measured to probe the evolution of the internal energy distribution of the stored ion ensemble. We find that significant heating of the electrosprayed ions occurs upon their extraction from the ion source, and that the radiative cooling rate is consistent with the prediction of a simple harmonic cascade model of vibrational relaxation.

Several small polycyclic aromatic hydrocarbons (PAHs) with closed-shell electronic structure have been identified in the cold, dark environment Taurus Molecular Cloud 1. We measure efficient radiative cooling through the combination of recurrent fluorescence (RF) and IR emission in the closed-shell indenyl cation (C9H7+), finding good agreement with a master equation model including molecular dynamics trajectories to describe internal-energy-dependent properties for RF. We find that C9H7+ formed with up to Ec=5.85 eV vibrational energy, which is ≈2 eV above the dissociation threshold, radiatively cool rather than dissociate. The efficient radiative stabilization dynamics are likely common to other closed-shell PAHs present in space, contributing to their abundance.

Product distributions and dynamics of low-collision-energy mutual neutralisation reactions involving even simple molecular ions are largely unknown. Reactions which involve oxygen ions, e.g., O2+ with O−, are expected to be important in atmospheric phenomena such as sprites and in high-pressure air or oxygen discharges. Here we show, by combining cryogenically stored-and-merged ion beams with coincident product-imaging techniques, that the O2+ with O− mutual neutralisation reaction results predominantly in dissociation of the O2+ molecule. Three competing reaction pathways yields both O(3P) (84%) and O(1D) (16%) products, but no O(1S) products. Analysis of the momentum-correlated dynamics of the reaction reveals the dominance of two-step mechanisms involving the 3pλu and 3sσg Rydberg states of O2. Furthermore, use of the 16,18O2+ isotopologue shows that the reaction products strongly depend on the vibrational levels of the O2+ ion for the channel leading to two O(1D) products.

We measured the spontaneous and photoinduced decays of anionic gold clusters, , with sizes ranging from 𝑁=2 to 13 and 15. After production in a sputter ion source, the size-selected clusters were stored in the cryogenic electrostatic ion-beam storage ring DESIREE, and their neutralization decays were measured for storage times between 0.1 and 100 s. The dimer was observed to decay by electron emission in parallel to neutral atom emission at long times, implying a breakdown of the Born-Oppenheimer approximation, analogous to the behavior of copper and silver dimers. Radiative cooling is observed for all other cluster sizes. The decays of clusters 𝑁=3,6,8–13,15 show only a single radiative cooling time. For 𝑁=6–13 the cooling times have a strong odd-even oscillation with an amplitude that decrease with cluster size and with the even 𝑁 having the faster cooling. We compare our results with previous measurements of radiative cooling rates of the corresponding cationic gold clusters, , which also show an odd-even effect with a similar oscillation amplitude but at orders of magnitude shorter timescales and out of phase with the anions. The tetramer and pentamer both show two cooling times, which we tentatively ascribe to different structural forms at different ranges of high angular momenta of the ions in the and beams. For , the shape of the decay curve suggests that the cluster cools by emission of low-energy photons. The calculated limit on photon energies strongly suggests that cooling is by vibrational transitions in this case. For , time-resolved studies of photoinduced decays were performed to track the evolution of the internal energy distribution. We conclude that the radiative cooling is dominated by sequences of vibrational transitions in the IR. The laser-enhanced neutralization rate of was exponential, in contrast to its spontaneous decay rate, indicating that the cluster had already been cooled to a very narrow internal energy distribution at 120 ms as the total (integrated) laser-enhanced intensity was independent of the laser firing time at later times. The unimolecular rate constants decreased from 500 s⁻¹ when laser excited at 0.12 s to 40 s⁻¹ when laser excited at 0.62 s.

The radiative cooling of naphthalene dimer cations, (C₁₀H₈)₂⁺ was studied experimentally through action spectroscopy using two different electrostatic ion-beam storage rings, DESIREE in Stockholm and Mini-Ring in Lyon. The spectral characteristics of the charge resonance (CR) band were observed to vary significantly with a storage time of up to 30 seconds in DESIREE. In particular, the position of the CR band shifts to the blue, with specific times (inverse of rates) of 0.64 s and 8.0 s in the 0–5 s and 5–30 s storage time ranges, respectively. These long-time scales indicate that the internal energy distribution of the stored ions evolves by vibrational radiative cooling, which is consistent with the absence of fast radiative cooling via recurrent fluorescence for (C₁₀H₈)₂⁺. Density functional based tight binding calculations with local excitations and configuration interactions (DFTB-EXCI) were used to simulate the absorption spectrum for ion temperatures between 10 and 500 K. The evolution of the bandwidth and position with temperature is in qualitative agreement with the experimental findings. Furthermore, these calculations yielded linear temperature dependencies for both the shift and the broadening. Combining the relationship between the CR band position and the ion temperature with the results of the statistical model, we demonstrate that the observed blue shift can be used to determine the radiative cooling rate of (C₁₀H₈)₂⁺.

We have studied the stability of C₅₉ anions as a function of time, from their formation on femtosecond timescales to their stabilization on second timescales and beyond, using a combination of theory and experiments. The C^-₅₉ fragments were produced in collisions between C₆₀ fullerene anions and neutral helium gas at a velocity of 90 km s⁻¹ (corresponding to a collision energy of 166 eV in the center-of-mass frame). The fragments were then stored in a cryogenic ion beam storage ring at the DESIREE facility, where they were followed for up to 1 minute. Classical molecular dynamics simulations were used to determine the reaction cross section and the excitation energy distributions of the products formed in these collisions. We find that about 15% of the C^-₅₉ ions initially stored in the ring are intact after about 100 ms and that this population then remains intact indefinitely. This means that C₆₀ fullerenes exposed to energetic atoms and ions, such as stellar winds and shock waves, will produce stable, highly reactive products, like C₅₉, that are fed into interstellar chemical reaction networks.