We have studied the mutual neutralization reaction of CO⁺ with O⁻ at a collision energy of ≤0.1 eV under single-collision conditions. We observe both fully dissociative (31.5 ± 1.8 %) and nondissociative (68.5 ± 1.8 %) charge transfer, involving at least five different electronically excited states in CO. From product momentum analysis, we find that the dynamics are governed by electron transfer processes at large O⁻-CO⁺ separations and the competition between predissociation and radiative decay in CO Rydberg states. In the case of nondissociative charge transfer, for one of the reaction channels we observe strong vibrational excitation (4 ≤ v ≤ 8) in the product CO molecule. These data are expected to be useful for modeling astrophysical and planetary atmospheric environments where CO⁺ and O⁻ are present.

The molecule, 2-cyanoindene, C₁₀H₇N (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-1. 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.

Rationale

The molecular anion WF₅⁻ constitutes an isobaric interference for the mass spectrometric detection of the HfF₅⁻ anion. The latter is expected to form an extremely sensitive way of detecting the radionuclide ¹⁸²Hf at natural levels using, for example, accelerator mass spectrometry.

Methods

Photoinduced detachment of WF₅⁻ molecular anions was studied in the Double ElectroStatic Ion Ring ExpEriment (DESIREE), with the ring cooled to cryogenic temperatures (~13 K). Photon energies from 2.5 to 4.5 eV were applied using an OPO laser system for excitation. This energy range covers the threshold for the removal of an electron from that anion species.

Results

Photoinduced reactions were observed which indicate the existence of an energy threshold for WF₅⁻ anions above which the neutralization yield increases strongly. In that range, the data of the neutral signal could be fitted by a linear function of photon energy E, yielding a threshold energy E_th = 3.13 ± 0.01 eV. This result constitutes a substantial improvement both in the range and the resolution of the photon energy as compared to previous data and provides now a rather precise value for that threshold.

Conclusions

This knowledge is required for any attempts to suppress WF₅⁻ anions by photodetachment in the ultrasensitive detection of the very rare radioisotope ¹⁸²Hf using the ¹⁸²HfF₅⁻ analyte anion; for that, the ¹⁸²WF₅⁻ anion would be a stable isobaric mass interference for analyses by accelerator mass spectrometry. The radionuclide ¹⁸²Hf is considered of great interest in various astrophysical environments such as nucleosynthetic processes.

Mutual neutralization reactions play a subtle but crucial role in atmospheric chemistry. The intrinsic gap between typical ionization energies and electron affinities allows cation–anion reactions to produce a broad range of neutral products. Specifically, mutual neutralization of NO⁺ and NO₃^– ions in the presence of water has been proposed to play a key role in the formation of atmospheric nitrous acid (HONO) and, consequently, also OH radical formation. Nevertheless, the mechanisms and products of molecular anion–cation reactions are largely unknown. Here, we present a detailed experimental study of the isolated NO⁺ and NO₃^– reactions, using three-dimensional coincidence imaging of the neutral products of low-energy collisions in merged cation and anion beams. We found that while 15 product channels are energetically accessible, the reaction proceeds exclusively via a single nonadiabatic pathway yielding NO + NO₂ + O. Momentum correlation analysis revealed an intricate nonadiabatic mechanism, initiated by a long-range electron transfer at ∼6 Å distance between the ions, resulting in vibrationally hot NO and an electronically excited NO₃ (²E′) intermediate that undergoes subsequent dissociation, attributed to a conical intersection with the lower lying NO₃ (²E″) state. The mechanistic picture of NO⁺ + NO₃^– neutralization and identification of specific intermediates provides a basis for considering the competing processes that in the presence of water can lead to HONO formation.

We combine cryogenic storage of fullerene anions up to minutes with laser photo-detachment spectroscopy and measure the electron affinities to be 2.684(3) eV for C60 and 2.7665(3) eV for C70, which settle long-standing issues concerning these values. We find that C−70 cools more efficiently than C−60 and that this is due to differences in photon emission from electronically excited states populated by inverse internal conversion (recurrent fluorescence). We also find that intramolecular vibrational redistribution is no longer effective at low internal energies of C−60 or C−70. Radiative cooling becomes extremely slow below intramolecular vibrational redistribution decoupling energies of 0.32(2) and 0.13(3) eV for C−60 and C−70, respectively.

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.

The negative ion of lanthanum has one of the richest bound-state spectra observed for an atomic negative ion and it has been proposed as a promising candidate for laser cooling, which has not yet been achieved for any negatively charged system. In the present study, radiative decays of low-lying excited states of La⁻ were experimentally investigated at the DESIREE cryogenic electrostatic ion-beam storage ring facility using state-selective photodetachment over storage times of up to 800 s. Independently, the transition rates for the relevant states were calculated using a high-precision hybrid theoretical approach. The measured lifetime of the lowest excited state of La⁻ () of 132(20) s agrees very well with the theoretical calculations and previous predictions. However, a puzzling discrepancy exists for the next-higher state , with the experiments yielding a lifetime ∼4 times shorter than calculated, highlighting the challenges for this complex ion. The results provide valuable insights into the prospects for laser cooling La⁻.

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.

Time-dependent unimolecular dissociation rates of the C₁₂H₈ isomers acenaphthylene (ACY) and biphenylene (BPY) cations were measured using a cryogenic electrostatic ion beam storage ring. The neutral, cyano-functionalized tracers of ACY, but not of BPY, have been identified in the interstellar molecular cloud TMC-1 by radioastronomy. For both polycyclic aromatic hydrocarbons (PAHs), dissociation is rapidly quenched by recurrent fluorescence (RF). Master equation simulations including RF rate coefficients based on ab initio molecular dynamics calculations reproduce the measured dissociation rates. Only marginal differences in the survival probabilities of ACY and BPY in TMC-1 are indicated by these results, with both cations being stable for vibrational energies up to about 7.6 eV, which is 3 eV above the dissociation threshold energy.