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 [Formula: see text] 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.

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.

The molecule, 2-cyanoindene, C _10 H _7 N (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.

We have studied low-energy mutual neutralization in reactions of Si-with Na+ and with K+ ions at the merged-beam double ion-beam storage-ring facility DESIREE. We made measurements with and without a significant fraction of the Si-ions in the metastable 2D level. The latter was achieved by state-selective photodetachment. The data reveal contributions from spin-forbidden mutual-neutralization reactions, i.e., violations of Wigner's spin-conservation rule. For the Na+ + Si-case, the dominating such reaction channel is obscured when metastable Si-ions are present in the beam.

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.

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 + 12(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.

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., O 2 + 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 O 2 + with O- mutual neutralisation reaction results predominantly in dissociation of the O 2 + 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 16,18 O 2 + isotopologue shows that the reaction products strongly depend on the vibrational levels of the O 2 + ion for the channel leading to two O(1D) products.

Radiative lifetimes of three elements of the nitrogen group have been experimentally investigated at the Double ElectroStatic Ion Ring Experiment (DESIREE) facility at Stockholm University. The experiments were performed through selective laser photodetachment of excited states of P$^-$, As$^-$ and Sb$^-$ ions stored in a cryogenic storage ring. The experimental results were compared with theoretically predicted lifetimes, yielding a mixture of very good agreements in some cases and large discrepancies in others. These results are part of our efforts to map out the lifetimes of all excited states in negative ions. This data can be used to benchmark atomic theories, in particularly with respect to the degree of electron correlation that is incorporated in various theoretical models.

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 C-60(+) + C-60(-) -> C-60(* )+ C-60 for collisions at interstellar-like conditions. Methods. We studied the MN reaction between C-60(+) and C(60)(- )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 C-60 and electron recombination with C-60(+). 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.

We have studied the stability of C$_{59}$ 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 C$_{60}$ fullerene anions and neutral helium gas at a velocity of 90 km/s (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 one 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 found that about 15 percent 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 C$_{60}$ fullerenes exposed to energetic atoms and ions, such as stellar winds and shock waves, will produce stable, highly reactive products, like C$_{59}$, that are fed into interstellar chemical reaction networks.

We have studied the mutual neutralization reaction of vibronically cold NO+ with O- at a collision energy of approximate to 0.1 eV and under single-collision conditions. The reaction is completely dominated by production of three ground-state atomic fragments. We employ product-momentum analysis in the framework of a simple model, which assumes the anion acts only as an electron donor and the product neutral molecule acts as a free rotor, to conclude that the process occurs in a two-step mechanism via an intermediate Rydberg state of NO which subsequently fragments.

We measured the product -state distribution and its dependence on the hydrogen isotope for the mutual neutralization between 16 O + and 1 , 2 H - at the double electrostatic ion -beam storage ring DESIREE for centerof -mass collision energies below 100 meV. We find at least six product channels into ground -state hydrogen and oxygen in different excited states. The majority of oxygen products populate terms corresponding to 2 s 2 2 p 3 ( 4 S degrees )4 s with 5 S degrees as the main reaction product. We also observe product channels into terms corresponding to 2 s 2 2 p 3 ( 4 S )3 p . Collisions with the heavier hydrogen isotope yield a branching into these lower excited states smaller than collisions with 1 H - . The observed reaction products agree with the theoretical predictions. The detailed branching fractions, however, differ between the theoretical results, and none of them fully agree with the experiment.

We measured the product-state distribution and its dependence on the hydrogen isotope for the mutual neutralization between O+16 and H−1,2 at the double electrostatic ion-beam storage ring DESIREE for center-of-mass collision energies below 100 meV. We find at least six product channels into ground-state hydrogen and oxygen in different excited states. The majority of oxygen products populate terms corresponding to 2s22p3(4S∘)4s with S∘5 as the main reaction product. We also observe product channels into terms corresponding to 2s22p3(4S)3p. Collisions with the heavier hydrogen isotope yield a branching into these lower excited states smaller than collisions with H−1. The observed reaction products agree with the theoretical predictions. The detailed branching fractions, however, differ between the theoretical results, and none of them fully agree with the experiment. Published by the American Physical Society 2024

We measured the spontaneous and photoinduced decays of anionic gold clusters, Au-N(-), 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, Au-N(+), 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 Au-4(-) and Au-5(-) beams. For Au-7(-) , 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 Au-5(-) , 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 Au-5(-) 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.

High-precision measurements of the electron affinities (EA) of the three stable isotopes of silicon, $^{28}$Si, $^{29}$Si and $^{30}$Si, have been performed at the cryogenic electrostatic ion-beam storage ring DESIREE. The quantum states of the ions were manipulated using laser depletion, and the ions were photodetached by laser photodetachment threshold spectroscopy. These EA values are the first reported for $^{29}$Si$^-$ and $^{30}$Si$^-$ and provide a reduced uncertainty for $^{28}$Si$^-$. The resulting EAs are $EA(^{28}$Si$) = 1.38952201(17)$ eV, $EA(^{29}$Si$) = 1.38952172(12)$ eV and $EA(^{29}$Si$) = 1.38952078(12)$ eV, with the corresponding isotope shifts $IS(^{29-28}$Si$) = 0.29(16)$ micro eV and $IS(^{30-28}$Si$) = 1.23(16) $ micro eV. In addition to these measurements, the resolution and signal-to-background level was sufficient to reveal the hyperfine structure splitting in the $^{29}$Si$^-$ isotope, which we report to be $1.8(4) micro eV.

The radiative decay of excited states of the negative ion of rhodium, Rh^-, has been investigated experimentally and theoretically. The experiments were conducted at the Double ElectroStatic Ion Ring Experiment (DESIREE) facility at Stockholm University using selective photodetachment from a stored ion beam to monitor the time evolution of the excited state populations. The lifetimes of the Rh^- ^3F_3 and ^3F_2 fine structure levels were measured to be 3.2(6) s and 21(4) s, respectively. An additional, previously unreported, higher-lying bound state of mixed ^1D_2+^3P_2+(4d^95s)^1D_2+^3F_2 composition was observed and found to have a lifetime of 10.9(8)s. The binding energy of this state was determined to be in the interval 0.1584(2) eV < E_b < 0.2669(2) eV, using laser photodetachment threshold (LPT) spectroscopy. An autodetaching state with a lifetime of 480(10) microseconds was also observed. Theoretical calculations of the excited-state compositions, energies, and magnetic-dipole transition lifetimes were performed using the multiconfiguration Dirac-Hartree-Fock and relativistic configuration interaction methods. The calculated lifetimes of the ^3F_3 and ^3F_2 fine structure levels are in excellent agreement with the measured values. The present study should provide valuable insights into electron correlation effects in negative ions and forbidden radiative transitions.

Mutual neutralization of hydronium (H 3 O + ) and hydroxide (OH − ) ions is a very fundamental chemical reaction. Yet, there is only limited experimental evidence about the underlying reaction mechanisms. Here, we report three-dimensional imaging of coincident neutral products of mutual-neutralization reactions at low collision energies of cold and isolated ions in the cryogenic double electrostatic ion-beam storage ring (DESIREE). We identified predominant H 2 O + OH + H and 2OH + H 2 product channels and attributed them to an electron-transfer mechanism, whereas a minor contribution of H 2 O + H 2 O with high internal excitation was attributed to proton transfer. The reported mechanism-resolved internal product excitation, as well as collision-energy and initial ion-temperature dependence, provide a benchmark for modeling charge-transfer mechanisms.

We have studied the stability of the smallest long-lived all carbon molecular dianion (C2-7 ) in new time domains and with a single ion at a time using a cryogenic electrostatic ion-beam storage ring. We observe spontaneous electron emission from internally excited dianions on millisecond timescales and monitor the survival of single colder C2- 7 molecules on much longer timescales. We find that their intrinsic lifetime exceeds several minutes-6 orders of magnitude longer than established from earlier experiments on C2- 7 . This is consistent with our calculations of vertical electron detachment energies predicting one inherently stable isomer and one isomer which is stable or effectively stable behind a large Coulomb barrier for C2- 7 -> C-7 thorn e- separation.