Henning Zettergren

Rationale: The molecular anion WF ₅ ⁻ constitutes an isobaric interference for the mass spectrometric detection of the HfF ₅ ⁻ anion. The latter is expected to form an extremely sensitive way of detecting the radionuclide ¹⁸² Hf at natural levels using, for example, accelerator mass spectrometry. Methods: Photoinduced detachment of WF ₅ ⁻ 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 WF ₅ ⁻ 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 WF ₅ ⁻ anions by photodetachment in the ultrasensitive detection of the very rare radioisotope ¹⁸² Hf using the ¹⁸² HfF ₅ ⁻ analyte anion; for that, the ¹⁸² WF ₅ ⁻ anion would be a stable isobaric mass interference for analyses by accelerator mass spectrometry. The radionuclide ¹⁸² Hf is considered of great interest in various astrophysical environments such as nucleosynthetic processes.

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.

The photo-induced detachment of WF5 molecular anions was investigated in the Double ElectroStatic Ion Ring ExpEriment (DESIREE) for photon energies from 2.5 eV to 4.5 eV, using an OPO laser system for excitation. This energy range covers the threshold for the removal of an electron from that anion species. Photo-induced 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, resulting in a threshold energy Eth = 3.1310+-009 eV. In accelerator mass spectrometry, this possible suppression of WF5 anions by photodetachment could be very beneficial for the efficient detection of the extremely rare radioisotope 182Hf using the 182HfF5 analyte anion; for this, the 182WF5 anion constitutes a pronounced stable interfering isobar. The radionuclide 182Hf is of great relevance in astrophysical environments as it constitutes a potential candidate to study events of nucleosynthesis that may have taken place in the vicinity of the solar system several million years ago.

Context . Sulfur allotropes are proposed as a contributor to the missing atomic sulfur in the interstellar medium, in particular octasulfur, regarded as the most stable sulfur allotrope. Models of sulfur chemical pathways in the ISM strongly rely on the stability of octasulfur. Aims . We investigate the de-excitation dynamics of hot S ₈ ⁺ ions in the gas-phase, addressing experimentally the competition between fragmentation, vibrational cooling, and recurrent fluorescence. By combining the experiments with calculations of rate coefficients, we elucidate the survival probability of octasulfur, placing constraints on the interstellar environments where the allotrope can be detected. Methods . Experiments are performed at the cryogenic ion beam storage ring DESIREE at Stockholm University. Thermally hot S ₈ ⁺ ions are injected into the storage ring, where the time-dependent fragmentation yield is measured. In addition, using inputs from quantum chemical calculations, the energy-dependent rate coefficients of fragmentation, vibrational cooling, and recurrent fluorescence are modeled. The dynamics of the excited-state populations in S ₈ ⁺ is studied based on a master equation formalism, providing the survival probability of the ions as a function of internal energy. The analysis is also extended to the neutral S ₈ allotrope. Results . We observe that radiative cooling is efficient on a time scale of 95 ms, corresponding to internal energies in the range of 1.1-1.4 eV. Experimentally, two fragmentation channels are observed, following the pathways S ₈ ⁺ → S ₅ ⁺ + S ₃ and S ₈ ⁺ → S ₆ ⁺ + S ₂ , in agreement with previous studies. From the computations, these channels have dissociation energies of 1.8 and 1.4 eV, respectively. The modeled rate coefficients predict very slow vibrational cooling, whereas those of recurrent fluorescence are consistent with the experiment. Recurrent fluorescence is expected to proceed from an electronic state at 0.76 eV, computed by time-dependent density functional theory. The analysis reveals a survival probability of less than 1% at internal energies higher than 1.70 eV. In contrast, calculations predict that recurrent fluorescence is not a competitive cooling channel in neutral S ₈ , which promptly fragments above 1.4 eV. Conclusions . Recurrent fluorescence is predicted to be a competitive cooling channel for S ₈ ⁺ , providing a stabilizing pathway for ions in hot environments. Based on simulated survival probabilities, an upper limit of 1100 K is found for the temperature of interstellar environments where S ₈ ⁺ could be detected. However, recurrent fluorescence is not effective for neutral S ₈ , which will fragment already at internal energies close to its dissociation limit.

By combining three-dimensional imaging with merged-beam techniques, the quantum-states distributions and reaction dynamics in the low-collision-energy mutual neutralisation reaction of H+ 2 with H− were determined for the first time. Use of the mixed isotopologue HD+ in a cryogenic double ion-beam storage ring allowed to study the effect of rovibrational cooling on the reaction, and revealed competition between triplet-state driven molecular fragmentation (≈ 72%) and singlet-state stabilization (≈ 28%), with all reactions producing fluorescence. A new theoretical approach is presented, which yields results in good agreement with the experiment, and thereby provides new rate coefficients and a predictive emission spectrum for the reaction. These results are expected to be crucial for the modelling of hydrogen plasma in, for examples, ion sources, plasma torches, fusion experiments and reactors.

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 D 2 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.

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 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 C 60 and 2.7665(3) eV for C 70 , 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.