Infrared spectra of C60+ and C120+, obtained via helium messenger spectroscopy, are reported. For C60+, new absorption features have been found just above the discrete vibrational spectrum of the ion. The absorption profile, which is broad and contains little structure, is assigned to one or more electronic absorption transitions and is in good agreement with predictions from time-dependent density functional theory. It seems likely that the transitions observed correspond to excitation from the 2A1u electronic ground state to one or both of the low-lying 2E1u and 2E2u electronic states previously identified as dark states of C60+. These states presumably become optically bright through vibronic coupling and specifically the Jahn–Teller effect. In the case of C120+, the simplest positively charged oligomer of C60, we present the first vibrational spectrum of this ion. Through a comparison with theory, vibrational features are best explained by a peanut-shaped structure for C120+, maintained by covalent bonding between the two C60 units. We have also discovered electronic transitions for C120+, which, similar to C60+, lie just above the vibrational spectrum.

We investigate dissociative electron attachment to 5-fluorouracil (5-FU) employing a crossed electron-molecular beam experiment and quantum chemical calculations. Upon the formation of the 5-FU⁻ anion, 12 different fragmentation products are observed, the most probable dissociation channel being H loss. The parent anion, 5-FU⁻, is not stable on the experimental timescale (~140 µs), most probably due to the low electron affinity of FU; simple HF loss and F⁻ formation are seen only with a rather weak abundance. The initial dynamics upon electron attachment seems to be governed by hydrogen atom pre-dissociation followed by either its full dissociation or roaming in the vicinity of the molecule, recombining eventually into the HF molecule. When the HF molecule is formed, the released energy might be used for various ring cleavage reactions. Our results show that higher yields of the fluorine anion are most probably prevented through both faster dissociation of an H atom and recombination of F⁻ with a proton to form HF. Resonance calculations indicate that F⁻ is formed upon shape as well as core-excited resonances.

Master equation modelling within the AWATAR ansatz reveals dissociation kinetics in hydrated silver cations, Ag ⁺ (H ₂ O) _n , n = 4–6.

Many doubly charged heteronuclear dimers are metastable or even thermodynamically stable with respect to charge separation. Homonuclear dicationic dimers, however, are more difficult to form. He₂ ²⁺ was the first noble gas dimer predicted to be metastable and, decades later, observed. Ne₂ ²⁺ is the only other dicationic noble gas dimer that has been detected so far. Here, we present a novel approach to form fragile dicationic species, by post-ionization of singly charged ions that are embedded in helium nanodroplets (HNDs). Bare ions are then extracted by colliding the HNDs with helium gas. We detect homonuclear doubly charged dimers and trimers of krypton and xenon, but not argon. Our multi-reference ab initio calculations confirm the stability of Kr₂ ²⁺, Kr₃ ²⁺, Xe₂ ²⁺, Xe₃ ²⁺, and Ar₂ ²⁺, but put the stability of Ar₃ ²⁺ towards dissociation to Ar⁺ + Ar₂ ⁺ into question.

The fullerenes C ₆₀ and C ₇₀ have been detected in various interstellar environments, and the cation C 60 + has been identified as a carrier of at least four of the diffuse interstellar bands (DIBs). Based on the presence of fullerenes in space, it is plausible that certain fullerene analogs are abundant in interstellar environments as well. In this context, we present the first electronic laboratory spectra of the analogs C ₆₀ H ₂ O ⁺ , C ₆₀ H ⁺ , C ₆₀ D ⁺ , and C ₆₀ Mg ⁺ . Furthermore, the electronic C 60 + spectrum is remeasured and assignments of the observed transitions are proposed. In the spectrum of C ₆₀ H ₂ O ⁺ , several distinct absorption features could be detected between 10,300 and 10,800 cm ⁻¹ , whereas the analogs C ₆₀ H ⁺ , C ₆₀ D ⁺ , and C ₆₀ Mg ⁺ show a broad absorption in the visible region between 17,000 and 25,000 cm ⁻¹ . None of the detected absorption features in these analogs of C 60 + could be assigned to DIBs.

Trichloroacetic acid (TCA) is an integral part of the atmosphere as well as a prototypical small polyhalogenated organic acid. Here, we investigate dissociative electron attachment (DEA) to the TCA molecule and small (TCA) _N clusters combining cluster beam experiments with quantum chemical calculations exploiting genetic algorithm search and structure clustering. We report negative ion mass spectra at electron energies of 2 eV and 6.8 eV as well as energy dependent ion yields from 2 eV to 10 eV. The chlorine anion Cl⁻ formation is by far the dominating channel at all energies in both isolated molecule and clusters. The calculations suggest that the C–Cl bond breaks spontaneously in the ground state of the transient negative anion. For a single molecule, about an order of magnitude less abundant CCl 2 − , CCl₂COO⁻, and CCl₃COO⁻ions are observed. These ions correspond to the HCl + CO₂, HCl, and H abstraction reactions, respectively. In the cluster environment, the major cluster ion fragments after the Cl⁻ion correspond to the H abstraction, with other fragmentation pathways (HCl abstraction) being strongly suppressed or not observed at all (HCl + CO₂). Our calculations substantiate the experimental observations and show how the hydrogen bond network efficiently controls the dissociation pathways in molecular clusters.