[en] A relatively simple setup for collection and detection of light emitted from isolated photo-excited molecular ions has been constructed. It benefits from a high collection efficiency of photons, which is accomplished by using a cylindrical ion trap where one end-cap electrode is a mesh grid combined with an aspheric condenser lens. The geometry permits nearly 10% of the emitted light to be collected and, after transmission losses, approximately 5% to be delivered to the entrance of a grating spectrometer equipped with a detector array. The high collection efficiency enables the use of pulsed tunable lasers with low repetition rates (e.g., 20 Hz) instead of continuous wave (cw) lasers or very high repetition rate (e.g., MHz) lasers that are typically used as light sources for gas-phase fluorescence experiments on molecular ions. A hole has been drilled in the cylinder electrode so that a light pulse can interact with the ion cloud in the center of the trap. Simulations indicate that these modifications to the trap do not significantly affect the storage capability and the overall shape of the ion cloud. The overlap between the ion cloud and the laser light is basically 100%, and experimentally >50% of negatively charged chromophore ions are routinely photodepleted. The performance of the setup is illustrated based on fluorescence spectra of several laser dyes, and the quality of these spectra is comparable to those reported by other groups. Finally, by replacing the optical system with a channeltron detector, we demonstrate that the setup can also be used for gas-phase action spectroscopy where either depletion or fragmentation is monitored to provide an indirect measurement on the absorption spectrum of the ion.

[en] An ongoing discussion revolves around the question of what effect hydrogenation has oncarbon backbone fragmentation in polycyclic aromatic hydrocarbons (PAHs). In order to shedmore light on this issue, we have measured absolute single carbon loss cross sections incollisions between native or hydrogenated pyrene cations (C₁₆H⁺_10+m, m = 0, 6, 16) and He as functions of center-of-massenergies down to 20 eV. Classical molecular dynamics (MD) simulations give further insightinto energy transfer processes and also yield m-dependent threshold energies for prompt(femtoseconds) carbon knockout. Such fast, non-statistical fragmentation processesdominate CH_x-loss for native pyrene (m = 0), while much slowerstatistical fragmentation processes contribute significantly to single-carbon loss for thehydrogenated molecules (m =6 and m =16). The latter is shown by measurements of large CH_x-loss crosssections far below the MD knockout thresholds for C₁₆H⁺₁₆ and C₁₆H⁺₂₆. Graphical abstract: .

[en] We have measured the spontaneous neutral particle emission from beams of copper cluster anions (Cu"−_N, N = 3 − 6) stored in an electrostatic storage ring. These experiments have show that multiple structural isomers may be present in the beam. (paper)

[en] Action spectroscopy provides key insights into the nature of electronic transitions of coordination complexes such as porhyrin-containing biochromophores like chlorophyll or transition metal complexes such as tris(bipyridine)ruthenium. (paper)

[en] Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) have been suggested to catalyze the formation of H₂ in certain regions of space, but it remains unclear under which circumstances this mechanism is viable given the reduced carbon backbone stability of superhydrogenated PAHs. We report a laboratory study on the stability of the smallest pericondensed PAH, pyrene (C₁₆H_10+N, with N = 4, 6, and 16 additional H atoms), against photodestruction by single vacuum ultraviolet photons using the photoelectron–photoion coincidence technique. For N = 4, we observe a protective effect of hydrogenation against the loss of native hydrogens, in the form of an increase in the appearance energies of the ${\mathrm{C}}_{16}{\mathrm{H}}_9^{+}$ and C₁₆H₈ ⁺ daughter ions compared to those reported for pristine pyrene (C₁₆H₁₀). No such effect is seen for N = 6 or 16, where the weakening effect of replacing aromatic bonds with aliphatic ones outweighs the buffering effect of the additional hydrogen atoms. The onset of fragmentation occurs at similar internal energies for N = 4 and 6, but is significantly lower for N = 16. In all three cases, H-loss and C_mH_n-loss (m ≥ 1, carbon backbone fragmentation) channels open at approximately the same energy. The branching fractions of the primary channels favor H-loss for N = 4, C_mH_n-loss for N = 16, and are roughly equal for the intermediate N = 6. We conclude that superhydrogenated pyrene is probably too small to support catalytic H₂-formation, while trends in the current and previously reported data suggest that larger PAHs may serve as catalysts up to a certain level of hydrogenation.

[en] Systematic molecular dynamics simulations based on the self-consistent charge density functional tight-binding method have been performed for the molecular collisions inside clusters of fullerene molecules, after prompt atom knockouts by the keV ions. The thermodynamic and kinetic stabilities, as well as the bonding features, have been investigated for the most stable dumbbell dimers C"+_1_8, C"+_1_9 and C"+_2_0. (paper)

[en] Radiative cooling of carbon cluster anions C_2n+1⁻ (n = 3–5) is investigated using the cryogenic electrostatic ion storage ring DESIREE. Two different strategies are applied to infer infrared emission on slow (milliseconds to seconds) and ultraslow (seconds to minutes) timescales. Initial cooling of the ions over the millisecond timescale is probed indirectly by monitoring the decay in the yield of spontaneous neutralization by thermionic emission. The observed cooling rates are consistent with a statistical model of thermionic electron emission in competition with infrared photon emission due to vibrational de-excitation. Slower cooling over the seconds to minutes timescale associated with infrared emission from low-frequency vibrational modes is probed using time-dependent action spectroscopy. For C₉⁻ and C₁₁⁻, cooling is evidenced by the time-evolution of the yield of photo-induced neutralization following resonant excitation of electronic transitions near the detachment threshold. The cross-section for resonant photo-excitation is at least two orders of magnitude greater than for direct photodetachment. In contrast, C₇⁻ lacks electronic transitions near the detachment threshold.