No abstract

The helium-tagging technique was employed to record absorption spectra of cold anthracene cations and protonated anthracene. The evaluation of the spectra of the chromophore with a different number of attached He atoms allows getting the precise band positions of the molecular ions in the gas phase. The positions of the two most intense bands of anthracene, suitable for astrophysical detection, were found to be λ max = 3478.9 ± 1.8 Å and λ max = 7068.9 ± 5.7 Å. A considerable shift of the red band position compared to a previous measurement was attributed to a temperature effect. No coincidence of the absorption bands in astrophysical observational spectra was found. This allows estimating the upper limit for the abundance of anthracene cations per H nuclei <10−9 along the HD 183143 line of sight. We discuss possible reasons for such a low abundance of this molecular ion.

Much of our knowledge about dynamics and functionality of molecular systems has been achieved with femtosecond time-resolved spectroscopy. Despite extensive technical developments over the past decades, some classes of systems have eluded dynamical studies so far. Here, we demonstrate that superfluid helium nanodroplets, acting as thermal bath of 0.4 K temperature to stabilize weakly bound or reactive systems, are well suited for time-resolved studies of single molecules solvated in the droplet interior. By observing vibrational wavepacket motion of indium dimers (In2) for over 50 ps, we demonstrate that the perturbation imposed by this quantum liquid can be lower by a factor of 10-100 compared to any other solvent, which uniquely allows to study processes depending on long nuclear coherence in a dissipative environment. Furthermore, tailor-made microsolvation environments inside droplets will enable to investigate the solvent influence on intramolecular dynamics in a wide tuning range from molecular isolation to strong molecule-solvent coupling. Introduction.A comprehensive understanding of mechanisms to convert solar energy into other energy forms is a prime objective for many research fields, with potential impact on light harvesting applications or the modelling of photoprotection in biomolecules. Despite extensive investigations, the primary processes triggered by photoexcitation of molecular systems remain insufficiently understood, as they proceed on pico-to sub-femtosecond time scales and involve concerted motion of electrons and nuclei in a complex manner. Insight into the functionality and dynamics of photoactive systems can be obtained in a unique way with femtosecond laser spectroscopy (1), revealing information about, for example, photofragmentation dynamics (2), molecular chirality (3), non-adiabatic coupling dynamics of 2 electrons and nuclei (4), charge transfer (5), or electron dynamics (6). Recent experiments on chemically and biophysically relevant molecules suggest that nuclear motions and in particular their coherences have strong influence on the electronic evolution of the system (7); examples include prototypical molecules for photosynthesis (8-10) and photovoltaics (11).The evolution of a molecular system after photoexcitation strongly depends on its immediate environment, with significant differences between isolated systems, where photodynamics can be precisely studied (12, 13), and the system in its real-world environment in condensed phase. Isolated molecules can be produced in a seeded supersonic expansion (14), where investigations are, however, often prevented by fragmentation resulting from excess energy during photoexcitation, or simply by the fact that weakly bound systems cannot be produced.This harmful vibrational energy can be dissipated to a thermal bath by embedding molecules in a high-pressure buffer gas (15) or a cryogenic matrix (16). As a disadvantage, influences of the environment on intrinsic dynamics can be severe and disentangling intra-and intermolecular dynamics i...

The adsorption of up to ∼100 helium atoms on cations of the planar polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, fluoranthene, and pyrene was studied by combining helium nanodroplet mass spectrometry with classical and quantum computational methods. Recorded time-of-flight mass spectra reveal a unique set of structural features in the ion abundance as a function of the number of attached helium atoms for each of the investigated PAHs. Path-integral molecular dynamics simulations were used with a polarizable potential to determine the underlying adsorption patterns of helium around the studied PAH cations and in good general agreement with the experimental data. The calculated structures of the helium–PAH complexes indicate that the arrangement of adsorbed helium atoms is highly sensitive toward the structure of the solvated PAH cation. Closures of the first solvation shell around the studied PAH cations are suggested to lie between 29 and 37 adsorbed helium atoms depending on the specific PAH cation. Helium atoms are found to preferentially adsorb on these PAHs following the commensurate pattern common for graphitic surfaces, in contrast to larger carbonaceous molecules like corannulene, coronene, and fullerenes that exhibit a 1 × 1 commensurate phase.

Helium clusters attaching the recently experimentally observed sulphur hexafluoride SF6+ and sulphur pentafluoride SF5+ ions are investigated in a combined experimental and theoretical effort. Mass spectrometry ion yields are obtained...