Speaker: Dominik Pentlehner (Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Germany) Title: Electronic Spectroscopy in a Pulsed Helium Droplet Beam Time: Today, 15.15 Place: Fysisk Auditorium Abstract: In the last decade, Helium nanodroplets became a popular matrix for the spectroscopy of single atoms, molecules and clusters. The droplets are characterized by a fast cooling of the embedded species to a temperature of only 0.37 K and their gentle interaction with the dopants. In many cases the electronic spectra of molecules inside the droplets resemble the corresponding spectra of the free molecules recorded in a supersonic jet. Characteristic differences compared to gas phase spectra are a solvent shift, phonon wings (PW) and in some cases also a splitting or broadening of zero phonon lines (ZPL). Our research aims to understand these characteristic signatures which are certainly due to the interaction between the dopant and the helium droplet. Therefore, we investigated a series of model compounds to systematically study the influence of the molecular structure on the helium-dopant interaction. The talk will start with an introduction to the electronic spectroscopy in helium droplets. Electronic spectra of a series of anthracenes will be discussed with particular emphasis on low frequency vibrational modes (torsions) of methyl and phenyl substituents. In the case of extended Franck-Condon progressions in these modes, which indicate that a conformational change is induced by the electronic excitation, extensive line broadening is observed in the helium droplets. For short Franck-Condon progressions the transitions remain sharp. A model to explain the broadening by an enhanced energy dissipation into the droplet leading to a homogeneous broadening of transitions will be discussed. From the linewidth we estimate the time constant for the energy dissipation to be about 0.2 ps in the case of 9-Phenylanthracene and 0.5 ps for 2- Methylanthracene. Peter Staanum and Nicolai Nygaard