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