Dr. Ricarda Riechers (2008-2011)
B.Sc. Lars Christiansen (2011 Guest)
Dr. Dominik Pentlener (2007-2010)
Dr. Alexander Vdovin (2006-2008 Humboldt fellow)
Dr. Rudolf Lehnig (2002-2004)
B.Sc. Joshua Sebre (2004 Guest)
Dipl. Phys. G. Michael Poetzl (2006)
Dr. Jonathan Goretzki (2007 Guest)
Spectroscopy in Superfluid Helium Nanodroplets
Superfluid helium nanodroplets are one of the most gentle host system with exceptional cryogenic capability .
Single molecules, molecular complexes and van der Waals clusters are readily prepared at a
temperature of only 0.37 K inside these droplets. Superb heat conductivity and
vanishing viscosity allows for recording MW, IR, and electronic spectra with rotational
resolution. The mobility of the dopant species inside the superfluid droplets allows for
studying bimolecular chemical reactions. Nevertheless, the weak interaction between dopant and
droplet can be observed in high resolution spectroscopic experiments. We concentrate on the
vibrational and (if possible) rotational fine structure of electronic spectra of molecules
in superfluid helium droplets. Our goals are to study the interaction between dopant and host
in order to learn about one of the weakest interaction forces and on microsolvation in superfluid
helium and, finally, to provide details on superfluidity in a microscopic scale . The ideal
conditions in helium droplets are exemplified in the spectra of porphycene showing tunneling
splitting of double hydrogen tunneling for all three isotopic variants (cf. figure) .
Photochemistry in Superfluid Helium Nanodroplets
Photochemical processes such as intramolecular charge transfer (CT) or protontransfer (PT) can be
initiated by electronic excitation of the molecule. Both processes are indicated by a significant
frequency shift of the emission with respect to the excitation. While in the gas phase the molecular
system use to be hot after CT or PT, in helium droplets vibrational excitation is dissipated into the
helium droplet and released by evaporative cooling prior to radiative decay. Thus, structural
information of the transfer species can be obtained from dispersed emission spectra taken in
helium droplets. The influence of solvent systems
on the intramolecular chemistry can be studied in a molecular scale by designing
complexes of well defined stoichiometry. As an example the ESIPT of 3-hydroxyflavone has been
investigated and in sddition the influence of water molecules  including isotopic variants.
Techniques and Equipment
Fluorescence excitation and dispersed emission of electronic transitions are
recorded for molecules, molecular complexes and van der Waals clusters doped
into superfluid helium nanodroplets.
Helium droplets are generated in a continuous source and in a pulsed source.
The latter was developed in our laboratory in cooperation with Uzi Even from
Tel Aviv University, Israel .
The first provides droplets from 103 to 107 atoms and the latter from 104
Doping is carried out by the pick-up procedure. The dopant species is provided
in a heatable gas cell on axis to the droplet beam. Single
particle and controlled homogeneous and heterogeneous multiple particle doping is possible.
Frequency tunable laser systems in pulsed and cw mode are used for recording electronic spectra.
Fluorescence detection is accomplished by photomultipliers, dispersed emission by grating spectrographs
and CCD camera. Quadrupole mass spectrometer as well as a bolometer are detectors additionally operated
in the lab.