Helium nanodroplets isolation: From collective excitations in organic aggregates to ultra-short time studies at millikelvin temperatures |
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| Frank Stienkemeier | |
| Universität Freiburg | |
| Helium nanodroplets have been employed to form aggregates of atoms and molecules at ultra-cold temperatures for quite some time. It has been shown that peculiar structures can be produced that have not been observed before, and that in spectroscopic studies unprecedented spectral resolution can be achieved. We apply this technique to nanostructures of organic semiconductors in order to study collective excitations in such systems. In particular, complexes of PTCDA molecules show new spectral features which can be assigned to excitonic transitions. Comparison with model calculations provides insight into the interaction between the molecules, vibronic modes and their influence on the electronic properties. In order to probe the superfluid state of helium droplets and to understand energy dissipation mechanisms, dynamical processes have been studied by femtosecond pump-probe techniques. The dynamics of vibrational wave packets in small molecules with variable coupling strengths to the helium environment provides information on decoherence and energy transfer in molecule – bath systems. Finally the strong-field ionization dynamics of rare-gas-doped helium nanodroplets is studied using few-cycle femtosecond laser pulses. Efficient double-ionization of the helium atoms in the droplets is observed when doping the droplets with only a few heavy rare-gas atoms. Pump-probe measurements with identical few-cycle pulses reveal the resonantly enhanced formation of singly and doubly ionized helium atoms at delay times in the range of a few hundreds of femtoseconds. |