The main focus of our research is the investigation of primary processes in photo-excited molecules
and organic systems. Many of these processes proceed on the time scale of the nuclear motion,
typically within tens or at most hundreds of femtoseconds (1 fs = 10-15 s = 0.000 000 000 000 001 s).
For the experiments we use ultrashort laser pulses with center wavelengths that can be freely chosen
throughout the UV and visible. In recent years we were able to make significant contributions to
the generation, characterization and applications of such pulses. We pioneered noncollinearly
phase matched optical parametric amplifiers (NOPAs) and gradually decreased the obtainable pulse
duration to the 5 fs regime. The direction of our work on pulse sources is given by the needs of
the molecular dynamics, and the time resolved molecular spectroscopy directly benefits from the
evolving possibilities of optical pulse technology. The ultrahigh time resolution allows us to
observe coherent nuclear wavepacket motion and to pin down multidimensional reaction coordinates.
Due to the large spectral range accessible to us, characteristic spectral features can be found
for most transient species or states which reveal the nature and the fate of these species.
Typically, organic molecules have relevant absorption bands in the UV. Ultrashort pulses from
240 to 450 nm are generated by frequency doubling or sum frequency generation. In either scheme we
find it advantageous to use chirped pulses. To shorten the UV pulse length from typical values of
20 – 30 fs down to 7 fs, we resort to achromatic phase matching.
We work also on further simplifying the use of these cutting edge pulses in molecular
spectroscopy. This includes the measurement of the group velocity
mismatch (GVM) in organic solvents that tends to deteriorate the temporal resolution and the
increase of the NOPA repetition rate to 200 kHz to increase the detection sensitivity. A particularly
useful setup is a recently completed white light spectrometer with a probe continuum generated in
CaF2 that spans from 310 to 730 nm. We synchronize the pump probe delay and the wavelength
tuning already during the recording and can measure transient spectra with a residual chirp of less
than 50 fs. The pump wavelength can be freely chosen between 250 and 700 nm with a cross correlation
of typically 70 fs.
The novel pulse sources are used for the investigation of molecular processes ranging from
simple photophysical relaxations to complex chemical reactions. Many of the latter ones are
multistep and the slower kinetics proceed on the nano- to millisecond scale. To access these
time scales experimentally, we recently developed compact laser flash photolysis techniques
compatible with ultrafast pump-probe setups. They are based on readily available high power
light emitting diodes and diode lasers.
Our early work on the vibrational excess energy dependence of the ultrafast internal
conversion in azulene showed, that even for sizeable molecules in solution the signature
of the vibronic states is retained for times in the picosecond range. This is nicely
seen in the excited state intramolecular
proton transfer that proceeds within about 50 fs and is accompanied by persistent
vibrational wavepacket motion
of the keto product in selected modes. The analysis of the wavepackets allowed us to
unambiguously decipher the underlying microscopic process. This is possible not only
for a single proton, but also for double proton transfer.
The lasting vibronic coherence will also allow future control experiments, which we will
conduct with newly developed shaped UV pulses with 20 fs substructures.
The electron transfer is investigated in triarylmethane lactones. It can proceed as fast as
50 fs, with a strong dependence on the polarity and viscosity of the solvent. Future work is
aimed at the additional influence of selected substitutions of the molecules that can be used to
tailor the local electron density. Beyond the variation of the solvent environment, we have
started to investigate chromophores incorporated into nano- and meso-porous zeolites. We find
that these provide not just physical constraints but can heavily influence the photochemical
In organic materials and in particular in
thin layer systems designed for organic electronics applications complex energy
transport and charge transfer processes govern the device performance. With transient
absorption spectroscopy we investigate the ultrafast primary steps and the transport
pathways. Related transport phenomena are also observed in multi-chromophoric supramolecules.
Large-scale molecular rearrangements initiated by electron transfer are typical for the
molecular switches we investigate and the dissociation of diaryl methanes. The latter are of
importance for the study of reactivity scales in organic chemistry and provide a wide playground
for the study of bimolecular reactions.
Many if not all of the outlined activities are only possible with the stimulating and successful
collaboration with our partners in Munich, Germany and the world.
The high degree of coherence of our NOPA pulses can
be utilized for gated heterodyne detected CARS microscopy. Phase locked pulses at three
independently selectable wavelengths are used in these experiments. The gating and heterodyning
allows the efficient suppression of nonresonant signal contributions in complex samples.
Some of the newly developed experimental techniques will in the future be used for heterodyne
detected phase-locked two-color pump-probe spectroscopy. This technique is closely related
to 2D optical spectroscopy and will render in-depth information of the couplings responsible
for ultrafast reactive processes.