Research topics of the group of Eberhard Riedle
Molecular dynamics with tunable ultrashort pulses
The main focus of our research is the investigation of primary processes in photoexcited 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.
For the very shortest pulses an extremely broad spectrum is needed and the various
spectral components have to be phase locked. In the visible we can generate pulses
with spectra spanning from 480 to 730 nm, or somewhat narrower ones that can be
tuned with 10 fs duration. The chirp management employed by us for pulse
compression is based on easy to handle prism compressors and specially designed
Brewster chirped mirrors. To fully characterize the pulses, we developed zeroadditional-phase spectral phase interferometry for direct electric-field reconstruction
(ZAP-SPIDER). For routine work we built a dispersion free autocorrelator that can
measure pulses as short as 10 fs with wavelengths ranging from 420 to 1600 nm.
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 behavior.
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.
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.
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.
Collaborations:
Bein / Mintowa, Mayr, Langhals, Kauffmann, Telle, Steinmeyer, Clark MXR
Karpiuk, Lischka / Regina, Willig, Hebling