Molecular dynamics with tunable ultrashort pulses

LMU München
Fakultät für Physik




(c) 2002 BMO

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.


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 zero-additional-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.

fs 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.

fs 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.

fs 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.

fs 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.