Christian Schriever - Research interests

molecules are the building blocks of life and matter. The microscopic properties of the molecules and the interactions between the molecules determine the macroscopic parameters of the substances they form. The development of new functional nano materials, technical substances or pharmaceuticals relies on a profound understanding of the underlying molecular structures, interactions and chemical and physical reactions. These reactions are often a complex sequence of elementary constitutive reactions on the time scale of a few ten to some hundred femtoseconds.
Our goal is to observe, understand and manipulate these primary steps.

Many ultrafast molecular processes are governed by intramolecular motions at the speed of skeletal vibrations as well as by the interaction with the surrounding medium. For complex molecules only little is known about the influence of the environment on the coherent wavepacket motions and how environmentally induced variations of the wavepacket motion change the outcome of the process. To understand the interplay between intra- and intermolecular contributions to the dynamics, we disentangle them by studying the same photoreaction both in the solvated and in the isolated molecule. It is essential to use the same probe process for both experiments; otherwise the wavefunction will be projected onto different manifolds of final states, resulting in different signatures even if there are no changes in the molecular dynamics.

dynamics in solution confinement in solvation shell collisions - dephasing		unperturbed dynamics in the gas phase investigation of the isolated molecule best comparable to quantum chemistry

Intramolecular atomic movements can be understood as a superposition of rotations, translations and vibrational modes of the molecule. By coherent superposition of several normal modes, vibrational wavepackets are formed which correspond to a highly localized spatial probability distribution of the system in a certain geometric configuration. While classical chemical reaction kinetics describe conformation changes as rate processes we could show that in the case of ultrafast processes a correct modeling has to be done in form of a wavepacket motion. The reaction can then be understood as a sequence of atomic motions along the reaction coordinate: the reaction path.

Ultrafast excited state intramolecular proton transfer (ESIPT) is the prototype of a chemical reaction since it comprises the breaking of a bond and the simultaneous formation of a new one. ESIPT systems show rich wavepacket dynamics and are especially suited to investigate the dynamics of hydrogen bonds, since they provide a well defined reactant geometry and the transfer of the hydrogen atom can be triggered by photoexcitation with an ultrashort laser pulse. For selected proton transfer systems we investigated the dynamics and kinetics of the ESIPT and subsequent internal conversion. Following these links, you can learn more about:

• the ESIPT is initiated by a contraction of the molecular skeleton: once the proton donor-acceptor distance is sufficiently reduced, the electronic configuration change form the enol to the keto conformation takes place .
• the ESIPT reaction path involves no tunneling contribution of the reactive proton .
• the ESIPT wavepacket dynamics in the presence of two parallel reaction pathways .
• the ESIPT is an ultrafast reaction on a potential energy surface with a large gradient: the environment has little impact on the reaction path of the proton transfer .
• the internal conversion subsequent to the proton transfer proceeds on a slower time scale and is largely depend on the interaction with the surrounding medium .

Experimental access to the time scale of the fastest intramolecular processes can be obtained by ultrashort, spectral tunable light pulses with the duration of a few femtoseconds. They can be used to trigger ultrafast photoreactions at a well defined point in time, control the propagation of the wavepacket and record its evolution in real time. It is essential that the experimental time resolution allows to resolve the fastest steps that are involved in the reaction. Only then it is possible to identify the relevant coordinates for the reaction and elucidate the reaction path. Following these links you can learn about the generation of ultrashort tunable and shaped light pulses and their application in spectroscopy:

• an ultrasensitive pump-probe spectrometer for transient absorption measurements in the gas phase and in solution with a temporal resolution of 20 fs and a sensitivity down to 1.1 x 10^-6 .
• tunable light pulses at 2 MHz from below 300 nm to 990 nm with durations down to 14 fs. .
• octave wide, gap-free tunable light pulses from a 3ω pumped MHz NOPA .
• shaped UV excitation pulses with 20 fs substructures .
• A novel ultra-broadband transient spectrometer based on a supercontinuum fiber laser that allows for the measurement of transient dynamics on the time scale from nanoseconds to seconds .

The results from the investigation of model systems can be applied to foster the understanding of large bio- or techno-relevant molecules. Laser driven wavepacket dynamics could be applied to excite local modes and selectively break bonds. The shaped UV pulses allow for experiments that make use of a propagation on several electronic surfaces. Even the combination of UV and near-infrared pulses should be in reach with the presented pulse generation methods.

Quantum mechanical calculations aid in the interpretation of the experimental results. The critical comparison of experimental and theoretical data allows for a constant improvement of the computational methods. It is almost in reach to develop functional molecules along certain specifications by means of molecular dynamics simulations. In future it should be possible to develop new pharmaceuticals that bind via H-bonds to the healthy form to the prion protein and thus stabilize it.

Franck-Condon factors allow to distiguish between reactive modes (excited by the reactive geometry change) and optical modes (excited by the electronic transition); moreover they can be used to determine the initial conformation in cluster quantum dynamics, to calculate electron transfer rates and singlet to triplet transition rates. We developed a software package - FC-Lab2 - for easy computation and evaluation of Franck-Condon factors.

But of course there is always some part of my works that is along Feynman's trails:
"Physics is like sex: Sure, there might be some practical outcome, but that's not why we do it!"

Download the online version of my PhD thesis as PDF (link). The summary is in German the research results are in English.