3.1 Dynamics of Condensed Phase Molecular Systems
Project coordinator(s): E. Nibbering, O. Kornilov
Topic 4: Electronic excited state dynamics in molecular model systems


The people involved: Johan Hummert, Nicola Mayer, Geert Reitsma, Oleg Kornilov, Marc J. J. Vrakking
Former team members: Markus Kubin, Julius Zielinski, Martin Eckstein, Iason Katechis, Kathrin Lange, Marcus Rosenblatt, Chung-Hsin Yang, Boris Peev


1. In these experiments ultrafast electronic dynamics of molecules in condensed and gas phase are investigated using the method of time-resolved photoelectron spectroscopy. The pump beam from the near-IR or tunable vis/VUV source excites the system under investigation, which is then probed by the ultrashort XUV pulses. This configuration helps to fully explore the nature of the dynamic processes induced by photoabsorption. The experimental apparatus has reached the time resolution of 35 fs and can ultimately reach sub-10fs resolution allowing to study ultrafast electron dynamics and its coupling to some of the fastest molecular vibrations, such as C=C stretch vibrations in conjugated molecules. The pilot study on dissociation and autoionization dynamics of nitrogen molecules have demonstrated the capabilities of the setup. Currently in the focus are studies of ultrafast internal conversion and isomerization with photoelectron spectroscopy which will help to follow molecular dynamics down to the ground state with sufficient time and energy resolution. The main long term goal of the project is to establish the time-resolved photoelectron spectroscopy as a tool for investigation of open quantum systems focusing on the effects of coherent dynamics in biological and biomimetic molecules, such as retinals, rhodopsins, fluorescent proteins and photosynthetic complexes.


2. Most conventional sources of high-order harmonic (HH) radiation produce short XUV pulses with broad spectra containing individual lines corresponding to odd harmonics of the fundamental laser photon energy (1.6 eV in the case of a femtosecond Ti:sapphire laser). For many applications it is desirable to have a much narrower photon energy spectrum and this can be achieved by selecting one harmonic out of the HH spectrum without significant changes in pulse durations. However, conventional single stage monochromators employed in the XUV spectral range introduce pulse-front tilt in the XUV pulses, which destroys the time resolution of the experiment. To overcome this difficulty we have constructed a novel time delay compensating monochromator, which consist of two identical stages in the mirrored configuration: the first stage selects the harmonic of interest and the second stage compensates the pulse-front tilt induced in the first stage ultimately achieving sub-10 fs time duration inherent to HH generation. The monochromator beamline has been comissioned recently demonstrating the following properties:
- Time resolution: 35 fs (cross-correlation between XUV (12.5 fs) and IR (33.5 fs)
- Spectral resolution: < 500meV
- Transmission: 3%-16% for 3 eV - 50 eV
- Continuous automated scans in the complete spectral range
- Long term stability: scan durations >36 hrs
- Up to 10^7 photons per pulse at 1 kHz repetition rate, 10 kHz rate possible.

Reference: M. Eckstein et al, arXiv:1604.02650 [physics.ins-det]


3. The simplest example of non-trivial dynamics in a quantum system is a discrete bound state coupled to a continuum of unbound states. In scattering or absorption experiments this system leads to well-known Fano resonances observed across many areas of physics. In this project we investigate molecular autoionizing resonances corresponding to ultrafast electronic decay of molecular Rydberg states. The method of time-, angular- and energy-resolved photoelectron spectroscopy allows us to unravel dynamics of a complex resonance in N2, which originate from two discrete states coupled to several continua and leads to the phenomenon of interference stabilization (also called interference narrowing). The study is continued both experimentally and theoretically looking at general interference phenomena in bound-unbound systems and possibility of controlling the dynamics with light.
Citation: Eckstein et al, J. Phys. Chem. Lett. 6, 419 (2015). Eckstein et al, Phys. Rev. Lett. 116, 163003 (2016), Eckstein et al., Faraday Discussions, 2016, DOI: 10.1039/C6FD00093B


4. In practice, molecular dynamics is always affected by the molecular environment, such as water, other solvents or a protein for the case of biologically important chromophore molecules. In this project we aim at investigation of such open quantum systems applying time-resolved spectroscopy to the condensed phase. For this purpose the XUV time delay compensating monochromator is coupled to a microliquid jet source delivering liquid samples to vacuum. This method allows us to observed dynamics in solvated molecules or molecular complexes. The setup has recently been commissioned as demonstrated by a time-resolved experiment on sideband generation in liquid water. In the first experiments water soluble azo-benzene derivatives are investigated. These molecules undergo rapid isomerization upon illumination by blue light and are related to other azo-benzene compounds used for as molecular switches. The ground state photoemission spectrum of such molecule, tartrazine, is shown and investigation of its isomerization dynamics and the solvent dependece are underway. Further molecule of interest is retinal, retinal, the chromophore of the vision protein rhodopsin. This molecule is responsible for the first step in the vision process. Absorption of light by a retinal leads to transformation of the 11-cis configuration to an all-trans configuration on 200 fs timescale. This molecule is known to change its relaxation dynamics drastically in different environments (solutions of different polarity, the protein), but the exact mechanisms remain elusive. In the experiments currently underway time-resolved photoelectron spectroscopy from liquid microjets is used to probe electronic structure of all-trans-retinal and its ultrafast evolution after excitation by a UV pulse. Another example of coherent dynamics and its influence on electronic transformations of molecules is observed in photosynthetic reaction centers. It is proposed that coherent excited state dynamics significantly influences functions of the light-harvesting molecules in these centers. Application of the time-resolved photoelectron spectroscopy to these systems will allow to determine absolute binding energies of all states involved in the dynamics, since photoelectron spectroscopy with XUV radiation, in contrast to all optical spectroscopies, can access every energy level of a molecule and does not have dark transitions. Further development in this direction includes 2D photoelectron spectroscopy with coherent pulse pairs.