3.1 Dynamics of Condensed Phase Molecular Systems

1. We investigate ultrafast electronic dynamics of molecules in condensed and gas phase are using the method of time-resolved photoelectron spectroscopy. The pump beam from the near-IR or tunable vis/VUV source excites the molecules, which are probed by ionization with wavelength-selected ultrashort XUV pulses and photoelectron or photoion detection. The XUV photoionization probe helps to explore the electronic relaxation dynamic in molecules along the full reaction coordinate and delivers information on binding energies of the states involved in the dynamics.

The experimental beamline delivering XUV pulses is based on the principle of high-order harmonic generation (HHG) . Most conventional HHG sources produce short XUV pulses with broad spectra containing individual lines corresponding to odd harmonics of the fundamental laser photon energy (1.56 eV in the case of the femtosecond Ti:Sa laser system used in this setup). However, for many photoionization studies, especially for photoelectron spectroscopy, it is desirable to have a narrow photon energy spectrum. In our beamline this is achieved by selecting one harmonic out of the HH spectrum using a time delay compensating monochromator, which compensates for pulse stretching due to diffractiona and deliveres wavelength-selected XUV pulses without significant increase of the pulse durations.  The monochromator beamline demonstrates 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 10⁷ photons per pulse at 1 kHz repetition rate, a laser source with 10 kHz rate is available.

2. Investigation of molecular dynamics in biological and biomimetic systems usually requires experiments in condensed phase, because molecular environment often affects the relaxation of the chromophore molecules. This is, for example, the case for retinal (the vision chromophore), where photoisomerization yields depend on the solvent. However, combining photoelectron spectroscopy with liquid solutions is not an easy task owing to the high vapour pressures of liquids, which degrade vacuum needed to detect photoelectrons. To overcome this problem we use the microliquid jet technique, which is also employed for transient absorption experiments in Topic 2 and 3. Jets of only 10-15 µm in diameter allow us to deliver solvated molecules to the interaction zone of the photoelectron spectrometer ("magnetic bottle" time-of-flight) with minimal effects of evaporation due to very small surface of these thin jets. The microliquid jet endstation combined with the XUV time delay compensating monochromator beamline allowed us for the first time to record time-resolved photoelectron spectra of an organic molecule, Quinoline Yellow, in aqueous solution at sub-10 mM concentration. Experiments with concentrations down to 100 µm are underway. This milestone opens the route to investigate electronic relaxation of many biochromophores and biomimetic systems and explore the dependence of the first relaxation steps on the molecular environment. In the current plans investigation of electronically-excited aminoacids and molecular switches, both based on azo-benzene and HBDI-type biomimetic switches.

3. Influence of the molecular environment on ultrafast dynamics not only has practical implications for relaxation of different chromophores, but also poses fundamental scientific questions on the role of bath (environment) memory effects in open quantum systems. We use the ultrafast tools of this project to get experimental insight in situations, where such memory effects may exists. The advantage of these tools lies in the fact that photoionization may often be considered as an instantaneous event and photoionization signals therefore render snapshots of the dynamics at the time of the arrival of the ionization pulse. We are looking are looking for the quantum-mechanical effects pertaining to open quantum systems both in solvated molecules, where environment plays the role of the bath, but also in large isolated molecules, where different substates or subsystems may behave classically.  The the latter category falls a recent experiment in highly excited naphthalene cation. We observed a surprizing effect of increased relaxation times with the increase of excitation energy (one would expect the relaxation to proceed faster for higher energies) and propose that this observation, together with similar experimental results on size-dependent relaxation in polyaromatic hydrocarbons, may be a manifestation of quantum-mechanical trapping and timescale separation, earlier proposed for unimolecular dissociation processes at dissociation thresholds.