Electronic structure and the ultrafast electron dynamics of transition metals and transition-metal complexes in water studied by time-resolved two-color pump-probe photoelectron spectroscopy

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The major objective of the proposed research is to experimentally study the ultrafast electron dynamics in transition metal complexes in water, initiated by the absorption of UV laser light. The initially created electronically excited metal complex relaxes through several unstable intermediate electronic configurations. Determination of the energies and lifetimes of these intermediates is crucial for understanding subsequent chemical reactivity, which involves the formation of new species through a delicate balance of ligand motion and solvent response. In particular, the iron complexes [Fe(CN)6]4-/3-, [Fe(tren(py)3]2+ and Fe(H2O)62+/3+ are promising candidates for charge transfer or spin flip processes. To follow these processes on their natural timescale, i.e., tens of femtoseconds to picoseconds, we propose to use a novel time-resolved 2-color pump-probe photoelectron spectrometer. Our targets are aqueous transition metal solutions in vacuum probed in a micron scale liquid microjet. An ultrashort (ca. 30 fs) pump pulse excites the molecular complexes in solution. A suitably delayed probe pulse (in the VUV) ionizes the electronically excited complex as it evolves. Emitted photoelectrons will be detected in vacuum using a time-of-flight electron spectrometer. From the resulting transient photoemission spectra we then can obtain information about the relaxation dynamics, and orbital energetics. The second major focus is on characterizing the angular distribution of photoelectrons emerging from valence orbitals at a given laser photon energy. We will establish if the Cooper-Zare formalism, that describes this angular dependence in the gas phase, holds for aqueous solutions near the water/vacuum interface. Extreme UV-photons (ca. 20 nm) from a high-harmonic generation source will be used to check the applicability of the Cooper-Zare formula at different probing depths and will also allow us to measure valence photoelectron spectra well below the ionization threshold.