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

该研究的主要目的是通过实验研究由紫外激光吸收引发的过渡金属配合物在水中的超快电子动力学。最初产生的电子激发的金属络合物通过几个不稳定的中间电子构型弛豫。这些中间体的能量和寿命的测定是至关重要的理解随后的化学反应，这涉及到通过配体运动和溶剂响应的微妙平衡形成新的物种。特别是铁配合物[Fe（CN）6]4-/3-、[Fe（tren（py）3]2+和Fe（H2O）6 2 +/3+是电荷转移或自旋翻转过程的有希望的候选者。在自然时间尺度上跟踪这些过程，即，几十飞秒到皮秒，我们建议使用一种新的时间分辨双色泵浦探测光电子能谱仪。我们的目标是过渡金属水溶液在真空中探测在微米尺度的液体微射流。超短型（Ultrashort）30 fs）泵浦脉冲激发溶液中的分子复合物。适当延迟的探测脉冲（在VUV中）使电子激发的复合物在其演化时电离。发射的光电子将在真空中使用飞行时间电子光谱仪检测。从所得到的瞬态光电子能谱，然后我们可以获得有关弛豫动力学和轨道能量学的信息。第二个主要焦点是表征在给定的激光光子能量下从价轨道出现的光电子的角分布。我们将建立如果库珀-扎尔形式主义，描述了这种角度依赖性的气相中，持有水/真空界面附近的水溶液。极端紫外光子（ca. 20 nm）的高次谐波产生源将被用来检查库珀-扎雷公式在不同的探测深度的适用性，也将使我们能够测量远低于电离阈值的价光电子谱。

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.