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

本研究的主要目的是通过实验研究由紫外激光吸收引发的水中过渡金属配合物的超快电子动力学。最初创造的电子激发金属配合物通过几个不稳定的中间电子构型松弛。确定这些中间体的能量和寿命对于理解随后的化学反应至关重要，这涉及到通过配体运动和溶剂反应的微妙平衡形成新物种。特别是，铁配合物[Fe(CN)6]4-/3-, [Fe(tren(py)3]2+和Fe(H2O)62+/3+是电荷转移或自旋翻转过程的有希望的候选物。为了在自然时间尺度（即几十飞秒到皮秒）上跟踪这些过程，我们建议使用一种新型的时间分辨双色泵浦探针光电子光谱仪。我们的目标是在微米尺度的液体微射流中探测真空中的水性过渡金属溶液。超短（约30秒）泵浦脉冲激发溶液中的分子复合物。一个适当延迟的探针脉冲（在VUV中）电离电子激发的复合物，因为它的演变。发射的光电子将在真空中用飞行时间电子能谱仪检测。从得到的瞬态光发射光谱中，我们可以得到关于弛豫动力学和轨道能量学的信息。第二个重点是表征在给定激光光子能量下从价轨道产生的光电子的角分布。我们将确定描述气相中这种角依赖关系的Cooper-Zare形式是否适用于水/真空界面附近的水溶液。来自高谐波源的极紫外光子（约20 nm）将用于检查Cooper-Zare公式在不同探测深度的适用性，并将使我们能够测量远低于电离阈值的价光电子能谱。

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.