The photoinduced relaxation dynamics of nucleobases and their thionated analogs have been investigated extensively over the past decades motivated by their crucial role in organisms and their application in medical and biochemical research and treatment. Most of these studies focused on the spectroscopy of valence electrons and fragmentation. The advent of ultrashort x-ray laser sources such as free-electron lasers, however, opens new opportunities for studying the ultrafast molecular relaxation dynamics utilizing the site- and element-selectivity of x-rays. In this review, we want to summarize ultrafast experiments on thymine and 2-thiouracil performed at free-electron lasers. We performed time-resolved x-ray absorption spectroscopy at the oxygen K-edge after UV excitation of thymine. In addition, we investigated the excited state dynamics of 2-tUra via x-ray photoelectron spectroscopy at sulfur. For these methods, we show a strong sensitivity to the electronic state or charge distribution, respectively. We also performed time-resolved Auger–Meitner spectroscopy, which shows spectral shifts associated with internuclear distances close to the probed site. We discuss the complementary aspects of time-resolved x-ray spectroscopy techniques compared to optical and UV spectroscopy for the investigation of ultrafast relaxation processes.

中文翻译：

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核碱基及其硫代类似物的时间分辨 X 射线光谱

核碱基及其硫代类似物的光诱导弛豫动力学在过去几十年中得到了广泛的研究，原因是它们在生物体中的关键作用及其在医学和生化研究和治疗中的应用。这些研究大部分集中在价电子和碎片的光谱上。然而，超短 X 射线激光源（例如自由电子激光器）的出现，为利用 X 射线的位点选择性和元素选择性研究超快分子弛豫动力学提供了新的机会。在这篇综述中，我们想要总结在自由电子激光器上进行的胸腺嘧啶和 2-硫氧嘧啶超快实验。我们在胸腺嘧啶紫外激发后在氧 K 边缘进行了时间分辨 X 射线吸收光谱。此外，我们通过 X 射线光电子能谱研究了 2-tUra 在硫上的激发态动力学。对于这些方法，我们分别对电子状态或电荷分布表现出很强的敏感性。我们还进行了时间分辨俄歇-迈特纳光谱，它显示了与接近探测位点的核间距离相关的光谱变化。我们讨论了时间分辨 X 射线光谱技术与光学和紫外光谱技术在超快弛豫过程研究中的互补性。