Direct Observation of the ππ* to nπ* Transition in 2-Thiouracil via Time-Resolved NEXAFS Spectroscopy

IF 4.6 2区化学 Q2 CHEMISTRY, PHYSICAL

The Journal of Physical Chemistry Letters Pub Date : 2025-04-15 DOI:10.1021/acs.jpclett.5c00544

Fabiano Lever, David Picconi, Dennis Mayer, Skirmantas Ališauskas, Francesca Calegari, Stefan Düsterer, Raimund Feifel, Marion Kuhlmann, Tommaso Mazza, Jan Metje, Matthew S. Robinson, Richard J. Squibb, Andrea Trabattoni, Matthew Ware, Peter Saalfrank, Thomas J. A. Wolf, Markus Gühr

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Abstract

The photophysics of nucleobases has been the subject of both theoretical and experimental studies over the past decades due to the challenges posed by resolving the steps of their radiationless relaxation dynamics, which cannot be described in the framework of the Born–Oppenheimer approximation (BOA). In this context, the ultrafast dynamics of 2-thiouracil has been investigated with a time-resolved NEXAFS study at the Free Electron Laser FLASH. Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) can be used to observe electronic transitions in ultrafast molecular relaxation. We performed time-resolved UV-pump/X-ray probe absorption measurements at the sulfur 2s (L1) and 2p (L2/3) edges. We are able to identify absorption features corresponding to the S2 (ππ*) and S1 (nπ*) electronic states. We observe a delay of 102 ± 11 fs in the population of the nπ* state with respect to the initial optical excitation and interpret the delay as the time scale for the S2 → S1 internal conversion. We furthermore identify oscillations in the absorption signal that match a similar observation in our previous X-ray photoelectron spectroscopy study on the same molecule.

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时间分辨NEXAFS光谱法直接观察2-硫脲嘧啶中π*到nπ*跃迁

过去几十年来，核碱基的光物理一直是理论和实验研究的主题，这是因为解决核碱基无辐射弛豫动力学步骤所带来的挑战，而这些步骤无法在玻恩-奥本海默近似（BOA）的框架内描述。在这种情况下，我们在自由电子激光器 FLASH 上进行了时间分辨 NEXAFS 研究，对 2-thiouracil 的超快动力学进行了研究。近边缘 X 射线吸收精细结构光谱（NEXAFS）可用于观察超快分子弛豫中的电子跃迁。我们在硫的 2s (L1) 和 2p (L2/3) 边缘进行了时间分辨紫外泵/X 射线探针吸收测量。我们能够确定与 S2（ππ*）和 S1（nπ*）电子态相对应的吸收特征。我们观察到 nπ* 态的种群相对于初始光激发有 102 ± 11 fs 的延迟，并将该延迟解释为 S2 → S1 内部转换的时间尺度。此外，我们还发现了吸收信号中的振荡现象，这与我们之前对同一分子进行的 X 射线光电子能谱研究中的类似观察结果相吻合。

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来源期刊

The Journal of Physical Chemistry Letters CHEMISTRY, PHYSICAL-NANOSCIENCE & NANOTECHNOLOGY

CiteScore

9.60

自引率

7.00%

发文量

1519

审稿时长

1.6 months

期刊介绍： The Journal of Physical Chemistry (JPC) Letters is devoted to reporting new and original experimental and theoretical basic research of interest to physical chemists, biophysical chemists, chemical physicists, physicists, material scientists, and engineers. An important criterion for acceptance is that the paper reports a significant scientific advance and/or physical insight such that rapid publication is essential. Two issues of JPC Letters are published each month.