Department of Physical Chemistry II, Ruhr University Bochum, 44801 Bochum, Germany.
Chemical Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720.
Proc Natl Acad Sci U S A. 2023 Feb 21;120(8):e2216480120. doi: 10.1073/pnas.2216480120. Epub 2023 Feb 15.
The photo-induced radiolysis of water is an elementary reaction in biology and chemistry, forming solvated electrons, OH radicals, and hydronium cations on fast time scales. Here, we use an optical-pump terahertz-probe spectroscopy setup to trigger the photoionization of water molecules with optical laser pulses at ~400 nm and then time-resolve the transient solvent response with broadband terahertz (THz) fields with a ~90 fs time resolution. We observe three distinct spectral responses. The first is a positive broadband mode that can be attributed to an initial diffuse, delocalized electron with a radius of (22 ± 1) Å, which is short lived (<200 fs) because the absorption is blue-shifting outside of the THz range. The second emerging spectroscopic signature with a lifetime of about 150 ps is attributed to an intermolecular mode associated with a mass rearrangement of solvent molecules due to charge separation of radicals and hydronium cations. After 0.2 ps, we observe a long-lasting THz signature with depleted intensity at 110 cm that is well reproduced by ab initio molecular dynamics. We interpret this negative band at 110 cm as the solvent cage characterized by a weakening of the hydrogen bond network in the first and second hydration shells of the cavity occupied by the localized electron.
光诱导水的辐射分解是生物学和化学中的一个基本反应，在快速时间尺度上形成溶剂化电子、OH 自由基和水合氢离子。在这里，我们使用光泵太赫兹探测光谱装置，用 ~400nm 的光学激光脉冲触发水分子的光离解，然后用 ~90fs 的时间分辨率的宽带太赫兹（THz）场来时间分辨瞬态溶剂响应。我们观察到三种不同的光谱响应。第一个是一个正的宽带模式，可以归因于一个初始弥散的、无定形的电子，半径为（22 ± 1）Å，其寿命很短（<200fs），因为吸收在 THz 范围之外发生蓝移。第二个具有约 150ps 寿命的新出现的光谱特征归因于一种分子间模式，它与由于自由基和水合氢离子的电荷分离而导致溶剂分子的质量重排有关。在 0.2ps 之后，我们观察到一个持续时间长的 THz 特征，在 110cm 处的强度耗尽，这可以通过从头分子动力学很好地再现。我们将 110cm 处的负带解释为溶剂笼的特征，这是由占据局部电子的空腔的第一和第二层水合壳中的氢键网络的弱化引起的。
