Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569, USA.
J Chem Phys. 2017 Aug 21;147(7):074504. doi: 10.1063/1.4985906.
The structure of the hydrated electron, particularly whether it exists primarily within a cavity or encompasses interior water molecules, has been the subject of much recent debate. In Paper I [C.-C. Zho et al., J. Chem. Phys. 147, 074503 (2017)], we found that mixed quantum/classical simulations with cavity and non-cavity pseudopotentials gave different predictions for the temperature dependence of the rate of the photoexcited hydrated electron's relaxation back to the ground state. In this paper, we measure the ultrafast transient absorption spectroscopy of the photoexcited hydrated electron as a function of temperature to confront the predictions of our simulations. The ultrafast spectroscopy clearly shows faster relaxation dynamics at higher temperatures. In particular, the transient absorption data show a clear excess bleach beyond that of the equilibrium hydrated electron's ground-state absorption that can only be explained by stimulated emission. This stimulated emission component, which is consistent with the experimentally known fluorescence spectrum of the hydrated electron, decreases in both amplitude and lifetime as the temperature is increased. We use a kinetic model to globally fit the temperature-dependent transient absorption data at multiple temperatures ranging from 0 to 45 C. We find the room-temperature lifetime of the excited-state hydrated electron to be 137±40 fs, in close agreement with recent time-resolved photoelectron spectroscopy (TRPES) experiments and in strong support of the "non-adiabatic" picture of the hydrated electron's excited-state relaxation. Moreover, we find that the excited-state lifetime is strongly temperature dependent, changing by slightly more than a factor of two over the 45 C temperature range explored. This temperature dependence of the lifetime, along with a faster rate of ground-state cooling with increasing bulk temperature, should be directly observable by future TRPES experiments. Our data also suggest that the red side of the hydrated electron's fluorescence spectrum should significantly decrease with increasing temperature. Overall, our results are not consistent with the nearly complete lack of temperature dependence predicted by traditional cavity models of the hydrated electron but instead agree qualitatively and nearly quantitatively with the temperature-dependent structural changes predicted by the non-cavity hydrated electron model.
水合电子的结构，特别是它主要存在于空腔内还是包含内部水分子，一直是近期争论的焦点。在论文 I [C.-C. Zho 等人，J. Chem. Phys. 147, 074503 (2017)]中，我们发现使用包含空腔和非空腔赝势的混合量子/经典模拟对光激发水合电子回到基态的弛豫速率的温度依赖性给出了不同的预测。在本文中，我们测量了光激发水合电子的超快瞬态吸收光谱随温度的变化，以检验我们模拟的预测。超快光谱清楚地表明，在较高温度下，弛豫动力学更快。特别是，瞬态吸收数据显示，在平衡水合电子基态吸收之外存在明显的过漂白，这只能通过受激辐射来解释。这种受激辐射成分与水合电子实验已知的荧光光谱一致，随着温度的升高，其幅度和寿命都减小。我们使用动力学模型对多个温度（0 至 45°C）下的温度相关瞬态吸收数据进行全局拟合。我们发现激发态水合电子的室温寿命为 137±40 fs，与最近的时间分辨光电子能谱 (TRPES) 实验非常吻合，并且强烈支持水合电子激发态弛豫的“非绝热”图像。此外，我们发现激发态寿命对温度有很强的依赖性，在 45°C 温度范围内变化略超过两倍。这种寿命的温度依赖性以及随着体相温度升高而加快的基态冷却速率，应该可以直接通过未来的 TRPES 实验观察到。我们的数据还表明，水合电子荧光光谱的红色一侧应该会随着温度的升高而显著减小。总的来说，我们的结果与传统水合电子空腔模型预测的几乎完全没有温度依赖性不一致，而是与非空腔水合电子模型预测的温度相关结构变化定性和几乎定量一致。
