氢氧化物水溶液中电子光剥离的超快动力学
Ultrafast Dynamics for Electron Photodetachment from Aqueous Hydroxide
摘要：
用泵浦-探测超快激光光谱研究了水溶液中氢氧化物负离子在200 nm单光子或337 nm和389 nm双光子激发下的电荷转移到溶剂（CTTS）反应。瞬态吸收动力学的水合电子，eaq-，已被观察到，从几百飞秒到600 ps，并研究作为氢氧化物浓度和温度的函数。双生子衰变动力学是双峰的，具有快指数分量（约。13 ps）和由于电子的扩散逃逸而导致的较慢的功率“尾部”。对于双光子激发，逃逸电子的外推分数是1.8倍高于200 nm激发（31%对17.5%，在25 oC，分别），由于电子分布的加宽。双光子电子分离是非常低效的;在400 nm处的相应吸收系数为< 4 cm TW-1 M-1（假设光分离的量子效率为1）。对于[OH-]之间的10 mM和10 M，几乎没有浓度依赖性的溶剂化电子动力学的时间曲线进行了观察。在较高的温度下，电子的逃逸分数以3 × 10 -3 K-1的斜率增加，并且紧密对的复合和扩散控制的解离变得更快。这两个过程的活化能分别为8.3和22.3 kJ/mol。采用Shushin的有心力场中扩散控制反应的半解析理论，建立了孪生动力学模型。讨论了这些结果对水的光电离的影响。
英文摘要：
Charge-transfer-to-solvent (CTTS) reactions of hydroxide induced by 200 nm monophotonic or 337 nm and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, eaq-, have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component (ca. 13 ps) and a slower power "tail" due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% vs. 17.5% at 25 oC, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is < 4 cm TW-1 M-1 (assuming unity quantum efficiency for the photodetachment). For [OH-] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3x10-3 K-1 and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed.
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