Physik-Department E11, Technische Universität München, D-85748 Garching, Germany.
J Am Chem Soc. 2011 Feb 2;133(4):790-5. doi: 10.1021/ja103866s.
The photodetachment of aqueous hydroxide (OH(−)(aq) and OD(−)(aq)) is studied using femtosecond pump−probe and pump−repump−probe spectroscopy. The electron is detached after excitation of the hydroxide ion to a charge-transfer-to-solvent (CTTS) state at 202 nm. An early intermediate is observed that builds up within 160 fs and is assigned to nonequilibrated OH−electron pairs. The subsequent dynamics are governed by thermalization, partial recombination, and dissociation of the pairs, yielding the final hydrated electrons and hydroxyl radicals. An additional pulse at 810 nm is used for secondary excitation of the intermediate species so that more insight is gained into the recombination process(es). Using this technique we observe a novel geminate recombination channel of OH with adjacent hydrated electrons. This channel leads to ultrafast quenching (700 fs) of almost half the initial number of radicals. The fast mechanism displays an isotope effect of 1.4 (for OD(−)(aq) quantum yield 35%, time constant 1.0 ps). This process was not observed in similar experiments on aqueous bromide and seems to be related to the special properties of the hydroxide ion and its local H-bonding environment. Our findings underline the high reactivity of the prehydrated electron.
采用飞秒泵浦-探测和泵浦-重泵-探测光谱法研究了水合氢氧根（OH(-)(aq)和 OD(-)(aq)）的光离解。在 202nm 处将氢氧根离子激发到电荷转移到溶剂（CTTS）态后，电子被离解。观察到一个早期的中间体，它在 160fs 内建立起来，并被分配给非平衡的 OH-电子对。随后的动力学由热化、对电子对的部分复合和离解控制，生成最终的水合电子和羟基自由基。在 810nm 处使用附加脉冲对中间物种进行二次激发，以便更深入地了解复合过程。使用这种技术，我们观察到 OH 与相邻水合电子的一种新的成对复合通道。这个通道导致近一半初始自由基的超快猝灭（700fs）。快速机制显示出 1.4 的同位素效应（对于 OD(-)(aq)量子产率为 35%，时间常数为 1.0ps）。在类似的溴化物水溶液实验中没有观察到这个过程，它似乎与氢氧根离子及其局部氢键环境的特殊性质有关。我们的发现强调了预水合电子的高反应性。
