Abteilung für Molekulare Physikalische Chemie, Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universität , Wegelerstraße 12, 53115 Bonn, Germany.
J Phys Chem B. 2014 Jan 9;118(1):265-77. doi: 10.1021/jp4103993. Epub 2013 Dec 17.
Femtosecond multiphoton ionization experiments have been conducted on ammonia over a wide range of temperature (225 K ≤ T ≤ 490 K) and density (0.18 g/cm(3) ≤ ρ ≤ 0.7 g/cm(3)), thereby covering the liquid and supercritical phases. The experiments were carried out with excitation pulses having a wavelength of 400 nm, and the ionization was found to involve two photons. Therefore, the total ionization energy in this study corresponds to 6.2 eV, which is roughly 2 eV below the valence-to-conduction band gap of the fluid. The ionization generates solvated electrons, which have been detected through their characteristic near-infrared resonance, and must be facilitated through a coupling to nuclear degrees of freedom of the liquid. The recombination of the solvated electron with the geminate fragments was found to obey predominantly single-exponential kinetics with time constants between 500 fs and 1 ps. Only a very minor fraction of the photogenerated electrons is able to escape from the geminate recombination. The results indicate that the majority of electrons are injected into suitable trapping sites located between the first and second solvation shells of the initially ionized ammonia molecules. Such configurations can be considered as instantly reactive and facilitate an ultrafast barrierless electron annihilation. This process is found to exhibit a pronounced kinetic isotope effect, which indicates that the electronic decay is accompanied by the transfer of a proton. The sequence of ionization and recombination events can therefore be described appropriately as a proton-coupled electron transfer (PCET) followed by a proton-coupled back electron transfer (PCBET).
飞秒多光子电离实验已在很宽的温度（225 K ≤ T ≤ 490 K）和密度（0.18 g/cm³ ≤ ρ ≤ 0.7 g/cm³）范围内对氨进行了研究，从而涵盖了液体和超临界相。实验中使用波长为 400nm 的激发脉冲进行，发现电离涉及两个光子。因此，本研究中的总电离能对应于 6.2eV，大致低于流体的价带至导带能隙 2eV。电离产生溶剂化电子，通过其特征近红外共振检测到这些电子，并且必须通过与液体的核自由度耦合来促进。发现溶剂化电子与孪生碎片的复合符合主要的单指数动力学，时间常数在 500fs 到 1ps 之间。只有很少一部分光生电子能够从孪生复合中逃脱。结果表明，大多数电子被注入到最初电离的氨分子的第一和第二溶剂化壳层之间的合适捕获位置。这种构型可以被认为是立即反应的，并促进超快无势垒电子湮灭。发现这个过程表现出明显的动力学同位素效应，这表明电子的衰减伴随着质子的转移。因此，电离和复合事件的顺序可以恰当地描述为质子耦合电子转移（PCET），随后是质子耦合反向电子转移（PCBET）。
