Femtosecond two-photon ionization and solvated electron geminate recombination in liquid-to-supercritical ammonia.
Femtosecond two-photon ionization and solvated electron geminate recombination in liquid-to-supercritical ammonia.
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DOI：
10.1021/jp211725r
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发表时间：
2012-02
期刊：
影响因子：
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通讯作者：
J. Urbanek;A. Dahmen;Joel Torres-Alacan;P. Königshoven;J. Lindner;P. Vöhringer
中科院分区：
文献类型：
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作者：
J. Urbanek;A. Dahmen;Joel Torres-Alacan;P. Königshoven;J. Lindner;P. Vöhringer
报道了首个飞秒泵浦探针对纯液态氨多光子电离产生的溶剂化电子的研究。最初的超快电离是用266 nm激光脉冲进行的，发现需要两个光子。用飞秒探针脉冲检测溶剂化电子，该探针脉冲共振，其特征近红外吸收波段约为1.7 μm。此外，研究了溶剂化电子在较宽的温度（227 K≤T≤489 K）和密度(0.17 g cm（-3)≤ρ≤0.71 g cm(-3)）范围内的双相复合动力学，从而涵盖了液体和溶剂的超临界相。电子在第一步中与最初的双光子电离产生的铵离子重新结合，从而形成瞬态离子对(e(am)（-）·nh4(+))，随后在第二步中与酰胺自由基反应以重组中性氨。逃逸概率，即溶剂化电子能够避免双相湮灭的比例，与离子初始复合的经典Onsager理论在定量上是一致的。当明确考虑到离子对介导的重组机制的顺序性时，Onsager模型提供了溶剂化电子的平均热化距离为6.6 nm，这强烈表明电离机制涉及流体的传导带。
The first-ever femtosecond pump-probe study is reported on solvated electrons that were generated by multiphoton ionization of neat fluid ammonia. The initial ultrafast ionization was carried out with 266 nm laser pulses and was found to require two photons. The solvated electron was detected with a femtosecond probe pulse that was resonant with its characteristic near-infrared absorption band around 1.7 μm. Furthermore, the geminate recombination dynamics of the solvated electron were studied over wide ranges of temperature (227 K ≤ T ≤ 489 K) and density (0.17 g cm(-3) ≤ ρ ≤ 0.71 g cm(-3)), thereby covering the liquid and the supercritical phase of the solvent. The electron recombines in a first step with ammonium cations originating from the initial two-photon ionization thereby forming transient ion-pairs (e(am)(-)·NH(4)(+)), which subsequently react in a second step with amidogen radicals to reform neutral ammonia. The escape probability, i.e., the fraction of solvated electrons that can avoid the geminate annihilation, was found to be in quantitative agreement with the classical Onsager theory for the initial recombination of ions. When taking the sequential nature of the ion-pair-mediated recombination mechanism explicitly into account, the Onsager model provides a mean thermalization distance of 6.6 nm for the solvated electron, which strongly suggests that the ionization mechanism involves the conduction band of the fluid.