Bragg Arthur E, Verlet Jan R R, Kammrath Aster, Cheshnovsky Ori, Neumark Daniel M
Department of Chemistry, University of California, Berkeley, California 94720, USA.
J Am Chem Soc. 2005 Nov 2;127(43):15283-95. doi: 10.1021/ja052811e.
The electronic relaxation dynamics of water cluster anions, (H(2)O)(n)(-), have been studied with time-resolved photoelectron imaging. In this investigation, the excess electron was excited through the p<--s transition with an ultrafast laser pulse, with subsequent electronic evolution monitored by photodetachment. All excited-state lifetimes exhibit a significant isotope effect (tau(D)2(O)/tau(H)2(O) approximately 2). Additionally, marked dynamical differences are found for two classes of water cluster anions, isomers I and II, previously assigned as clusters with internally solvated and surface-bound electrons, respectively. Isomer I clusters with n > or = 25 decay exclusively by internal conversion, with relaxation times that extrapolate linearly with 1/n toward an internal conversion lifetime of 50 fs in bulk water. Smaller isomer I clusters (13 < or = n < or = 25) decay through a combination of excited-state autodetachment and internal conversion. The relaxation of isomer II clusters shows no significant size dependence over the range of n = 60-100, with autodetachment an important decay channel following excitation of these clusters. Photoelectron angular distributions (PADs) were measured for isomer I and isomer II clusters. The large differences in dynamical trends, relaxation mechanisms, and PADs between large isomer I and isomer II clusters are consistent with their assignment to very different electron binding motifs.
利用时间分辨光电子成像技术研究了水簇阴离子(H₂O)ₙ⁻的电子弛豫动力学。在本研究中，用超快激光脉冲通过p←s跃迁激发多余电子，随后通过光剥离监测电子演化。所有激发态寿命都表现出显著的同位素效应（τ(D₂O)/τ(H₂O)约为2）。此外，对于两类水簇阴离子，即异构体I和异构体II，发现了明显的动力学差异，它们之前分别被指定为具有内部溶剂化电子和表面束缚电子的簇。n≥25的异构体I簇仅通过内转换衰变，其弛豫时间与1/n呈线性外推，趋向于本体水中50 fs的内转换寿命。较小的异构体I簇（13≤n≤25）通过激发态自剥离和内转换的组合衰变。异构体II簇的弛豫在n = 60 - 100范围内没有明显的尺寸依赖性，自剥离是这些簇激发后的一个重要衰变通道。测量了异构体I和异构体II簇的光电子角分布（PADs）。大的异构体I和异构体II簇在动力学趋势、弛豫机制和PADs方面的巨大差异与其被指定为非常不同的电子束缚模式一致。
