Department of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-Ku, Kyoto 606-8502, Japan.
J Chem Phys. 2019 Sep 21;151(11):114503. doi: 10.1063/1.5116818.
We report solvated electron (e ) formation dynamics from the conduction band of liquid methanol studied using femtosecond time-resolved photoelectron spectroscopy. Liquid methanol is excited with vacuum UV (9.3 eV) pump pulses, and the subsequent electron dynamics are probed with UV pulses. The photoelectron signal exhibits a short-lived component (τ = 85 fs) without spectral evolution followed by a long-lived component with continuous spectral evolution over tens of picoseconds. We ascribe the former to a superexcited state, most likely the Wannier exciton, and the latter to the ground electronic state of e . In order to extract accurate energetics from the observed photoelectron spectra, we employ a spectral retrieval method to account for spectral broadening and shifting due to inelastic scattering of photoelectrons in the liquid. The electron binding energy (eBE) of the initial trap state of an electron is determined to be about 1.5 eV, and its biexponential increase up to 3.4 eV is observed with time constants of 2 and 31 ps, which are greater than 0.27 and 13 ps observed for e created by the charge-transfer-to-solvent reaction from CHO to liquid methanol. The solvation dynamics of e created by the electron trapping exhibit a pseudoisosbestic point at a pump-probe delay time of around 15 ps, and the peak energy of the eBE distribution rapidly changes around that time. These results indicate that there exist two trap states, both of which exhibit increasing eBE with time; however, the eBE of the shallow trap state increases only up to 2.1 eV, and transformation to a deep trap state at 25 ps occurs to reach an eBE of 3.4 eV.
我们报告了使用飞秒时间分辨光电子能谱研究液态甲醇导带中溶剂化电子（e）形成动力学。用真空紫外（9.3 eV）泵浦脉冲激发液态甲醇，并用紫外脉冲探测随后的电子动力学。光电子信号表现出没有光谱演化的短寿命成分（τ=85 fs），随后是具有数十皮秒连续光谱演化的长寿命成分。我们将前者归因于超激发态，很可能是 Wannier 激子，后者归因于 e 的基态。为了从观察到的光电子谱中提取准确的能量，我们采用光谱检索方法来考虑光电子在液体中的非弹性散射引起的光谱展宽和移动。电子的初始俘获态的电子结合能（eBE）被确定为约 1.5 eV，并且观察到其双指数增加到 3.4 eV，时间常数为 2 和 31 ps，这大于从 CHO 到液态甲醇的电荷转移到溶剂反应产生的 e 的 0.27 和 13 ps。由电子俘获产生的 e 的溶剂化动力学在泵浦探测延迟时间约为 15 ps 时表现出拟等色点，并且 eBE 分布的峰值能量在该时间附近迅速变化。这些结果表明存在两个俘获态，两者都随着时间的推移表现出增加的 eBE；然而，浅俘获态的 eBE 仅增加到 2.1 eV，并且在 25 ps 时发生向深俘获态的转变以达到 3.4 eV 的 eBE。
