Solvated electron formation from the conduction band of liquid methanol: Transformation from a shallow to deep trap state
Solvated electron formation from the conduction band of liquid methanol: Transformation from a shallow to deep trap state
复制标题
DOI：
10.1063/1.5116818
复制
发表时间：
2019-09-21
影响因子：
4.4
通讯作者：
Suzuki, Toshinori
中科院分区：
文献类型：
--
作者：
Hara, Ayano;Yamamoto, Yo-ichi;Suzuki, Toshinori
用飞秒时间分辨光电子能谱研究了液态甲醇导带中溶剂化电子(esolv-)的形成动力学。用真空紫外光(9.3 eV)泵浦脉冲激发液态甲醇，并用紫外光脉冲探测随后的电子动力学。光电子信号呈现出一个没有光谱演化的短寿命分量(tau=85fs)，然后是一个长寿命分量，光谱连续演化数十皮秒。我们将前者归因于超激发态，很可能是Wannier激子，而后者归因于esolv-的基电子态。为了从观测到的光电子光谱中提取准确的能级，我们采用了一种光谱恢复方法来解释由于光电子在液体中的非弹性散射引起的光谱展宽和漂移。测得电子初始陷态的电子结合能(EBE)约为1.5 eV，随时间常数2和31ps的增加，EBE的双指数增加到3.4 eV，大于0.27和13ps的电子结合能是由CH3O-向液态甲醇的电荷转移反应产生的esolv。电子捕获产生的esolv的溶剂化动力学在泵浦-探测延迟时间约为15ps时出现一个假等色点，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陷阱态，它们都表现出EBE随时间的增加而增加，而浅陷态的EBE只增加到2.1 eV，在25ps时发生向深陷态的转变，EBE达到3.4 eV。
We report solvated electron (esolv-) 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 (tau = 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 esolv-. 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 esolv- created by the charge-transfer-to-solvent reaction from CH3O- to liquid methanol. The solvation dynamics of esolv- 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.