Ultrafast Spectroscopy and Interface Femtochemistry of Adsorbed Molecules on a Metal in Liquid

本研究旨在阐明在固体/液体界面上影响界面飞秒化学的主要因素。我们用超快瞬变反射率测量方法研究了金属中光激发电子与吸附在金属表面的分子在液态溶液中的超快相互作用。研究了在高氯酸或硫酸水溶液中，在电化学电位控制下，不同烷基长链的烷硫醇对Au(111)晶面进行界面修饰和修饰的情况下，Au(111)表面清晰可见，建立了一套高精度观察金属在液体中的超快瞬时反射率的实验装置。Au(111)表面的瞬时反射率在光激发后的最初4皮秒内呈现快速下降、快速上升和保持不变的偏移量。快速反射率变化的衰减速率常数随界面条件的变化在3~10(1/ps)之间变化，但与激发光强度无关。快速衰减归因于电子-电子和电子声子耦合引起的电子弛豫，而恒定偏移量则归因于声子温度升高。瞬时反射率与电化学势、液体侧溶质电解质种类、表面修饰的存在以及烷基硫醇界面改性剂的碳氢链长度有很小但明显的依赖关系。实验结果与固/液界面的原子和分子结构有关。结果表明，吸附在界面上的分子，包括水分子和吸附离子，在液/固界面的超快电子弛豫中起着重要的作用。

This study aims to clarify main factors dominating interfacial femtochemistry at a sold/liquid interface. We have applied ultrafast transient reflectivity measurements for investigating ultrafast interaction of photoexcited electrons in metal with molecules adsorbed on the metal immersed in liquid solutions. A well-defined surface of Au(111) was investigated in an aqueous solution of perchloric acid or sulfuric acid, under electrochemical potential control and with/without interface modification by alkanethiols having a long alkyl chain of different lengths.We successfully developed an experimental setup to observe ultrafast transient reflectivity of metal in liquid with a high precision. Transient reflectivity of the Au(111) surface showed rapid decrease followed by rapid increase and a remaining constant offset for an initial 4 picosecond after photoexcitation. Decay rate constants of the rapid reflectivity change ranged from 3 to 10 (1/ps) depending on the interface conditions but there observed no dependence on excitation light intensity. The rapid decay was attributed to electron relaxation by electron-electron and electron phonon couplings, while the constant offset was due to phonon temperature rise.The transient reflectivity showed small but apparent dependence on electrochemical potential, kind of solute electrolytes in the liquid side, existence of surface modification, and hydrocarbon chain length of the alkanethiol interface modifiers. The experimental results are considered in connection with atomic and molecular structure of the solid/liquid interface. It is concluded that molecules adsorbed on the interface, including water molecules and adsorbed ions, play significant rolls in dominating ultrafast electron relaxation at the liquid/solid interface.