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

本研究旨在阐明在液相界面中主导界面飞化学的主要因素。我们已经应用超快瞬态反射率测量来研究金属中光激电子与吸附在金属上的分子在液体溶液中的超快相互作用。研究了Au（111）在高氯酸或硫酸水溶液中，在电化学电位控制下，用不同长度的长烷基链烷硫醇进行界面修饰或不进行界面修饰，得到了良好定义的表面。我们成功地建立了一个实验装置，可以高精度地观察金属在液体中的超快瞬态反射率。Au（111）表面的瞬态反射率在光激发后的4皮秒内呈现先下降后上升的变化规律，并保持恒定的偏移量。反射率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，而恒定偏移是由于声子温度升高引起的。瞬态反射率对电化学电位、液侧溶质电解质种类、表面改性是否存在以及烷基硫醇界面改性剂的烃链长度的依赖性较小。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，包括水分子和被吸附的离子，在控制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.