Ultra-fast interfacial charge transfer probed using a core-hole clock implementation of resonant inelastic x-ray scattering (RIXS)

在分子和它耦合的表面之间的超快速电子转移在光收集装置中起着关键作用，如染料敏化太阳能电池和水分解光电化学电池（图1所示的水分解光阳极模型是一个主要的例子）。探测这些通常发生在低飞秒时间尺度上的电荷转移过程的一种优雅的方法是使用共振光发射（RPES）形式的共振核能级光谱。这种非辐射核空穴衰变技术依赖于监测共振激发电子参与核空穴衰变时发射的光电子。但它们的逃逸深度非常有限，因此该技术只能应用于电荷转移界面在表面的系统。原则上，在核孔衰变过程中发射的光子携带相同的信息。相应的辐射技术被称为共振非弹性x射线散射（RIXS）。本提案的目的是实现一种新的核-空穴时钟实现共振非弹性x射线散射（RIXS），以探测电子从染料分子的特定未占据分子轨道转移到其吸附表面的导带。使用这种方法，我们应该能够以与共振光发射（RPES）相同的方式在核孔寿命（几飞秒）的时间尺度上探测电荷转移动力学，只是这里我们使用的是光子进光子出技术。这种方法是在现实系统中探测超快速电荷转移的一种非常有前途的途径，在现实系统中，分子器件（如太阳能电池）的电荷转移界面通常被传输层或电解质所掩埋。本海外旅行资助提案中描述的一系列同步加速器实验旨在获得RPES和RIXS的核孔时钟实现的最终协调数据，以及后者在埋藏电荷转移界面中的应用。

Ultra-fast electron transfer between a molecule and a surface to which it is coupled plays a key role in light-harvesting devices such as dye-sensitised solar cells and water-splitting photoelectrochemical cells (the model water splitting photoanode shown in figure 1 being a prime example). An elegant way to probe these charge transfer processes, which typically happen on the low femtosecond timescale is the use of resonant core-level spectroscopy in the form of resonant photoemission (RPES). This non-radiative core-hole decay technique relies on monitoring the photoelectrons emitted when a resonantly excited electron participates in the core-hole decay. But these have a very limited escape depth so the technique can only be applied to systems where the charge transfer interface is the surface. In principle, the photons emitted during core-hole decay carry the same information. The corresponding radiative technique is known as resonant inelastic x-ray scattering (RIXS).The aim of this proposal is to realise a novel core-hole clock implementation of resonant inelastic x-ray scattering (RIXS) to probe the electron transfer from specific unoccupied molecular orbitals of a dye molecule into the conduction band of a surface to which it is adsorbed. Using this method we should be able to probe charge transfer dynamics on the timescale of the core-hole lifetime (a few femtoseconds) in the same way as resonant photoemission (RPES), only here we are using a photons-in, photons-out technique. This approach is a very promising route to probing ultra-fast charge transfer in realistic systems where the charge transfer interface of molecular devices such as solar cells are typically buried by a transport layer or electrolyte. The series of synchrotron experiments described in this overseas travel grant proposal aim to obtain the data for a definitive reconciliation of the core-hole clock implementations of both RPES and RIXS, and the application of the latter to a buried charge transfer interface.