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

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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.