Electronic charge transfer and screening on surfaces

Ultrafast charge transfer reactions at surfaces are of importance for many processes in physics, chemistry, and technology. Examples include electron transfer processes in dye-sensitised semiconductor nanoparticles and solar cells and charge transport in molecular electronics. MAP’s unique attosecond tools (A.1.2, A.1.3, C.1.1) will for the very first time provide direct access to real-time control and observation of these dynamics. Our project investigations fall into two major categories.

Real-time observation of charge transfer between substrate and adsorbates, core-excited (resonantly or non-resonantly) by an attosecond XUV/X-ray pulse, by means of monitoring the temporal evolution of decay electron spectra with the attosecond streak-recording detection scheme. The interference between adsorbate-substrate charge transfer and core hole decay will shape the wavepacket of the decay electron on the energy and the time scale. As compared with the core hole clock method, this provides information on the complete evolution of the interaction of the two processes, not only the integral ratio of their respective lifetimes. Initially, we will focus on physisorbates, chemisorbed simple molecules, and strongly-coupled radicals on metal and semiconductor surfaces, in accordance with the evolving spectral coverage of MAP’s attosecond sources (C.1.1). In the longer term, investigations of larger organic molecules and supramolecular assemblies (in coorperation with C.1.4) are planned.

In another class of experiments, electron transfer dynamics from photoexcited dye molecules into TiO₂ substrates will be studied (in cooperation with C.1.6) by exciting the dye molecules with few-cycle VIS/UV pulses and observing dynamic chemical shifts of inner-shell states with attosecond XUV/X-ray pulses. Atoms of the dye molecules with well-separable contributions to the photoemission spectrum will serve as local probes.

The envisioned experiments are challenging and require close collaboration between several experimental groups of disparate scientific backgrounds (A.1.2, A.1.3, C.1.1, C.1.4) and with experts in modelling these phenomena (C.1.6, C.2.6). If we succeed, the studies are likely to provide valuable new insight into microscopic processes in surface-based systems.