Ultrafast Charge Transfer Processes & Attosecond Spectroscopy
Charge transport between solid substrates and thin films adsorbed onto it is important for many topics, e.g. organic light emitting diodes (OLEDs), dye-sensitized solar cells, electrochemical reactions, electronically stimulated chemistry on surfaces, non-adiabatic chemistry, spectroscopy and molecular electronics. Charge transfer dynamics can be very fast. For the extreme case of strongly coupled chemisorbates transfer times may be well below 1 femtosecond. Apart from linewidth detection, 4 different approaches are commonly used to study these phenomena:
a) Two photon photoemission (2PPE).
b) The "core hole clock" method.
c) Laserpulses of attosecond duration in combination with a laser-based streak camera detection scheme.
d) Conductance measurements with the STM and break junction techniques.
All 4 experimental schemes are used by E20 and its collaborators for investigations in this field:
2PPE is the method of choice for charge transfer processes occurring on a time scale of ~10fs and more. In the past, we have in collaboration with U. Höfer, Universität Marburg extensively studied the decay of image potential states on bare metal surfaces, as well as on metal surfaces covered by thin insulating layers [Surf. Sci. 548, 13; Appl. Phys. A78, 131; Chem. Phys. Lett. 358, 502; Phys. Rev. Lett. 89, 046802; Phys. Rev. Lett. 88, 056805; Appl. Phys. B73, 865; Chem. Phys. 251, 123]. We will extend the investigation of these model systems with a special focus on sandwich layers of metal substrates and films of the light solidified rare gases Ne and He (see also "physisorbed layers").
The so called core hole clock method utilizes the well known lifetimes of inner shell vacancies (= core holes) as an internal time ruler (see "techniques" section for details). Our current interest is on charge transfer in self-assembled layers of organic material on metal, semiconductor and insulator surfaces with a special emphasize on biological sensors and dye-sensitized solar cells [S. Neppl et al., submitted to Phys. Rev. Lett.].
The combination of attosecond XUV pulses obtained from envelope controlled high intensity IR laser pulses by high harmonic generation in rare gas jets, and the laser-based streak camera detection scheme [M. Drescher and F. Krausz, J. Phys. B. Mol. Opt. Phys. 38 (2005) S727, and references therein] is the approach of the future for the investigation of electron dynamics with ultimate resolution in the time domain. Gas phase experiments have demonstrated the potential of this method: Opposite to the core hole clock approach, the entire evolution of the electron dynamics including coherent coupling of different channels is monitored. Transferring this method to the substrate-adsorbate interface is our main goal. The experiments will be pursuit in the excellence cluster "Munich Centre of Advanced Photonics" in collaboration with the Max-Planck Institute of Quantum Optics, and with the group of W. Domcke, Chemistry Department TU-München.
Following the progress in photon energy range and detection efficiency, a series of experiments is scheduled for the near future:
- Temporal evolution of direct photoemission. In previous experiments, photoemission from tungsten (100) surfaces showed differential time shifts in the attosecond range between core and valence electron signals. These measurements will be repeated under much better experimental conditions and extended to tungsten surfaces different from (100), and to larger photon energies, elucidating novel details of electron emission and transport.
- Interference effects in resonant photoemission. Interference of direct and indirect channels gives rise to Fano lineshapes. Theory predicts a temporal evolution of these profiles accessible only by measurements in the time domain. A text book example for such an effect on solid surfaces is the 6eV satellite of Ni which will be studied first. Similar measurements are planned for resonant emission from 4p levels of 4d transition metals and for the giant resonances of rare earth materials.
- Fast adsorbate-substrate charge transfer is a key phenomenon in dye-sensitized solar cells. Dye layers on TiO2 will be the samples studied experimentally and theoretically (Domcke et coworkersd). Preliminary experiments targeting the temporal evolution of charge transport will be performed on monolayers of PF3 adsorbed on metallic and semiconducting surfaces. Comparative measurements of P2s and P2p resonances will be interesting because of the very different lifetimes of these core holes.
- Electronic dynamics in supramolecular assemblies. Electron dynamics in larger molecules and self-assembled supramolecular nanosystems will be studied in C.1.4. Combination of TOF and STM techniques will be used for sample preparation and investigation. The goal is to monitor the movement of electrons in highly organized molecular architectures and nanotextured materials in real time.
Investigations of charge transfer dynamics requiring spatial resolution on an atomic scale can only be done with scanning microscopy techniques. Scientific questions and experimental techniques resemble those of tunneling spectroscopy and will be described there.