Interfaces are what separate one material from another. We sense the world through interfaces, whether touching the surface of a table or seeing the light reflecting off the edge of a glass. Many other interfaces are less visible but still have a place in our lives. For example, modern solar cells consist of thin layers where interfaces play an important role for charge separation.
A team of Foundry users led by Royce Lam from the University of California, Berkeley, and colleagues have developed an interfacial probe utilizing soft-X-ray pulses from a free electron laser. By aiming the pulses at a graphite sample, the scientists have shown that they can detect a nonlinear spectrographic signal that arises from the graphene layers near the surface of the graphite. This demonstration opens up a new field in interface studies, offering the possibility to track surface chemistry reactions with the femtosecond resolution provided by the very short x-ray pulses from free electron lasers.
The new interfacial probe relies on a process where two photons of the same energy are absorbed by an atom to generate a photon with double the original energy. This so-called second harmonic generation (SHG) can only occur in regions of a material that are asymmetric—specifically, regions that lack inversion symmetry. This asymmetry is necessary to counter interference effects that would cancel out the SHG emission. An interface is by nature asymmetric, which is why researchers have for many years used SHG observations to investigate interfaces based on optical laser techniques. Having a similar capability in X-rays would allow measurements of the electronic structure using core level spectroscopy, where the inner shell provides atom-specific sensitivity. However, X-ray-based SHG studies have lagged behind those in the optical regime due to a lack of sources for coherent, high-intensity X-rays, which are needed to generate multiple absorptions.
This work is one of the first to investigate SHG with soft x rays (between 100 eV and 1 keV). The most significant finding for future utilization is the observation that the SHG signal is strongly enhanced upon resonant excitation into the unoccupied states in graphite. This means that interface-selective x-ray absorption spectra could be measured in SHG mode. X-ray absorption spectroscopy is already a well-established technique for studying chemical bonding and structure of species on surfaces. Combining this soft-x-ray probe with an optical laser to pump the system would allow researchers to follow surface chemical reactions in real time with femtosecond resolution. This will be essential in the development of new catalysts for the chemical industry, but it may also help with efforts to improve fuel cells or to create artificial photosynthesis systems.