Table-top soft X-ray spectroscopy and excitation dynamics of photoactive molecular
systems  (SNSF Project)

The chemical environment of an active site in a molecule reveals information about its role and functionality in chemical reactions. Because of their short wavelength and consequently high photon energy X-ray-based spectroscopic techniques are ideally suited to provide analytical tools for characterizing the chemical environment of individual scattering centers in a compound and thus the whole electronic structures of materials. X-ray radiation is conventionally classified between “hard” and “soft”, depending if the photon energy is higher or lower than 5 keV (corresponding to 0.5 nm wavelength).

Both types of radiation can give insight into the local atomic environment of each separate element in a compound because of their elemental sensitivity, however, due to their much higher absorption cross section soft X-rays are preferred when the concentration of the interacting center is reduced, like in the case of thin films and interfaces or single molecules in solution. In the last decades, synchrotrons have provided invaluable sources of soft (and hard) X-rays, which eventually have led to significant progress in many areas of science and technology.

Such an impact is expected to grow even further with the development of X-ray free-electron lasers (FELs), which are able to produce coherent, intense, femtosecond bursts of X-rays, enabling the investigation of nuclear and electron dynamics in chemical reactions, materials and biological systems with unprecedented spatial and temporal resolution. Several X-ray FEL facilities are currently under construction, however due to the very large cost and scale, the number of facilities and consequently beamtime access is expect to remain extremely limited. There is a strong interest into the development of complementary table-top extreme ultraviolet (XUV) and soft X-ray sources, possibly even with less advanced performances, to allow a broader set of users to explore new applications.

Plasma-driven sources are of great interest because of the demonstration to produce XUV and soft X-ray pulses as short as a few picoseconds and with peak brilliances larger than synchrotrons (although still below FELs). It is proposed to develop in our laboratory at Empa a plasma-driven table-top XUV source as a novel analytical technique to characterize chemical and physical properties of single molecules in solution. The generation of soft X-rays pulses provides the additional possibility to design pump-probe schemes to study dynamics down to the picosecond time scale.

The project is organized along two main research lines: (1) experiments at large scale facilities will both bring novel scientific results on catalytic samples in solution and provide benchmark information for the (2) development of a table-top XUV and soft X-ray source with the final goal of addressing the same scientific cases studied at large scale facilities with a laboratory scale system. It is envisaged that such a source can be complementary to large scale facilities and possibly serve as a platform for sample testing and beamtime qualification to FELs. In particular the existence of an easily accessible laboratory-based setup is attractive both for a wide range of Swiss and international user groups, who can prepare experiments for SwissFEL, and for SwissFEL itself because it will increase the effectiveness of the PSI facility for one of the its expected fields of impact: ultrafast liquid-phase chemistry.