Speaker
Description
With the extremely intense short-wavelength pulses of free-electron lasers (FELs) and high harmonic sources (HHG), novel experiments with highest spatial and temporal resolution become possible. One key example is coherent diffraction imaging of individual nanoparticles. Here, the elastically scattered photons form an interference pattern, which encodes structural information in a 'snapshot'. This enables the in-situ study of fragile structures, for example of superfluid helium nanodroplets, and time-resolved investigations of ultrafast dynamics such as laser induced ultrafast melting in metal nanoparticles. Even changes in the electronic structure due to excitation and ionization alter the scattering response and can therefore be followed with diffraction imaging. Because electron dynamics occur on a timescale shorter than the typical tens of femtosecond pulse durations, we expect an exciting development from the current progress at X-ray FELs (e.g. at Swiss-FEL) and HHG sources towards high-intensity attosecond pulses.
In my talk I will present time-resolved diffraction experiments on silver clusters and helium nanodroplets using extreme ultraviolet (XUV) pulses. The comparably long wavelengths allow for the measurement of wide-angle diffraction patterns that contain three-dimensional information, thus enabling the structural characterization of superfluid spinning droplets and silver nanopolyhedra. I will also discuss time-resolved pump-probe experiments using optical laser pulses with moderate intensities to excite the isolated nanoparticles. We observe interesting switching dynamics in the ultrafast electronic response of helium nanodroplets and find indications of ultrafast melting and instable phase explosions in superheated silver clusters. These results demonstrate the capability of diffraction imaging to visualize ultrafast nanoscale dynamics in highly excited matter.
