Wednesday, November 1, 2017 at 3:30pm
Physical Sciences Building, 401
245 East Avenue
Electron probes for ultra-fast structural dynamics
Ultrafast electron diffraction and microscopy (UED/UEM) techniques have been extensively applied to condense matter and chemical science problems in the last decades, demonstrating an enormous potential for structural dynamics characterization. More recently high density relativistic probes generated by radiofrequency-based electron sources have been successfully applied to UED problems, pushing the temporal resolution limit well below the picosecond.
While the clear advantage of such technology is the generation of ultrashort pulses, potentially below 10 femtoseconds, the price to pay is a much more complex and expensive instrument, lower average electron flux and poor transverse coherence limiting the range of problems that can be addressed by the technique.
The main challenge for UED instrumentation relates to the transverse beam quality. The achievable transverse coherence length at the sample is of the order of few nanometers for beam sizes of tens-to-hundreds of micrometers. Such probes are orders of magnitude larger than what is achievable with static electron microscopes, limiting the type of samples that can be studied and the scientific questions that can be addressed. Filling the gap in spatial resolution between static and dynamics experiments would provide a leap in electron instrumentation, enabling the study of dynamics around local defects, boundaries and interfaces, and producing pulses of the right spatial scale to probe single molecules, non-periodic samples and nanostructures.
At LBNL we are working on HiRES, the next generation of UED instrumentation. The High Repetition-rate Electron Scattering beamline has been recently commissioned and it is now ready for its debut, with the promise of increasing the electron flux by 3 to 4 orders of magnitude, enabling ultrafast nanodiffraction experiments and providing access to the study of meso-scale specimens. Large coherence lengths and small spot size could be produced at expenses of number of electrons within the pulse. The MHz repetition rate uniquely available at HiRES can be used to pump-probe mesoscale molecules, unveil rare events, or can be traded for enhanced beam properties providing probe sizes below 100 nm and relative energy spreads in the 10- range.
Simulcast at Cornell: https://https://cornell.zoom.us/j/342920767