Techniques (to be updated)
Ultrafast Electron Nanocrystallography
This technique incorporates the nanoscale sample preparation & characterization, ultrahigh vacuum, fs laser optics, single electron counting, proximity-coupled electron optics, and pulsed electron diffraction to provide combined spatiotemporal resolutions truly at the atomic scale (0.01Å in distance, 500 fs in time). Experiments recently conducted at MSU have demonstrated its ability to follow the complete course of a nanoscale phase transformation of nanoparticles as small as 2nm, and transient molecular transport at the nanointerface. Such detailed studies are at the resolution and sensitivity limits of techniques that combine spatial and temporal resolutions.
Femtosecond optical spectroscopy can selectively follow electronic state evolution in the dipole accessible regime. But many dark processes, such as radiationless decay, charge transfer, isomerizations (including phase transformation) are difficult to be directly investigated by optical spectroscopy. Using femtosecond electron source new opportunities are open for not only time-resolved electron energy loss spectroscopy (as traditionally implemented, but now resolved in real time), but also electron energy gain spectroscopy which is made possible by electron gaining energy from short-lived excited states. As electron scattering does not follow rigid selection rule and is sensitive to local topography and chemistry, such new spectroscopy in the time domain will allow simultaneous monitoring of energy transfer at multiple levels, thus particularly useful in studying complex phenomena far from equilibrium.
Time-Resolved Electron Local Probe
Electrons can be focused into single domain in materials for imaging and spectroscopy and parsed by field object varying in time. By merging these capabilities with time resolution, it is possible to detail transient collective dynamics of field (electronic or magnetic) domains formed on the nanometer scale. Such studies are particular useful for developing functional materials. In collaboration with Martin Berz’s group, we are developing bright, ultrafast electron source with novel focusing and compression optics to achieve this goal.
Structure Solutions using Nanocrystallographic Method
Material structures on the nanometer scale do not have the long-range order as in the bulk phase. Exact structure solution is thus generally difficult. Combining electron micrographic determination, nano-crystal modeling, and radial distribution function method, three dimensional atomic structures can be determined. In collaboration with Simon Billinge (Columbia) and Phil Duxbury (MSU) groups, we developing methods for quantitative modeling of ultrafast diffraction data. Results will be benchedmarked with X-ray determination using synchrotron radiation source.