Principal Investigator Nuh Gedik
Direct determination of structural dynamics requires the ability of measuring atomic motions with angstrom scale spatial resolution. Conventional ultrafast optical spectroscopy based on measuring transient changes in optical constants is sensitive to dynamics of electronic excitations but can provide only indirect information about structural dynamics. The spatial resolution in these techniques is also limited to micron scales due to diffraction limit.
Ultrafast electron diffraction (UED) can directly couple to structural dynamics and provide sub-angstrom spatial resolution together with sub-picosecond temporal resolution. The principle of UED is similar to pump probe spectroscopy. An ultrafast laser pulse is split into two; the first part of the laser pulse is directly focused on to the sample to create a non-equilibrium state. To probe the induced structural change, the second part is frequency tripled and focused on to a photocathode generating an ultrafast electron packet via photoelectric effect. These electrons are then accelerated through a high voltage (typically through 30 keV, de Broglie wavelength = 0.07 Å) and diffracted from the sample.
The relative arrival time of the probing electron packet and the initiating laser pulse at the sample can be changed by changing the relative optical path-lengths of the two laser beams. Recording the diffraction pattern of the electron packet as a function of this time delay provides both the equilibrium structure and a movie of the structural evolution with sub-Angstrom spatial resolution (reaching ~0.001 Å level) and sub-picosecond temporal resolution.