Project |01 Development of a new UEM
Two different modes of operation are emerging in time-resolved TEM: one utilizing multiple femtosecond pulses to stroboscopically build up an image (“single-electron” mode) and the other a single-pulse method whereby a single nanosecond pulse of many electrons is used to record a single image (and diffraction). A resolution approaching ≤1 nm (≥10 nm in the single-pulse mode) is now possible with the stroboscopic method.
Yet, in the “single-electron” mode there exists a missing dynamic window from 100 ps to 20 ns, an essential time scale for the structural dynamics of matter. In the “single-pulse” mode, the spatial resolution of ≥10 nm is still not enough to take a movie of macromolecular structures. Most of these result from the inherent space-charge effect in the dense packet of photoelectrons and insufficient excitation energy available in the up-to-date version of UEM. To circumvent the limitations, the approach should follow either the new development of an instrument with dedicated electron optics from the ground up or the manipulation of driving optical pulses to generate space-charge free electron packets. Here, we are after the latter approach that will be the most cost-effective, risk-free, and the less time-consuming. The new UEM (n-UEM) in the ULSaN lab will be built with combining the two extreme capabilities of ultrafast stroboscopic imaging for reversible phenomena in UEM-2 and single-shot imaging for irreversible materials dynamics in DTEM with unprecedented spatiotemporal resolutions .
Kwon, Mohammed Phys. Chem. Chem. Phys. 2012, 14, 8974
Kwon, Zewail Proc. Natl. Acad. Sci. USA 2007, 104, 8703
Project |02
Project |02 Femtochemistry
The fundamental timescale of molecular motions lies in femtosecond time regime. With the aid of femtosecond-resolved optical spectroscopic methods, we are going to investigate elementary chemical and biophysical dynamics. To do so, we will set up a robust fluorescence upconversion spectrometer and a time-correlated single photon counter, for which the femtosecond laser system coupled to the n-UEM is shared as a common light source. The combination of the two spectrometers will allow us to watch the evolution of chemical reactions from the early moment of atomic motions on the timescale of femtoseconds to nano-to-microseconds when the reactions occur over high activation barriers.
The origianl image was taken from the Zhong group at the Ohio State University
Zhong, Pal, Zewail Chem. Phys. Lett. 2011, 503, 1
The origianl image was taken from the Zhong group at the Ohio State University
Project |03
Project |03 Femtobiology
One of key elements in chemistry and biology is water. There are growing lines of evidence indicating the importance of water dynamics in the function of biological architectures. Yet, molecular mechanisms describing how water exerts these biological activities are still missing in a physical chemist’s perspective. In this project, we seek to address this issue by characterizing the transient properties of biomolecule-associated water (i.e. biological water) in real time invoking the ultrafast spectroscopy. We plan to develop methodologies that monitor the energetics of hydrogen-bondings among biological water molecules and track their in situ properties during the conformational changes of biological structures.
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In the past two decades we have witnessed the impact of femtochemistry that probes time-dependent electronic or vibrational transition energies of chemical species in ultrafast reactions with the atomistic time resolution of femtoseconds. However, the spectroscopic information attained is indirect and intuitive structural information of transient species has been the subject of frontier researches in the new millennium.
The recent emergence of 4D electron microscopy, which uniquely combines the ultrafast time resolution of femtochemistry and ultrafine spatial resolution of TEM, has opened a new era for direct visualization of atomic and molecular motions of matter. Yet, because of limited function, observations have been made to periodic (crystalline) atomic and molecular architectures and non-periodic (amorphous) objects of hundreds nanometers or bigger. In the ULSaN lab, we develop an advanced ultrafast electron microscope with invulnerable capabilities that can circumvent present limitations and promote the new methodology to a powerful platform to directly image molecular and collective motions, dissect fundamental phenomena, and deliver new concepts for specific control and global function of matter.