Research

1. Ultrafast Structural Dynamics of Molecules

Investigating how atoms and molecules interact via chemical bonds to form new chemical compounds is fundamental to understand Chemistry. Past discoveries have laid the foundation for many practical applications such as creating new medicines, developing green materials and designing electronic devices. The ability to “watch” chemical reactions with atomic scale spatial resolution and sub-femtosecond temporal resolution can revolutionize our understanding of chemical transformations. This is expected to lead to new chemical reactions created through the control and manipulation of atoms and electrons in molecules on ultrafast time scales. Novel beam sources like x-ray free electron lasers (XFELs) have unprecedented brilliance, time resolution and tunability, offering new opportunities for observing chemical processes and exploring fundamental molecular properties that were hidden in the past.

We conduct ultrafast x-ray scattering experiments in gas phase and liquid phase using Synchrotrons/XFELs at national labs, aiming to track ultrafast quantum motions of nuclei and electrons in molecules in both space and time. We also collaborate with other teams in the world to conduct ultrafast electron scattering experiments using Megaelectronvolt Ultrafast Electron Diffraction (MeV-UED).

Ultrafast x-ray (top) and electron (bottom) scattering set-up. The reaction of molecule is initiated with a laser pump pulse, and the time-evolving molecular structure is probed by scattering using X-ray/electron probe pulses with a variable time delay. The scattering signals are recorded with a pixelated detector.

The Advanced Photon Source at Argonne National Laboratory

Linac Coherent Light Source at Slac National Accelerator Laboratory

2. Ultrafast Molecular Spectroscopy

We conduct ultrafast molecular spectroscopy including transient absorption spectroscopy, transient (magnetic) circular dichroism, time-resolved photoelectron imaging and photoelectron circular dichroism with our own table-top femtosecond laser systems and spectrometers. Ultrafast Spectroscopy and Scattering are complementary techniques. Scattering experiment is a direct probe of the molecular structure while Spectroscopy measures the energies of the molecule. Spectroscopic measurements thus can provide vital information about time-evolving electronic and vibrational states during chemical reaction dynamics.

One topic we are interested in is molecular chirality. It is an important molecular property that describes a molecule that cannot be superimposed on its mirror image by any translations or rotations. It plays an important role in many chemical processes such as biological activities and organocatalysis. Although two enantiomers of a chiral molecule have the same chemical properties, they show vastly different effects when reacting with other chiral molecules. Since only one of the two enantiomers typically exists in living organisms, one enantiomer can act as a healing drug while the other could be toxic to humans. Time-resolved circular dichroism will be applied to probe dynamical chirality. It measures the difference between the photoelectron angular distributions following photoionization by left circularly polarized and right circularly polarized light. It can thus provide sensitivity to the chirality of evolving system.

3. Theoretical Spectroscopy

We develop theoretical methods that can predict experimental signals accurately. Advanced theoretical tools for recovering desired molecular properties from measured data will be investigated. Molecular systems that are ideal for experiments will be suggested through quantum dynamical simulations.

We also design novel techniques that can observe new phenomena by manipulating attosecond electron dynamics, and develop new experimental concepts that can reveal chemical information unavailable otherwise.

We are developing vortex (twisted) x-ray diffraction as a promising imaging tool for unveiling molecular symmetry.

We proposed a new technique, vortex beam diffraction, that can directly image transient electronic coherences in molecules generated upon photoexcitation or near conical intersections.

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