Research

Ultrafast optical transient absorption (OTA) spectroscopy is an interdisciplinary area of research that spans various disciplines in chemistry, materials science, physics and even biology. Its development based on femtosecond (10-15 second) lasers has enabled direct insight into the mechanistic and kinetic details of photoreaction events that occur in the timescale from femtosecond to picosecond. Experiments of this type exploit so-called pump–probe schemes. The photoreactions (e.g. transient non-equilibrium states) in the system under investigation are first initiated by an ultrafast laser pulse (pump) and then a probe laser pulse delayed with respect to the pump pulse is sent to trace the evolution of the system during the photoreactions. Laser optical probing is extremely sensitive to the changes in the spectrum of the elementary photo-excitations, which in particular gives the electronic information at the transient intermediate states of the photoexcited system. Therefore, ultrafast OTA spectroscopy has become a very efficient tool for the study of kinds of photoinduced reactions, like insulator-to-metal transitions, spin-crossover, charge carrier dynamics, exciton coherence and dephasing, high-temperature superconductivity, and charge/spin-density-wave ordering where the optical response of the material is drastically modified in the photoexcited state. While ultrafast OTA spectroscopy provides valuable information on the electronic (i.e. e - and h+ ) dynamics, without special configuration or experimental design, this technique can not easily ambiguously distinguish the contribution from different types of charge carriers (i.e. e - and h + ) or from different atomic centers. Neither can it track the optically dark states, referred to these do not possess the transition strengths over the laser spectrum range, such as the structural distorted states caused by the localization of charge carriers (namely, polaron formation). As an important complementation, time-resolved (TR) X-ray techniques based on laser pump-X-ray probe ( in particular, TR-X-ray diffraction (TR-XRD) and TR-X-ray absorption & fluorescence spectroscopy (TR-XAS &XRF) ) are the ideal tools to interrogate the electron and hole charge distribution and structural change inside the material upon photoexcitation due to its intrinsic element specific, oxidation state and local structure sensitive characteristics. By combing these laser and X-ray spectroscopic techniques by using ultrafast laser and X-ray sources, we can easily simultaneously identify the electronic (e.g. e - and h+ ) and structural (e.g. photo-induced local disorder phase) information at the transient states of the photoexcited material in real time. Also, these techniques are capable of visualizing the photo-induced structural evolution details at the atomic scale with femtosecond time solution, which can catch light-trigged “ultrafast transient atomic movement movies” in the material. Please find more details of laser pump-laser / X-ray probe techniques with the related publications as below.

Ultrafast OTA spectroscopy:

Laser pump-laser probe: studying the electronic dynamics (i.e. charge transfer, electron-hole relaxation/recombination, Auger process, carrier multiplication, etc.) of materials upon photoexcitation, in the timescale of femtosecond(fs)/picosecond (ps) to microsecond (us).

∆A(λ, t)=〖A(λ, t)〗_pump-〖A(λ, t)〗_unpump

∆T(λ, t)=〖T(λ, t)〗_pump-〖T(λ, t)〗_unpump

where A and T are light absorption and transmission, respectively.

[1] C. Liu, Exciton relaxation and electron transfer dynamics of semiconductor quantum dots. (Ph.D. Thesis at University of Rochester, 2015)

[2] C. Liu, J. J. Peterson, T. D. Krauss, Uncovering Hot Hole Dynamics in CdSe Nanocrystals. J. Phys. Chem. Lett. (2014).

[3] C. Liu, F. Qiu, J. J. Peterson, T. D. Krauss, Aqueous Photogeneration of H2 with CdSe Nanocrystals and Nickel Catalysts: Electron Transfer Dynamics. J. Phys. Chem. B (2015).

Ultrafast laser pumped X-ray techniques:

Laser pump-X-ray Probe: Time-resolved (TR) X-ray scattering and spectroscopy, such as TR-X-ray diffraction (XRD), TR-X-ray absorption (XAS including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)), TR-X-ray florescence (XRF), etc. : concurrently studying the electronic and structural dynamics of materials in the timescale of picosecond (ps) to millisecond (ms) with atomic-level resolution (Angstrom) as the charges including electron and hole are moving in the materials after photoexcitation.

∆XRD/XAS = XRD/XAS(laser-on) - XRD/XAS (laser-off)

[1] Y. Wang$, C. Liu$, X. Zhang*, et al. Visualizing Light-Induced Microstrain and Phase Transition in Lead-Free Perovskites using Time-Resolved X-Ray Diffraction. J. Am. Chem. Soc. (2022). ($ equally)

[2] C. Liu*, et al. Direct Spectroscopic Observation of the Hole Polaron in Lead Halide Perovskites. J. Phys. Chem. Lett. (2020).

[3] C. Liu, X. Zhang*, et al. Asynchronous photoexcited electronic and structural relaxation in lead-free perovskites. J. Am. Chem. Soc. (2019).

[4] C. Liu, X. Zhang*, et al. Probing the impact of solvation on photoexcited spin crossover complexes with high-precision x-ray transient absorption spectroscopy. J. Am. Chem. Soc. (2017).

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