Abstract:

The introduction of pump-probe techniques to the field of X-ray absorption spectroscopy (XAS) has allowed the monitoring of both electronic and structural evolution between intermediates of photoreaction systems with unprecedented accuracy, both in time and in space. In this talk, I will at first present the TR-XAS technique at APS sector 11-ID-D, which has been developed in the ultrafast laser pump-synchrotron X-ray probe configuration. Then, I will present the application of our TR-XAS technique to measure two representative photoreaction sample systems. In the solution case of spin-crossover (low spin (LS) ↔ high spin (HS)) Fe monomer [1], we have precisely determined the light-induced Fe-N bond length change from LS to HS states in two different solvents. In the suspension example of lead-free perovskite nanocrystals [2], we have found that upon photoexcitation, the electron is delocalized but the hole is localized at Br atom via likely forming Br2 dimer as Vk center. At the same time, the hole localization results in a local structural distorted state, which is long-lived (~ 60 µs) compared to the recombination of charge carriers (~ 20 ns). Additionally, I will present our recent preliminary TR-XAS results of 2D and 3D lead perovskite thin films from our new home-built sample platform [3].

[1] C. Liu et al., J. Am. Chem. Soc., 139, 17518 (2017).   
[2] C. Liu et al., J. Am. Chem. Soc., 141, 13074 (2019).
[3] C. Liu et al., To be submitted.