Toward time-resolved laser T-jump/X-ray probe spectroscopy in aqueous solutions

Abstract

Most chemical and biochemical reactions in nature and in industrial processes are driven by thermal effects that bring the reactants above the energy barrier for reaction. In aqueous solutions, this process can also be triggered by the laser driven temperature jump (T-jump) method, in which the water vibrational (stretch, bend, or combination) modes are excited by a short laser pulse, leading to a temperature increase in the irradiated volume within a few picoseconds. The combination of the laser T-jump with X-ray spectroscopic probes would add element-specificity as well as sensitivity to the structure, the oxidation state, and the spin state of the intermediates of reactions. Here, we present preliminary results of a near infrared pump/X-ray absorption spectroscopy probe to study the ligand exchange of an octahedral aqueous Cobalt complex, which is known to pass through intermediate steps yielding tetrahedral chlorinated as final species. The structural changes of the chemical reaction are monitored with great sensitivity, even in the presence of a mild local increase in temperature. This work opens perspectives for the study of non-light-driven reactions using time-resolved X-ray spectroscopic methods.

Enhancing the analysis of disorder in X-ray absorption spectra: application of deep neural networks to T-jump-X-ray probe experiments

“…We then apply our optimised DNN to assist with the interpretation of the T-jump pump/X-ray probe experiments of Chergui et al, described in ref. 24.…”

“…We now turn our attention to analysing the T-jump pump/ X-ray probe data obtained 7 ns after arrival of the NIR T-jump pump pulse; these data are described in ref. 24 and reproduced in Fig. 8.…”

“…Towards this goal, Chergui et al 24 recently used T-jump pump/X-ray probe experiments to investigate the NIR-driven ligand exchange processes of Co 2+ in chlorinated aqueous solution (Fig. 1).…”

“…d ' d) transition and, because the transient spectrum in ref. 24 was recorded 7 ns after the arrival of the T-jump pump pulse, it is likely that intramolecular relaxation of any electronically-excited complex would have already been complete, i.e. the excess electronic excitation energy would have already been thermalized as heat to the environment.…”

“…We demonstrate that our DNN is capable of predicting quantitatively the Co K-edge XANES spectra of arbitrary unseen/ 'out-of-sample' Co X-ray absorption sites. To trial our approach in a practical setting for the first time, we use our DNN concertedly alongside an ab initio MD ensemble strategy to obtain a deep insight into the T-jump pump/X-ray probe experiments of Chergui et al 24 on the ligand exchange processes of Co 2+ in chlorinated aqueous solution.…”