Anisotropy enhanced X-ray scattering from solvated transition metal complexes

Abstract

Time-resolved X-ray scattering patterns from photoexcited molecules in solution are in many cases anisotropic at the ultrafast time scales accessible at X-ray free-electron lasers (XFELs). This anisotropy arises from the interaction of a linearly polarized UV-Vis pump laser pulse with the sample, which induces anisotropic structural changes that can be captured by femtosecond X-ray pulses. In this work, a method for quantitative analysis of the anisotropic scattering signal arising from an ensemble of molecules is described, and it is demonstrated how its use can enhance the structural sensitivity of the time-resolved X-ray scattering experiment. This method is applied on time-resolved X-ray scattering patterns measured upon photoexcitation of a solvated di-platinum complex at an XFEL, and the key parameters involved are explored. It is shown that a combined analysis of the anisotropic and isotropic difference scattering signals in this experiment allows a more precise determination of the main photoinduced structural change in the solute, i.e. the change in Pt-Pt bond length, and yields more information on the excitation channels than the analysis of the isotropic scattering only. Finally, it is discussed how the anisotropic transient response of the solvent can enable the determination of key experimental parameters such as the instrument response function.

Solution Structure and Ultrafast Vibrational Relaxation of the PtPOP Complex Revealed by ΔSCF-QM/MM Direct Dynamics Simulations

“…54,98 c Obtained in ethanol by X-ray absorption measurements. 53,97 98 As seen in Table 2, the Pt-Pt distance predicted by the ∆SCF-QM/MM BOMD simulations agrees to a very good extent with the experimental S1…”

Simulation of ultrafast excited-state dynamics and elastic x-ray scattering by quantum wavepacket dynamics

“…Although XDS obtained by ultrafast pump-probe experiments contains an anisotropic component as well, decomposition of the experimental signal into isotropic and anisotropic parts allows comparison to the signal simulated by the present approach. 47,48 Utilizing nuclear pair densities, the isotropic scattering signal is written as 49…”

Ultrafast X-Ray Scattering Measurements of Coherent Structural Dynamics on the Ground-State Potential Energy Surface of a Diplatinum Molecule

“…This is the case here, as the dσ * pσ absorption peak has a transition dipole moment aligned along the Pt-Pt axis along which the Pt nuclei contract following photoexcitation. The difference scattering signal from such a distribution of solute molecules is described by [47,48,50] ∆S(Q, t) = ∆S 0 (Q, t) + P 2 (cos(θ q ))∆S 2 (Q, t);…”

“…Assuming that the solute in each of the vibrational ensembles es and gs, hole can be represented by a single average structure, the isotropic ∆S 0 and anisotropic ∆S 2 parts of the solute contributions to the difference scattering signal are calculated from [49,50]:…”