Towards structural dynamics in condensed chemical systems exploiting ultrafast time-resolved x-ray absorption spectroscopy

We present the case for exploiting time-resolved x-ray absorption to study structural dynamics in the liquid phase. With this aim in mind and considering the large differences between absorption coefficients in the optical and the x-ray domains as well as the x-ray absorption cross sections due to unexcited species, we have estimated the anticipated signal-to-noise ratio $\mathrm{(S/N)}$ under realistic conditions with femtosecond laser pump pulses and synchrotron radiation x-ray probe pulses. As a model system, we examine ${\mathrm{I}}^{-}$ photodetachment in water and detect the appearance of laser-generated neutral I atoms by their x-ray near-edge absorption structure (XANES) and by their extended x-ray absorption fine structure (EXAFS). While the $\mathrm{S/N}$ ratio critically depends on the photolysis yield, which itself is governed by the optical absorption cross section, the optimum sample concentration varies in a complex fashion as a function of pump laser intensity and optical absorption cross section. However, concentrations yielding near total absorption of the pump laser deliver quite optimum $\mathrm{S/N}$ ratios. The calculations presented here provide guidelines for the implementation of time-resolved x-ray absorption experiments in condensed phase chemical systems.