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

The authors present the case for exploiting time-resolved x-ray absorption to study structural dynamics in the liq. phase. With this aim in mind and considering the large differences between absorption coeffs. in the optical and the x-ray domains as well as the x-ray absorption cross sections due to unexcited species, the authors have estd. the anticipated signal-to-noise ratio (S/N) under realistic conditions with femtosecond laser pump pulses and synchrotron radiation x-ray probe pulses. As a model system, the authors examine I- photodetachment in H2O 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 S/N ratio critically depends on the photolysis yield, which itself is governed by the optical absorption cross section, the optimum sample concn. varies in a complex fashion as a function of pump laser intensity and optical absorption cross section. However, concns. yielding near total absorption of the pump laser deliver quite optimum S/N ratios. The calcns. presented here provide guidelines for the implementation of time-resolved x-ray absorption expts. in condensed phase chem. systems. [on SciFinder (R)]