SOLVATION DYNAMICS USING ULTRAFAST X-RAY ABSORPTION SPECTROSCOPY

摘要

Solvation dynamics aims to incorporate the active role of the solvent in chemical and biochemical reactions and its relevance stems from the fact that most of chemistry and biology takes place in liquids. A full description of the electronic and molecular changes associated with these reactions includes time-resolved structural information, which can be obtained using transient X-ray absorption spectroscopy. Carrying out optical pump and X-ray probe experiments using a synchrotron X-ray source involves the drawback that time resolution is limited by the shortest achievable particle bunch length and therefore lies in the picosecond time domain. However, it can be considerably improved applying the slicing scheme where a femtosecond laser pulse is co-propagated with an electron bunch such that the energy of the electrons within the spatially overlapping region will be modulated due to their interaction with the laser field. Suitable X-ray optics subsequently allows for the extraction of femtosecond X-ray pulses but on the other hand roughly the same factor gained in X-ray pulse duration is lost in flux, which increases the integration time significantly. A feasibility study for a particular system should include a realistic estimation of the expected integration time. This can be done by calculating the signal to noise ratio S/AS in dependence of the total number of necessary X-ray photons which in turn can be related to the integration time. Neglecting any pump-pulse-induced solvent or counter-ion excitation the transient signal can be written in a simple form and its noise can be estimated applying error propagation on S(I~p; l~(unp)) where the errors on the pumped and unpumped intensity values can be determined in the shot-noise limit. The sample specific influence on the integration time shows up in the fraction of molecules that can be excited during the pump process.