Ultrafast Optical and X-ray Absorption Studies of Solvation Dynamics

Solvation dynamics, the process of solvent reorganization upon electronic excitation of a solute, is central to our understanding chemical reactions in liquid phase. Ultrafast optical studies of solvation dynamics have so far been carried out on polyatomic molecules, which have internal degrees of freedom. This property does not allow the unambiguous extraction dynamics of the solvent shell. Because of their atomic character (i.e. lacking internal degrees of freedom) and of their solvent sensitive absorption bands (the so-called CTTS or charge-transfer-to-solvent bands), atomic halides represent ideal systems for the study of electronic solvation dynamics. Although these systems have received detailed attention in femtosecond optical studies, very little has been learned about the response of the caging solvent, which results from the fact that optical tools do not extract structural movement in a direct fashion. In this thesis, we combined ultrafast laser and structure-sensitive X-ray spectroscopies to probe in real-time the formation and decay of a nascent iodine atom created by photodetachment of a valence electron from the parent iodide anion. Optical pump-probe experiments are used to assess the photoproduct concentrations on time scales ranging from femtoseconds to nanoseconds. We also carried out detailed optical studies employing 1- and multiphoton detachment of the valence 5p electron from iodide, and confirmed an ultrafast thermal heating of the entire sample on a ps time scale. Static L1 and L3-edge X-ray absorption spectra of aqueous iodide have been recorded and analysed in terms of simulations based on classical, quantum mechanical molecular mechanics (QMMM) and density functional theory (DFT) molecular dynamics. QMMM yields the solvent shell structure that best fits the EXAFS spectrum. Picosecond X-ray absorption near edge structure (XANES) spectra were recorded from 50 ps up to several tens of nanoseconds. They were analyzed with respect to the different photoproducts observed on these time scales, delivering for the first time new spectra for the intermediate reaction products, I0 and I-2. By analyzing the transient extended x-ray absorption fine structure (EXAFS) data of the iodine atoms, we derived a dramatic expansion of ∼0.6 Å of the solvent shell with respect to that of iodide. Femtosecond XANES studies reveal an increase in the binding energies of the 5p and 2s, with respect to those at 50 ps. An increase in the 2s →5 p transition probability is consistent with the increase of the ionization energy of 2s electron of iodine atom at 300 fs, as compared to 50 ps. The 2s →5 p transition probability is found to decrease by ∼1.1 times from 300 fs to 50 ps.

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