Van Thai Pham

A high-repetition rate scheme for synchrotron-based picosecond laser pump/x-ray probe experiments on chemical and biological systems in solution

We present the extension of time-resolved optical pump/x-ray absorption spectroscopy (XAS) probe experiments towards data collection at MHz repetition rates. The use of a high-power picosecond laser operating at an integer fraction of the repetition rate of the storage ring allows exploitation of up to two orders of magnitude more x-ray photons than in previous schemes based on the use of kHz lasers. Consequently, we demonstrate an order of magnitude increase in the signal-to-noise of time-resolved XAS of molecular systems in solution. This makes it possible to investigate highly dilute samples at concentrations approaching physiological conditions for biological systems. The simplicity and compactness of the scheme allows for straightforward implementation at any synchrotron beamline and for a wide range of x-ray probe techniques, such as time-resolved diffraction or x-ray emission studies.

Structural Determination of a Photochemically Active Diplatinum Molecule by Time‐Resolved EXAFS Spectroscopy

Metallica: A large contraction of the Pt-Pt bond in the triplet excited state of the photoreactive [Pt(2)(P(2)O(5)H(2))(4)](4-) ion is determined by time-resolved X-ray absorption spectroscopy (see picture). The strengthening of the Pt-Pt interaction is accompanied by a weakening of the ligand coordination bonds, resulting in an elongation of the platinum-ligand bond that is determined for the first time.

Retrieving photochemically active structures by time-resolved EXAFS spectroscopy

Describing the nature and structure of molecular excited states is important in order to understand their chemical reactivity and role as intermediates in photochemical reactions. The recent implementation of x-ray absorption spectroscopy in the ultrafast time domain allows studying the electronic and structural dynamics of photochemically active molecules in solutions. In this work we present the structural determination of a photoexcited diplatinum molecule, [Pt2(P2O5H2)4]4-, which plays a photocatalytic role in important chemical conversions. A novel analysis of time-resolved EXAFS spectra based on the fitting of the experimental transients obtained from optical pump/x-ray probe experiments has been performed to derive a contraction of 0.31(5) A of the two Pt atoms and a ligand expansion of 0.010(6) A. The former is assigned to the formation of a transient Pt-Pt bond in the excited state, while the latter indicates a concomitant weakening of the Pt-ligand coordination bonds.

Probing the Transition from Hydrophilic to Hydrophobic Solvation with Atomic Scale Resolution

We use ultrafast x-ray absorption spectroscopy to determine the solvent structure change upon laser abstraction of the electron from I-. The transition from hydrophilic to hydrophobic occurs over 4 ps during which a transient I-OH2 species is formed.

Impurity-Localized Electronic Changes in Eu:SrGa_2S_4 Studied by Ultrafast X-Ray Absorption Spectroscopy

The time-dependent electronic structure of Eu:SrGa2S4 after excitation by a femtosecond laser pulse is investigated by time-resolved x-ray absorption spectroscopy. We observe a transient shift in the Eu L3 edge consistent with Eu ionization.