E0040
Time-Resolved X-Ray Absorption Spectroscopy. Melanie
Saes, Wojciech Gawelda, Maik Keiser, Alexander Tarnovski, Christian Bressler,
Majed Chergui, Inst. de Physique de la Matière Condensée, Unive.
de Lausanne, CH-1015 Lausanne-Dorigny, Switzerland.
With the advent of femtosecond lasers in the 1980’s it
has become possible to follow chemical reactions on the time scale of nuclear
motion. However, optical pump-probe techniques are based on spectroscopic
observables, which can be inverted to structures in only very few cases. X-ray
absorption spectroscopy has the advantage of delivering structures via EXAFS
(extended x-ray absorption fine structure), but it also delivers electronic
structure via XANES (X-ray absorption near-edge structure). This last point is
important as in many cases it is the electronic changes which drive chemical
reactions. We will illustrate the extraction of structural and dynamical
information by laser-only methods on condensed phase molecular systems that are
studied in our labs. This will serve to stress the advantages and limitations of
such methods, and therefore the need to use x-rays.
We have developed a new tool to study the electronic and
structural changes in light-driven processes based on picosecond time-resolved
X-ray absorption (XAS), in which an ultrashort laser pulse excites the molecule
and a picosecond X-ray pulse from a synchrotron probes the light-induced
changes. Crucial to this experiment is the reduction of noise, which we will
present. Then we will present the first X-ray absorption data on the transient
species [Ru(bpy)2(bpy)-]2+ at the Ru
L2 and L3-edges, which indicate the oxidation state change
of Ru(II) to Ru(III) following photo-excitation of aqueous
[Ru(bpy)3]2+. The kinetics of the excited Ru-complex will
be discussed and the feasibility of future time-resolved absorption measurements
with this setup evaluated.
This work was funded by the Swiss National Science Foundation
via Contracts No. 2000-059146.99 and No.620-66145.01, by the Swiss Light Source
(Villigen) and the Advanced Light Source (Berkeley).