Physica Scripta. Vol. T115, 102–106, 2005 PicosecondTime-Resolved X-Ray Absorption Spectroscopy of Solvated Organometallic Complexes W. Gawelda 1 , C. Bressler 1,* , M. Saes 1,2 , M. Kaiser 1 , A. N. Tarnovsky 1 , D. Grolimund 2 , S. L. Johnson 2 , R. Abela 2 and M. Chergui 1 1 Laboratoire de Spectroscopie ultrarapide, ISIC-FSB-BSP, Ecole Polytechnique F´ ed´ erale de Lausanne, CH-1015 Lausanne, Switzerland 2 Swiss Light Source, Paul-Scherrer Institut PSI, CH-5232 Villigen, Switzerland Received June 26, 2003; accepted February 10, 2004 pacs numbers: 78.47.+p, 78.70.Dm, 61.10.Ht, 82.20.−w, 82.50.kx Abstract We describe an experimental setup, which measures transient chemical changes of photoexcited solutes in disordered systems via time-resolved X-ray Absorption Spectroscopy (XAS) with picosecond temporal resolution. The setup combines a femtosecond amplified laser with picosecond X-ray pulses at beamline 5.3.1 at the Advanced Light Source in Berkeley, USA. New results on time-resolved XAS used for probing both the electronic and the geometric modifications of a photoexcited tris-(2,2 ′ -bipyridine) ruthenium (II), [Ru II (bpy) 3 ] 2+ in water solution are presented. 1. Introduction With the advent of time-resolved optical spectroscopy and the rapid development of femtosecond lasers, it has become possible to monitor the transient chemical changes occurring during an ongoing chemical reaction on fundamental timescales, which range from femtoseconds for molecular vibrational motions up to nanoseconds and microseconds for slower diffusion- controlled processes. In addition, since all chemical reactions take place on the atomic level, the complete study of transient chemical structures requires both ultrafast temporal resolution and atomic-level spatial resolution, usually in the picometer range. Femtosecond laser spectroscopy provides the necessary time- resolution to observe fully the photoinduced time evolution of a chemical system under investigation however it is less sensitive in detecting transient molecular structures. On the other hand, there are many well-established structure- sensitive experimental techniques, such as Electron Diffraction, X-ray Absorption Spectroscopy (XAS) and X-ray or Neutron Diffraction, which are useful in determining the structures of complex many-body systems with atomic-level spatial resolution. Therefore, combining the advantages of ultrahigh atomic-level resolution and ultrafast temporal resolution of these techniques, one could visualize nuclear and electronic motions during the chemical reaction and capture the transient molecular structures. Among these structural tools, XAS offers distinct advantages, when applied to chemical and biological systems in the condensed phase. It is element specific and allows the measurement of the local electronic structure of an atom of interest in a disordered medium, such as a liquid, by X-ray Absorption Near Edge Structure (XANES) and the local geometric structure, including nearest neighbor distribution and relevant bond distances, by Extended X-ray Absorption Fine Structure (EXAFS). Ultrafast Time-Resolved X-Ray Absorption Spectroscopy is a novel experimental technique which has not found wide-spread use yet. Although this scheme was originally proposed many * E-mail: Christian.Bressler@epfl.ch years ago [1–3], up to now, only a few time-resolved XAS measurements were carried out successfully, both in liquid [4–9] and gas phase systems [2] with nanosecond or longer temporal resolution. Only recently, Chen et al. [10] carried out a sub- nanosecond measurement of structural dynamics of an excited Copper (I) Diimine complex using both XANES and EXAFS and we reported on picosecond time-resolved XANES studies of a Ruthenium (II) tris-bipyridine complex in water solution reaching the sub-100 picosecond pulse-limited time resolution [11]. In our laser-pump X-ray-probe experiment, a femtosecond laser pulse starts a chemical reaction and a delayed X-ray pulse probes the photoinduced changes in a system. In full analogy to a laser-only experiment, by scanning the time delay between pump and probe pulse one can observe the time evolution of the transient chemical species. Synchrotrons are currently the brightest and the most stable pulsed X-ray sources. Moreover, they produce radiation that can be tuned over an extremely broad range up to the hard X-ray regime, e.g. hundreds ofkeV, however, the delivered X-ray pulse widths are usually limited to the tens of ps range, except for recent breakthrough experiments carried out by Schoenlein et al., where a successful experimental scheme providing femtosecond synchrotron pulses was realized [12]. The feasibility of time resolved XAS depends on the number of available X-ray photons per data point. This depends on the X- ray pulse intensity and on the repetition rate of the measurement, which is typically in the 1 kHz range, when using femtosecond amplified lasers. On the other hand, synchrotrons operate at much higher repetition rate of usually 100–500 MHz, which means that after synchronization of both sources we reduce the available X- ray photon flux by more than 5 orders of magnitude. Therefore, these experiments require a very sensitive detection scheme given that the X-ray pulse intensity at 3rd generation X-ray beamlines is rather weak, i.e. 10 4 –10 6 photons/pulse. Furthermore, there are other experimental challenges imposed by the nature of the experiment itself and a careful optimization of the sample geometry is required. The details on both sample and detection scheme optimization can be found elsewhere [13]. 2. Experimental Setup The experiments are carried out at bend magnet beamline 5.3.1 of the Advanced Light Source in Berkeley, USA, which operates between 2.5 and 12keV. We use a special filling pattern of the storage ring called camshaft mode (Fig. 1). It consists of a close-packed multibunch train followed by a 100ns empty section, in which a ten-fold more intense single electron bunch is placed, whose radiation we use in the pump-probe measurement. The beamline provides a hard X-ray photon flux of about Physica Scripta T115 C  Physica Scripta 2005