Maik Kaiser

A setup for ultrafast time-resolved x-ray absorption spectroscopy

We present a setup which allows the measurement of time-resolved x-ray absorption spectra with picosecond temporal resolution on liquid samples at the Advanced Light Source at Lawrence Berkeley National Laboratories. The temporal resolution is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the experiment and present a single-pulse detection scheme.

Ultrafast time‐resolved X‐ray absorption spectroscopy of chemical systems

X-ray absorption spectroscopy (XAS) is a well-established technique to probe the electronic and structural properties of a large class of systems, ranging from solid-state materials to proteins [1]. X-ray spectra are characterized by a series of edge features, which arise due to the extraction of a core electron from the inner K, L, M, etc. shells to the ionization limit. Zooming into one of the edges reveals a variety of fine structure near the edge, the so-called XANES or X-ray absorption near edge structure, while at higher energies above the edge the so-called EXAFS or extended X-ray absorption fine structure, can be observed. In many-body systems, these features are caused by the presence of neighboring atoms and lie in the fact that the photoelectron scatters off the neighboring atoms and the backscattered wave interferes with the outgoing wave. This interference gives rise to weak modulations of the otherwise smoothly varying absorption cross-section as a function of Xray energy. The proper analysis of these features delivers information about the local geometric structure to high accuracy. For studying chemical properties, XAS has the following advantages:

A fast X-ray chopper for single-bunch extraction at synchrotron sources

A new X-ray chopper system based on air-bearing technology was developed in a joint project between the Paul Scherrer Institut and DESY. The X-ray chopper wheel of 150 mm diameter has a slit width of 50 µm, which corresponds to an opening time of 230 ns at 30 000 r min−1. The wheel is equipped with either one or two slits at slightly different radii, resulting in repetition frequencies of 500 and 1000 Hz. To reduce air friction the wheel is operated in a reduced-pressure environment. The chopper is controlled by a specially developed phase lock loop controller, which is synchronized to the master bunch clock of the synchrotron. The very small and constant friction of the bearing and the high mechanical accuracy of all mechanical components in combination with the high-precision controller resulted in a very small long-term jitter of less than 3 ns. The chopper is capable of isolating X-rays generated from a single bunch of the future PETRA III storage ring at DESY operated in 40-bunch mode.

Ultrafast x-ray spectroscopy for structural dynamics studies in chemistry and biology

Time-resolved x-ray absorption fine structure (XAFS) spectroscopy with picosecond temporal resolution is a new method to observe electronic and geometric structures of short-lived reaction intermediates. It combines an intense femtosecond laser source synchronized to the x-ray pulses delivered into the microXAS beamline of the Swiss Light Source (SLS). We present key experiments on charge transfer reactions as well as spin-crossover processes in coordination chemistry compounds next to solvation dynamics studies of photogenerated atomic radicals.

Femtosecond X-ray science at the Swiss Light Source

unmounting and centering are all integrated into Blu-Ice through an intuitive custom robot tab; response over internet connections is reasonable from home (or café) wifi networks. Remote connection also allows data processing without the bottleneck of transferring the data home. Successful remote collection has enabled the MBC to institute an (almost) on-demand scheduling paradigm where members request beamtime as needed in blocks of time from 4 to 48 hours while beamline visits and Service Crystallography fill in the gaps. An MBC on-call list is available for beamtime to fill unused shifts and another ad-hoc beamtime request system is available for non-members. Remote collection has the benefit of easing access to the synchrotron beamline, encouraging the Mentor/Student relationship during data collection and provides a teaching platform that may otherwise be unavailable to crystallography labs.

Picosecond and femtosecond X-ray absorption studies of the photoinduced spin change in Fe complexes

We developed a combined imaging and dynamic light scattering (DLS) system for routine measurements in droplets of multi-well plates as well as in gel tubes used for protein crystallization. The system is of high value for rapid identification of good crystallization conditions. Today automated methods to crystallize macromolecules are widely used and can easily generate thousands of crystallization droplets. Nevertheless the evaluation of crystallization experiments to find optimal growth conditions remains a bottleneck. Therefore we have investigated methods to improve the process of evaluating results and finding crystal growth conditions. One method is DLS, the second is the use of combined white/UV illumination for determination of whether crystal-like objects are biomolecular and identification of crystals in crystallisation set-ups. Up to now, it has been impossible to determine the particle size directly in protein solution droplets because of size and configuration constraints. We have developed a CCD camera-based imaging instrument and combined a laser source and a detector to perform DLS measurements in situ. The plate-screening system allows to monitor and evaluate the entire crystallization process in an automated way. For example the stages of nucleation and the progress of crystal growth without disrupting the course of equilibration can be analyzed. The data provide information to understand in greater detail the process of crystal initiation and growth and will allow further optimisation, thereby leading to better crystals. Finally we will also describe a method to support the identification of protein crystals, exploiting the fact that most proteins and other biomolecules fluoresce when illuminated with UV light.

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.

Structural dynamics and electronic structure changes probed with lasers and X-rays

Several new results and opportunities for ultrafast x-ray absorption spectroscopy (XAS) are demonstrated. It is possible to work with highly dil. solns. in transmission mode without dramatic loss of signal-to-noise ratio. This is very promising as one can envision the study of samples, for which large concns. are impossible to reach. Moreover, it is possible to scan the time delay between the laser pump pulse and the x-ray probe pulse, and therefore follow the evolution of the system from the start. The operation of an optical x-ray cross-correlator is also demonstrated. The time resoln. is not a limiting factor and the expts. are feasible with sources of shorter x-ray pulses, provided the flux is not too low. [on SciFinder (R)]