| People involved: | Wilfred Fullagar |
| Former members: | Sophie Canton, Jens Uhlig, Monika Walczak, Niklas Gador, Erhan Cengiz |
This project is related to the following Fields, Subjects and Techniques:
Time resolved structural measurements lead to an intimate understanding of the processes occuring in photoexcited systems, which typically take place in the subpicosecond time regime. To catch a snapshot of molecules undergoing such fundamental processes, correspondingly short pulses of radiation are needed. A short pulse of light is an effective way to trigger many fundamental chemical processes. After a suitable delay the system can be interrogated using a second pulse, which may be a very different form of radiation. Structural measurements require that this second pulse has a wavelength comparable to bond lengths. Elsewhere we motivate the use of polychromatic X-rays (or through them, electrons in EXAFS experiments). Measuring at different delay times allows reconstruction of the evolution of the molecular structure following the first pulse.
There are numerous approaches to the production of subpicosecond bursts of X-ray radiation. The common feature is the acceleration or deceleration of charged particles, in most cases electrons. At sub-relativistic energies, ultrashort laser pulses can be used to generate hot electrons with sufficient energy for bremsstrahlung X-ray production. High Harmonic Generation can also be considered as a form of coherent bremsstrahlung radiation, capable of delivering collimated sub-femtosecond X-ray pulses. When the charged particles are moving at relativistic velocities, there is usually a corresponding collimation of the radiation they produce. Among the possibilities to generate X-ray pulses are bend magnet/wiggler radiation, undulator/free electron laser radiation, Smith-Purcell radiation, transition radiation, parametric X-radiation, channel radiation, Thomson scattering and reverse Compton scattering. To a large extent, the characteristics of the electron beam dictate the properties of the resulting X-rays. The necessary relativistic electron beams are conventionally generated using linear accelerators in large scale facilities. However, entirely laser-based plasma wakefield acceleration approaches have been established in recent years. The latter approaches allow far more compact and affordable apparatus in considerably more widespread facilities. Betatron oscillations of electrons in the latter schemes also leads to ultrashort X-ray pulses.
Several of these X-ray generation approaches are being pursued in Lund and we are very much involved in the associated developments, especially in the context of their potential for ultrafast pump-probe Laue crystallography and EXAFS. Meanwhile, the main focus of our work is a small tabletop X-ray source based on a moderate power femtosecond laser. Detectors play a novel and critical role in the practical use of this source, which we believe we are the first to propose. Laser requirements for this approach are not particularly demanding, such that suitable systems are already in widespread use in most laboratories undertaking ultrafast optical measurements. One foreseeable arrangement for pump-probe EXAFS measurements using this apparatus is shown below.
General overview and motivation for X-ray based measurements
X-ray absorption spectroscopy
X-ray Diffraction and the argumentation for broad bandwidth
Overview and motivation for ultrafast X-ray measurements (This page)
Developments done on ultrafast X-ray sources in Lund
Developments on X-ray Detectors
Measurements done on ultrafast synchrotron user facilities
Steady state measurements done on synchrotrons