|People involved:||Wilfred Fullagar, Villy Sundström, Dharmalingam Kurunthu, Ujjwal Mandal, Fredrik Parnefjord|
|Former members:||Jens Uhlig, Monika Walczak, Sophie Canton, Robert Smith, Niklas Gador|
|Involved facilities:||Laser plasma source, Synchrotron facilities|
This technique has the following projects (and possibly other techniques) related to it:
What knowledge is needed to characterize function of a biomolecule, molecular material or a chemical reaction mechanism? Structure has long been considered to be the key information and several powerful techniques like X-ray crystallography and multidimensional NMR have been developed, following the notion that "seeing is believing", to provide equilibrium (static) structures of molecules. At the same time function of a molecular system implies change of structure and in order to characterize and understand how a biomolecule or material works it is necessary to know how and why the structural changes occur. The How is related to finding out precisely which structural changes occur, which atoms are involved, how are they affected, which bonds are broken, how does energy and charge flow through the molecule, and what are the temporal characteristics of the changes. The Why is associated with energetics and interactions; what is the energy landscape that connects reactants, intermediates and products and how do molecules interact with each other and with their environment? Ultimate understanding of a process enables the control of it. The wish to advance our understanding of molecular systems from equilibrium structures to function and eventually to enable reaction control, can be seen to require development of experimental techniques that resolve structural dynamics.
Distances between chemically bonded atoms are ~10-10 m. The wavelength of visible light (~10-6 m) is extremely large in comparison. When a visible light wave interacts with two bonded atoms, each atom sees approximately the same value of lights electrical field, so that light waves scattered from the two atoms have almost the same phase. The result is that scattered visible light cannot be directly used to determine the spatial relationship of two bonded atoms. This situation changes when the wavelength becomes similar to the distance between objects. In addition to X-rays, the de Broglie relationship between momentum and wavelength (p=h/λ) indicates that any moving particle has a corresponding wavelength. Electrons, neutrons, other subatomic particles and even entire molecules can be given a speed such that their de Broglie wavelength is approximately the distance between atoms. The nature of the particle-sample interaction differs in each case, but de Broglie wavelengths comparable to inter atomic distances can be exploited for chemical structure determination, this being the basis for electron and neutron structure techniques.
For massive particles, momentum and energy changes in a scattering sample can be revealed by angularly resolved time of flight techniques; additional opportunities exist if the particle is charged. Elastic momentum changes reveal molecular structure, while inelastic scattering can reveal molecular dynamics. For photons (eg X-rays) which have no mass or charge, knowing the momentum change requires knowledge of the photon energy (Q = 4p/l.sinq; l relates to photon energy via E = hc/l). So, attention has been given to acquisition of X-ray detectors with prospects of spatially resolved and energy resolved individual photon scattering measurements. Such detectors open the way to efficient in-house molecular measurements using ultrabrief but weak polychromatic X-ray sources in pump-probe setups.
The "hot warm cold" logo comes from the combination of a hot source and a cold detector to study matter at intermediate temperatures. The plot below shows some representative Planck blackbody radiation curves (from "Hot-Warm-Cold" Chemical Physics seminar, May 9 2008).
An in-house approach to ultrafast X-ray absorption spectroscopy developed in this group has used the flow of broadband X-ray photons from a suitably hot source. This is spectrally modulated by a sample. Modulations contain information about the scattering of electron de Broglie waves that are stimulated within the sample, which in turn relates to its molecular structure. Electron scattering is fast at the atomic level, so that molecular dynamics measurements are feasible in conventional pump-probe laser setups. A detector collects photons with very high efficiency and, if sufficiently cold, can have enough X-ray spectral resolution to measure the modulations.
The life of a photon can reveal much in this "hot-warm-cold" scheme. Statistical observation of quantised thermal flow is an approach that can expose molecular dynamics. It complements (reverses) Boltzmann's statistical molecular dynamics approach to thermodynamics.
A collection of apparatus used during source explorations (2004-2008).
General overview and motivation for X-ray based measurements (This page)
X-ray absorption spectroscopy
X-ray Diffraction and the argumentation for broad bandwidth
Overview and motivation for ultrafast X-ray measurements
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