New Scientific Opportunities at the European Synchrotron Radiation ...

European Synchrotron Radiation Facility Long-Term Strategy 7 July 2006
world-leading investigations exploit the time structure (down to ~100 picoseconds) of
special storage ring operation modes. These studies will explode in importance when
the new XFELs become available, as demonstrated by the ground breaking work at
the TTF2 facility at DESY, in the VUV, and in the X-ray region at the SPPS at SLAC.
Instrument optimisation and new beamlines at the ESRF will permit both diffraction
and spectroscopic investigations, setting the scene for sub-picosecond XFEL science
in the decades after 2010.
Picosecond time-resolved diffraction has proved to be a very powerful tool to track
the structure of molecules as they change shape and composition during a chemical
reaction. These experiments are performed in a pump and probe fashion:
femtosecond laser pulses excite a sub-set of molecules in the sample, and delayed
X-ray pulses probe the structure at a given delay. The diffraction patterns are
recorded on a CCD detector and by varying the time delay, from 100 picoseconds to
milliseconds, a real-space film can be made by Fourier inverting the diffraction
patterns. Laue diffraction studies are usually conducted in the 4-bunch mode of
storage ring operation, which gives the highest number of photons in a single shot.
The best example is the recording of the dissociation of CO from the myoglobin
complex MbCO. One of the key questions to address in the next few years is the role
of water molecules in protein dynamics. Indeed, new experiments have been able to
resolve water motions in certain proteins.
Solution Phase X-ray Diffraction (SPXD) is another field where the ESRF has made
important contributions. These experiments use a moving sample, which makes it
possible to study irreversible reactions at high frequency, ~1000 Hz. Using the
intense pink beam from a single harmonic of a short period in-vacuum undulator it is
now possible to acquire diffraction spectra, with 100 ps time resolution, at a rate of 20
per hour. The spectra measure the laser-induced change in atom-atom correlations
during a chemical reaction. The experiments have shown the signatures of the
formation and breakage of bonds, geminated and non-geminated recombination of
products, the solvent structure around the products and the hydrodynamics of the
solvent medium (i.e. the change in temperature, pressure and density). These
experiments are analysed using molecular dynamics simulations (MD), classical
hydrodynamics and statistical theory of liquids.
Interest is turning towards larger and more complex molecules. The recent results
obtained for the allosteric transition of haemoglobin show that studies can be
extended to macromolecular assemblies. There are strong indications that protein
dynamics (folding/unfolding) can be studied in the natural environment in this way.
Nano-scale crystals should also be mentioned, where plasmons can be excited by
laser pulses. The electron energy is transported, via the electron-phonon interaction
to the lattice, and coherent vibrations have been observed in powder diffraction. Very
small nano-particles with diameters down to 5 nm are also likely to be paarticularly
interesting as an intermediate between a molecule and a crystal (giving rise to Debye
scattering rather than crystal diffraction). The physics of such many-body molecules
will undoubtedly become a fascinating area of research.
An increased effort must be made to further develop time-resolved sub-nanosecond
X-ray scattering and spectroscopy through beamline development and by exploiting
new storage ring operating modes to provide new possibilities for these techniques.
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