Perspectives for XPCS at the ERL Source
Anders Madsen
European Synchrotron Radiation Facility
In my talk I will present examples of X-ray Photon Correlation Spectroscopy (XPCS) measurements performed at the Troika beamline, ID10A of the ESRF and will discuss the limits of what is currently feasible. XPCS is a scattering technique which relies on the coherence properties of radiation generated by high-brilliance 3rd generation synchrotron sources. Presently XPCS is a signal-to-noise limited technique meaning that the source, the optics, the detector, and also the sample set the limits for the Q-range over which the correlation function can be measured. Hence nowadays XPCS is mostly performed on systems that scatter strongly, i.e. in the vicinity of Bragg points (or rods), or in the forward direction (SAXS) from disordered systems consisting of larger objects. In the case of weakly scattering samples, only slow dynamics can be characterized because their study necessitates the use of a (slow) CCD detector. In my talk I will discuss some of the limiting factors we encounter today.
For XPCS at the ERL I propose to study the Q-dependent a and b relaxations in a molecular liquid glass former at inter- and intra molecular distances (0.1-1000 nm). In the bulk liquid state such samples are perfectly disordered with only structural correlations seen at for instance the characteristic C-C distance etc. This doesn’t change a lot upon approaching Tg because the glass transition is a dynamic transition with only little or no effect on the static behavior. Mesoscopic heterogeneities have been observed in some cases upon approaching the glassy state with typical domain size ranging from 5-1000 nm, but the electronic contrast of these domains is small, and their origin remains a controversial issue. The dynamics of glass forming liquids are often studied in the frequency domain (by inelastic scattering techniques) where slow (>~0.1ns) dynamics cannot be resolved. Hence for many systems it is not possible today to measure the Q-behavior of the a-relaxation near Tg with existing XPCS set-ups. Alternatively, such glass formers are studied by techniques without Q-resolution (dielectric spectroscopy, NMR etc.) where a lot of information is lost. In my opinion this is a widely unexplored and interesting field, and here XPCS measurements could bring new experimental evidence, for instance to test some of the predictions of mode coupling theories.
