BROAD ENERGY BAND-PASS TECHNIQUES FOR PERTURBATION CRYSTALLOGRAPHY; FIRST RESULTS.

H. Graafsma¹, G.W.J.C. Heunen¹, A. Puig-Molina¹, S. Scheres³, C. Shulze² and D. Bourgeois¹

¹ European Synchrotron Radiation Facility, BP220, Grenoble, France;
² Swiss Synchrotron Light Source, PSI Switserland;
³ Bijvoet Center for Biomolecular Research, Utrecht University, the Netherlands.

One of the most exciting developments in X-ray diffraction crystallography are in the area of time resolved and perturbation diffraction studies. With the development of powerful synchrotron sources combined with new detector technologies, new and unprecedented opportunities exists to study not just the static structures of important materials but also to study these materials in action.

We have developed techniques where use is made of a broad energy band-pass beam to study the influence of an external electric field on the atomic structure of AgGaS₂ and LiNbO₃. The use of a broad energy band-pass beam (or pink beam) greatly increases the data collection speed compared to standard monochromatic techniques, while giving a much lower x-ray background than standard white-beam (or Laue) techniques. The broad energy band was generated in various ways.

The use of a graded multi layer results in an energy bandwidth of 1.4 % with a Gaussian shaped intensity distribution. The multi layer is extremely easy to use and to align, the Gaussian intensity distribution, however, makes it less suitable for perturbation studies. Details and results will be presented.

A combination of a bent Laue crystal with bending magnet or wiggler radiation can produce a beam with 4% energy bandwidth and a perfectly flat intensity distribution. The characteristics of the produced beam are ideal for perturbation or time-resolved studies. A disadvantage is the alignment procedure, which is delicate and critical. Also not all photons of the incoming beam are focussed onto the sample. Details of the set-up and first results of a study of the influence of an electric field on AgGaS₂ will presented.

For both the graded multi-layer and the bent Laue crystal a zero-dimensional high count-rate Germanium detector was used in combination with a digital lock-in amplifier.

As third possibility a combination of a total reflecting X-ray mirror for the high energy cut-off and Cu-absorbers for the low energy cut-off was tested, using a CCD detector coupled to an X-ray image intensifier. The set-up is easy to install and use, but has a low efficiency, greatly reducing the available flux. This set-up was tested using a LiNbO₃ crystal in an external electric field. Details and result will be presented.