W0091

Undulators at high brilliance synchrotron x-ray facilities such as the Advanced Photon Source (APS) provide intense highly collimated x-ray beams which, when coupled with pump-probe crystallographic techniques, open new possibilities to study transient chemical phenomena. The high spectral brilliance of the source results in a very high photon flux density at the sample, enabling high-resolution monochromatic time-resolved crystallographic (TRC) measurements in the microsecond time domain. ChemMatCARS, a National Synchrotron Resource for Chemistry and Materials Science, is presently commissioning an undulator-based TRC capability at the APS for applications in small molecule crystallography. In the time resolved experiments performed at the facility, two CCD frames are taken consecutively covering the same angular range. The first frame is taken with the pump-laser on while the second is taken under dark conditions. The procedure continues much like a standard crystallography measurement with the difference that two sets of data are collected for a given sample. The relative change in integrated intensity of each of the reflections is used to refine the change in molecular geometry upon excitation. The heart of the system is a mechanical x-ray beam chopper¹ consisting of a brass wheel with slots machined around its circumference. The wheel is spun at a high rate and depending on its rotational speed and slot size, will produce differing x-ray-beam frequencies and pulse widths. The temporal width of the chopped x-ray pulse and the repeat frequency produced by the wheel are tailored to the lifetime of the molecule under study. A description of the facility and its capabilities will be presented as well as plans for the future.