Time-resolved (TR) X-ray crystallography at advanced light sources is a frontier area bound to undergo dramatic new developments with the advent of ultrabright X-ray free-electron lasers (XFELs). Earlier TR studies of chemical systems in which atomic resolution was reached have used monochromatic radiation for which methods are well developed and a reasonable accuracy can be achieved (Coppens, Benedict et al.
; Cailleau et al.
). The use of monochromatic data avoids the wavelength dependence of both the scattering process and the detector response and does not suffer from the broadening of the reflection maxima inherent in the Laue technique. However, with a narrow bandwidth of monochromatic radiation only a very small fraction of the photons in the synchrotron beam are productively used, even at undulator-equipped beamlines. Thus a longer exposure time and therefore more pulses are required, which limits the time resolution that can be achieved. Furthermore, the longer exposure time implies more extensive laser exposure, thereby enhancing the temperature increase of the sample due to light absorption.
It follows that for picosecond-timescale TR diffraction at synchrotron sources the use of polychromatic radiation is imperative. To achieve this goal we have developed a number of methods to improve the accuracy and interpretation of Laue measurements. They include the RATIO technique in which the measured I
ratios are used in combination with a set of monochromatic data collected at the same temperature (Coppens et al.
), photo-Wilson plots to estimate the temperature increase due to the laser exposure of the sample (Schmøkel et al.
), the definition of R
factors specific for dynamic structure crystallography (Coppens, Kamiński & Schmøkel, 2010
), and a scaling technique for relative scaling within multi-crystal data sets collected on the same substance. The scaling is required for calculation of the photodifference maps using all available reflections. The technique is based on the moduli of the absolute fractional intensity change and is described in §2.3
. We describe a single-pulse Laue diffraction experiment of a binuclear RhI
complex in which the techniques summarized above have been applied and a comparison of the results with theoretical calculations. A preliminary account of the results of this study has been published (Benedict et al.