Tracking Atomic Positions in Molecular Reactions by Picosecond X-ray Scattering at the ESRF

Abstract

The enormous gain in intensity from undulators has led to an equivalent gain in time resolution and some techniques can now be performed routinely at synchrotrons at a time resolution of 100 picoseconds, a limit set by the X-ray pulse length. In the simplest time-resolved experiment, the evolution of a process is recorded by the detector just as a video camera records a football match. In diffraction and scattering experiments, however, the time-resolution of large-area CCD detectors, which are essential for capturing a large fraction of the signal, is milliseconds at best. This is significantly longer than the fundamental times in physics, chemistry, and biology: the femto-, pico-, and nanosecond time scales. The pump-probe method is often used to improve the time resolution in diffraction and scattering experiments at the expense of much longer acquisition times. The samples are usually initiated by short laser pulses and the ensuing structural evolution recorded with “flashes” of X-rays at a given delay. By measuring many delays, the snapshots can be stitched together into a film. The highest time resolution from a synchrotron is obtained when the X-ray flash is generated by a single bunch of electrons. For those experiments that can use the pink beam, Laue diffraction for example, high-quality signals can sometimes be recorded in one pulse, as shown in Figure 1.