Molecular Dynamics Probed by Short X-ray Pulses from a Synchrotron

The intensity of single X-ray pulses from third generation synchrotrons like the ESRF, APS and Spring8 is now so high that it is feasible to conduct time resolved experiment with diffraction, scattering and spectroscopic techniques on samples ranging from small molecules to proteins, with 100 ps time resolution. This limit is dictated by the X-ray pulse length from a synchrotron. In a time-resolved X-ray experiment one first needs to initiate the process of interest in the sample and then open the X-ray shutter and record the signal as a function of time. In the experiments shown in this review, the signals are recorded on a large CCD detector in order to record the signal efficiently in space. Unfortunately, the time-resolution of current CCDs is still too slow, 100 μs at best, to be useful for monitoring molecular processes on molecular time scales. The only way to explore faster phenomena is to use the pump-probe method where the time resolution is obtained from varying the pump-probe delay. In the examples discussed here, the samples were excited by picosecond or nanosecond laser pulses and the excited samples were probed by 100 ps X-ray pulses from an undulator.