Instrumentation for photocrystallographic experiments of transient species.

Instrumentation for stroboscopic time-resolved diffraction studies at low temperatures is described. Exciting laser light is delivered to the crystal through an optical fiber. During the diffraction experiment, fluorescence from the sample is focused onto a fiber optic bundle surrounding the laser-light fiber, and monitored by a photodiode. A rotating slotted disk produces a pulsed X-ray beam with pulse frequencies suitable for the study of molecular excited states with lifetimes of 10 micros or longer. Synchronization of the laser-pump/X-ray-probe pulses is achieved through a trigger signal from a photosensor mounted on the rotating disk, or from an X-ray sensitive photodiode inserted in the beamstop. For the study of shorter-lived species the time structure of the synchrotron beam is to be used. Equations are derived for the maximum and average fractional excited-state populations as a function of lifetime, pulse frequency and the fraction of molecules being excited by the laser pulse.