Instrumentation for photocrystallographic experiments of transient species

Abstract

Instrumentation for stroboscopic time-resolved diffraction studies at low temperatures is described. Exciting laser light is delivered to the crystal through an optical ®ber. During the diffraction experiment,¯uorescence from the sample is focused onto a ®ber optic bundle surrounding the laserlight ®ber, 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 ms 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.

Excited-state structure by time-resolved X-ray diffraction

“…Time-resolved diffraction measurements were performed at the X3 beamline at the National Synchrotron Light Source, using instrumentation speci®cally designed for time-resolved stroboscopic pump-probe experiments (Fullagar et al, 2000). X-ray pulses are produced by inserting a rotating chopper wheel in the X-ray beam (!…”

Synchrotron ultrafast techniques for photoactive transition metal complexes

“…Previous reports highlighted a 3 A 2u -symmetry lowest-lying triplet state [204,205], with a measured lifetime of 9.8 µs in deoxygenated water [206][207][208] and a quantum yield of approximately 100% [208]. The geometry of such a long-lived triplet state was thoroughly investigated in the crystal phase using both ultrafast [209][210][211] and stationary-state X-ray methods [212,213]. The crucial structural parameter is the contraction of the Pt-Pt bond distance, which , from analysis of TR-XXS data collected at 100 ps and 1 µs after laser excitation at 267 nm [199].…”

Revisiting Bragg's X-ray microscope: Scatter based optical transient grating detection of pulsed ionising radiation

“…Non-collimated beams can achieve a similar outcome, corresponding, as with the use of polychromatic radiation, to the sacrifice of X-ray source brilliance in one or other degree of the source's Liouville phase space [82]. Nevertheless, this work revisits the use of brilliant monochromatic X-ray sources [83], which provide two-dimensional Ewald-sphere cuts of reciprocal space that can realise or anticipate ultrafast structure studies [38,84], and for which a lesser or greater degree of control may be associated with the stimulus [85]. Extrapolating successful crystallographic results [38] to single-molecule scattering proposals at XFELs [86], a level of optimism can be appreciated in terms of available photons and measurement times.…”