W0099

Stroboscopic Single-Crystal Diffraction of Molecular Excited States at Atomic Resolution. Philip Coppens,^a Ivan Vorontsov,^a Tim Graber,^b Guang Wu,^a Milan Gembicky^a and Y.-S. Chen^c,^aDept. of Chemistry, State Univ. of New York at Buffalo, Buffalo, NY, ^bThe Center for Advanced Radiation Sources, The Univ. of Chicago, Chicago, IL, ^cPhysics Dept., Univ. of Toledo, Toledo, OH.

This work is directed at removing the limitation of X-ray diffraction analysis to ground state structure determination by extending the field to the measurement of non-equilibrium transient species. A stroboscopic pump-probe technique has been developed in which the X-ray detector is only exposed to the probing radiation for a short period after the occurrence of an exciting laser pulse. The subsequent analysis is based on the response ratios, defined as the fractional change of the intensity of each reflection upon light exposure. In the first study at NSLS, the shortening of the Pt-Pt distance upon excitation of the [Pt₂(pop)₄]^4- ion (pop = pyrophosphate, (H₂P₂O₅)^2-) was measured and then compared with DFT results obtained with different functionals.¹ Subsequent experiments, with a new experimental setup at the third generation Advanced Photon Source,² reveal a geometry change upon photoinduced metal-to-ligand charge transfer (MLCT) in the complex [Cu(I)(dmp)(diphos)]PF₆ (dmp=dimethylphenanthroline, diphos=1,2-bis(diphenylphosphino)ethane). Since differential techniques are employed the method is sufficiently sensitive to provide geometry information on species present at a level as low as 2%. Further experiments are in progress.