TCBG Seminar

Ultrafast X-Ray Diffraction and Spectroscopy

Professor Kent Wilson
Department of Chemistry and Biochemistry
University of California
San Diego, CA

Monday, April 21, 1997
3:00 pm (CT)
3269 Beckman Institute


What we really would like to know in order to fundamentally understand chemical and biochemical reactions is how the atoms and electrons are moving as the reaction happens. While we can measure optical spectra on the ultrafast time scale during which bonds are broken and made, unfortunately we cannot directly invert these spectra to what we most want to know: the nuclear and electronic dynamics. There are measurements which are directly invertible to where the electrons and atoms are: x-ray diffraction and (somewhat less directly) x-ray absorption (EXAFS and chemical shifts). Up to now, these measurements have been orders of magnitude too slow to follow the making and breaking of bonds. We are developing techniques to carry x-ray diffraction and absorption to the sub-picosecond regime. We produce ultrafast x-ray pulses by driving solid density plasmas with intense light pulses (20 fs, 150 mJ, 20Hz, 3 W average power). We can produce either line radiation (for example Cu Ka) for diffraction or quasi-continua for absorption spectroscopy. We have demonstrated that we have enough x-ray flux by taking diffraction patterns from several types of static crystals and are now working on dynamic spectra during light induced processes. While x-ray diffraction can measure the global distribution of electron density and the positions of atoms, x-ray absorption can measure the local distribution, about a single chosen type of atom, of electrons (by chemical shift) or of atoms (by EXAFS). We have used ultrafast absorption, in a collaboration with J.-C. Kieffer and his group, to follow the photodissociation of sulfur hexafluoride with 1.5 ps resolution. This talk will be given with a multimedia presentation, as a combination of text, equations, images, video, computer graphics and animation, and sound, all controlled by the lecturer. The material is presented using a Macintosh computer, video compression board, and a high resolution LCD color projector.

Tea and coffee will be served in R3151 Beckman Institute at 2:15pm.

Main TCBG Seminars page