Due to recent technological developments simultaneous atomic resolution in space and time to follow microscopic dynamics is within reach. X-ray free electron lasers and other similar sources under construction world wide can deliver the appropriate light pulses. In a parallel development, intense vacuum ultruviolet pulses with duration in the attosecond regime can be created. Finally, the next generation synchrotrons will reach sub-picosecond time resolution. This opens a completely new parameter regime for light-matter interaction, namely time-resolved X-ray dynamics.
Ultrashort laser pulses can follow chemical reaction kinetics in real time, but extracting quantitative information on the evolving molecular and electronic structure from optical measurements remains a major challenge. Therefore, ultrafast spectroscopy seeks new probes to provide a deeper understanding of chemical reactions at the level of atoms and electrons.
Ultrashort X-ray pulses are such new probes, and in recent years time-resolved laser spectroscopy and established X-ray methods have been combined to create new tools to directly probe the local electronic and molecular structure in time and energy. Hard X-rays are typically used to probe atomic arrangements through scattering and K-edge spectroscopy, while soft X-rays are sensitive to valence-charge distributions and hold tremendous potential for following the formation and dissolution of chemical bonds in real time. The information gleaned from ultrafast X-ray probes is essential to advance our understanding of the cooperative relationship between electronic charge distributions, atomic rearrangement, and the formation of new molecular structures. It is particularly effective for understanding molecular dynamics in solution, where much important chemistry occurs and where the solvent environment substantially influences reaction dynamics through interaction with the valence charge distribution.
- construction and development of an ultrafast X-ray source; the pulsed X-ray source is based upon the generation of a dense plasma via intense (greather than 1015 W/cm2) laser excitation of solid targets, specifically a copper wire.
Preliminary X-ray spectrum of this source is shown on the right. The two narrow peaks are the copper K-alpha and K-beta lines. The lines are riding atop the bremsstrahlung continuum background. Current the x-ray source is located in open air resulting a limited optical pulse focussing parameters and relatively low x-ray yield. It is anticipated that the generated x-ray pulses are sub-picosecond. Diagnostic tests are now underway, including X-ray yield optimization and x-ray pulse length.