Using spatially coherent x-ray sources for measuring ultrafast atomic motions and bio-medical imaging

Guest lecture with Dr. Christoph Rose-Petruck from the Chemistry Department at
Brown University

Abstract
Ultrafast high-intensity laser pulses incident upon condensed matter targets can generate high-density plasmas that emit x-ray pulses with sub-picosecond temporal structure, significant spatial coherence, and high brightness. Such laser-driven plasma x-ray sources are used for structural measurements of various solvated transition metal complexes by XAFS spectroscopy. Static and ultrafast XAFS measurements will be presented. Structural parameters of solvated Fe(CO)5 were measured by XAFS spectroscopy and are compared to FTIR data and Density Functional Theory calculations of the solvated complexes. The results show that Fe(CO)5 forms weak complexes with a single solvent molecule, with potentially significant consequences for ultrafast bi-molecular reaction in solution.

The high spatial coherence of laser-driven plasma x-ray sources can also be used for in-line x-ray holographic imaging of objects and ultrafast density waves in materials. This imaging method relies on the differential phase-shift of x-rays causing image contrast even in the absence of any x-ray absorption. This concept is similar to optical in-line holography but is done with ~ 20-keV x-rays. Holographic tissue images acquired with an ultrafast laser plasma source are presented. Example images of tumor-infiltrated livers in a murine model are presented and discussed. A further refinement of this imaging method relies on ultrasound waves simultaneously applied to the tissues during x-ray imaging. Due to the tissues’ differential acoustic impedance, differential movements are induced that can be detected with high contrast.