Visualizing ultrafast chemical dynamics with X-rays

Abstract

We live in a world bathed in light. Light drives photosynthesis and is responsible for the oxygen environment that enables the diversity of life found on our planet. Photochemistry provides the means to harness light energy for productive function through the movement of charge, a change in molecular shape, or the cleavage of a bond. Photochemistry also cleaves DNA and produces damaging free radicals. Photochemical processes—both harnessed and destructive—are unavoidable. Ultrafast transient spectroscopies in the UV-visible and infrared regions of the spectrum have allowed the detailed probing and characterization of many photochemical processes occurring on a wide range of timescales. These probes, however, provide only indirect insight into the electronic and structural dynamics of photochemical transformations. The development of X-ray free electron lasers (XFELs) and tabletop high harmonic sources have now provided the tools required to interrogate directly the electronic and structural changes that take place on the very fastest timescales of chemistry. These measurements provide insight into photochemistry, but also more generally into the coupling of electronic and nuclear degrees of freedom in chemistry. It is becoming possible to visualize chemical reactions with both electronic and atomic resolution on timescales from attoseconds to seconds. In PNAS, Bacellar et al. (1) combine femtosecond X-ray absorption (XAS) and X-ray emission (XES) spectroscopies to address an outstanding question in heme protein photophysics: Does the decay of photoexcited ferric heme involve a cascade of iron-centered spin states or is it characterized more simply by subpicosecond internal conversion to the ground state (2⇓⇓–5)? This question is addressed by Bacellar et al. on a room temperature protein sample, at modest concentration, with ultrafast time resolution, and exploiting the sensitivity of X-rays to both electronic and structural dynamics. Fe K-edge XES probes the 2p→1s (Kα) and 3p→1s (Kβ) transitions of … [↵][1]1Email: rsension{at}umich.edu. [1]: #xref-corresp-1-1