About ultrafast imaging and structural dynamics

The world is not static. At every length and time scale, there are characteristic motions that ultimately occur through the anharmonic coupling between different degrees of freedom within the object of interest. These characteristic motions thus hold the secrets to the primary forces as well as to all the details of the various couplings responsible for the (complex) dynamics inherent to the system.

With respect to the discipline of Chemistry, one of the great dream experiments is to directly watch the interplay between atomic and electronic motions during a chemical event. Similarly in Biology, where chemical processes drive numerous biological functions, biologists have based a number of key concepts on static structures in which the relative atomic motions connecting different functionally relevant forms are envisaged. For both Chemistry and Biology, the key event is the nuclear sampling of the barrier, a critical point separating reactant and product channels. This so-called transition state is the central concept in Chemistry, extending to and equally used in Biology. However, for other than simple few atom systems we do not have a rigorous structural connection to this critical point. This structural and dynamical information is critical to rational control of biological processes and advancing chemistry to the level of selectivity found in Nature. There are similar concepts in Physics with respect to cooperative phenomena where the barrier crossing events are best described in collective coordinates with characteristic lengths scales rather than a single critical point.

The ultimate limit...