R. J. Dwayne Miller, Professor, Max Planck Group for Atomically Resolved Dynamics, Department of Physics, University of Hamburg, Germany, and The Centre for Free Electron Laser Science, and Departments of Chemistry and Physics, University of Toronto, Canada.
One of the grand challenges in science is to watch atomic motions as they occur during structural changes. In the fields of chemistry and biology, this prospect provides a direct observation of the very essence of chemistry and the central unifying concept of transition states in structural transitions. From a physics perspective, this capability would enable observation of rarefied states of matter at an atomic level of inspection, with similar important consequences for understanding nonequilibrium dynamics and collective phenomena. This experiment has been referred to as "making the molecular movie". Due to the extraordinary requirements for simultaneous spatial and temporal resolution, it was thought to be an impossible quest and has been previously discussed in the context of the purest form of a gedanken experiment. With the recent development of femtosecond electron pulses with sufficient number density to execute single shot structure determinations, this experiment has been finally realized (Siwick et al. Science 2003). Previously thought intractable problems in attaining sufficient brightness and spatial resolution, with respect to the inherent electron-electron repulsion or space charge broadening, has been solved. The first studies focused on simple systems. With recent advances in source brightness even weakly scattering organic molecules and even solution phase systems have been opened up to atomic exploration, as will be documented by several movies of atomically resolved structural dynamics.
In this respect, one of the marvels of chemistry and biology is that despite the enormous number of possible nuclear configurations, chemical processes reduce to a few key modes. The “magic of chemistry” is this enormous reduction in dimensionality in the barrier-crossing region that makes chemical concepts transferrable. Recent studies using an order of magnitude brighter, rf compressed, pulses have given the first the first direct atomic view of the barrier crossing processes and the distillation of chemistry to projections along a few principle reaction coordinates (Gao et al Nature 2013). The molecular motions illustrate that the process is mediated by strong mode coupling and near perfect correlation in which the lowest frequency directs atomic traffic (renormalizes the potential energy surface) in the barrier-crossing region.
These new developments will be discussed in the context of developing the necessary technology to directly observe the structure-function correlation in biomolecules ¾ the fundamental molecular basis of biological systems.