Physical Society Colloquium

Mapping Atomic Motions with Ultrabright Electrons:
The Chemists' Gedanken Experiment Enters the Lab Frame

Dwayne Miller

University of Toronto / Max Planck Institute for the Structure and Dynamics of Matter

One of the grand challenges in science is to watch atomic motions on the primary timescales of structural transitions, i.e. to watch atoms move in real time. This prospect would provide a direct observation of the very essence of chemistry and the central unifying concept of transition states that links chemistry to biology. 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 ultrabright electron sources capable of literally lighting up atomic motions, this experiment has been realized (Siwick et al.. Science 2003).� The first studies focused on relatively simple systems. Further advances in source brightness have opened up even complex organic systems and solution phase reaction dynamics to atomic inspection. Recent studies have given the first direct atomic view of barrier crossing processes and the distillation of chemistry to projections along a few principle reaction coordinates (Gao et al. Nature 2013, Jean-Ruel et al. JPC B 2013, Miller Ann. Rev. Phys. Chem. 2014, Miller Science 2014). The molecular motions illustrate that the process is mediated by strong mode coupling and near perfect correlation in which the lowest frequency mode directs atomic traffic (renormalizes the potential energy surface) in the barrier crossing region. These observations solve a long standing riddle (to RJDM at least). Despite the large number of degrees of freedom, unique mode structure, and near astronomical possible nuclear configurations, chemistry can be generalized into various classes of reaction mechanisms. We can now directly observe the operating physics. The “magic of chemistry” is this enormous reduction in dimensionality in the barrier crossing region that makes chemical concepts transferrable.

These new developments will be discussed in the context of developing the necessary technology to directly observe the structure-function correlation in biomolecules — to give the most fundamental (atomic) basis for understanding biological systems at the molecular level.