  A major impediment to a more complete understanding of barrier crossing and
other single-molecule processes is the inability to directly visualize the
trajectories and dynamics of atoms and molecules in reactions. Rather, the
kinetics are inferred from ensemble measurements or the position of a
transducer (e.g. an AFM cantilever) as a surrogate variable. Direct
visualization is highly desirable. Here, we achieve the direct measurement of
barrier crossing trajectories by using optical microscopy to observe position
and orientation changes of pairs of Ag nanoparticles in an optical ring trap,
i.e. passing events. A two-step mechanism similar to a bimolecular exchange
reaction is revealed by analysis that combines detailed knowledge of each
trajectory, a statistically significant number of repetitions of the passing
events, and the driving force-dependence of the process. We find that while the
total event rate increases with driving force, this increase is only due to
increased rate of encounters. There is no drive force-dependence on the rate of
barrier crossing because the key motion for the process involves a radial
motion of one particle as a thermal (random) fluctuation allowing the other to
pass. This simple experiment can readily be extended to study more complex
barrier crossing processes by replacing the spherical metal nanoparticles with
anisotropic ones or through more intricate optical trapping potentials.
