title: Dark structures in molecular radiationless transitions determined by ultrafast diffraction.
abstract: The intermediate structures formed through radiationless transitions are termed "dark" because their existence is inferred indirectly from radiative transitions. We used ultrafast electron diffraction to directly determine these transient structures on both ground-state and excited-state potential energy surfaces of several aromatic molecules. The resolution in space and time (0.01 angstrom and 1 picosecond) enables differentiation between competing nonradiative pathways of bond breaking, vibronic coupling, and spin transition. For the systems reported here, the results reveal unexpected dynamical behavior. The observed ring opening of the structure depends on molecular substituents. This, together with the parallel bifurcation into physical and chemical channels, redefines structural dynamics of the energy landscape in radiationless processes.
authors.affiliation: Laboratory for Molecular Sciences, Arthur Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
authors.forename: Ramesh
authors.initials: R
authors.lastname: Srinivasan
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Feenstra
authors.forename: Sang Tae
authors.initials: ST
authors.lastname: Park
authors.forename: Shoujun
authors.initials: S
authors.lastname: Xu
authors.forename: Ahmed H
authors.initials: AH
authors.lastname: Zewail