Dark Structures in Molecular Radiationless Transitions Determined by Ultrafast Diffraction
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.
Additional Information© 2005 American Association for the Advancement of Science. Received 9 November 2004; accepted 21 December 2004; Published online 6 January 2005. Supported by NSF and the U.S. Air Force Office of Scientific Research.
Supplemental Material - Srinivasan.SOM.pdf