Ultrafast Electron Diffraction. 4. Molecular Structures and Coherent Dynamics
Ultrafast electron diffraction (UED) is developed, in this and the accompanying paper, as a method for studying gas-phase molecular structure and dynamics on the picosecond (ps) to femtosecond (fs) time scale. Building on our earlier reports (henceforth referred to as 1-3), we discuss theoretical and experimental considerations for the approach. Specifically we show that the use of rotational and vibrational coherences can add a new dimension to structural determination of gas-phase species. In addition to the internuclear separations of the molecular sample, the spatial alignment reflected in the scattering pattern contains bond angles and rotational constants for both excited-state and ground-state species. Vibrational coherence effects are also observable, and the motion of the wave packet is revealed by the change of the diffraction pattern with time, thus yielding the molecular dynamics. UED provides the temporal evolution of the reaction coordinate directly and is well-suited for studies of global structure changes on this time scale. Paper 5 details our experimental studies with UED and the current time resolution of the apparatus.
© 1994 American Chemical Society. Received: January 13, 1994. This work was supported by a grant from the US. Air Force Office of Scientific Research and the National Science Foundation. The authors thank Dr. Scott Kim and Dr. Marcos Dantus for their stimulating discussions on ultrafast electron diffraction, and Hakno Lee and Dr. Juen-Kai Wang for their help. We thank Dr. Ralf Friichtenicht for assisting in the translation of some of the referenced (German) articles. J.C.W. acknowledges the National Science Foundation for providing a Graduate Student Fellowship.