ULTRAFAST ELECTRON-DIFFRACTION .4. MOLECULAR-STRUCTURES AND COHERENT DYNAMICS

Ultrafast electron diffraction (UED) is developed, in this and the acc ompanying paper, as a method for studying gas-phase molecular structur e and dynamics on the picosecond (ps) to femtosecond (fs) time scale. Building on our earlier reports (henceforth referred to as 1-3), we di scuss theoretical and experimental considerations for the approach. Sp ecifically we show that the use of rotational and vibrational coherenc es can add a new dimension to structural determination of gas-phase sp ecies. In addition to the internuclear separations of the molecular sa mple, the spatial alignment reflected in the scattering pattern contai ns bond angles and rotational constants for both excited-state and gro und-state species. Vibrational coherence effects are also observable, and the motion of the wave packet is revealed by the change of the dif fraction pattern with time, thus yielding the molecular dynamics. UED provides the temporal evolution of the reaction coordinate directly an d is well-suited for studies of global structure changes on this time scale. Paper 5 details our experimental studies with UED and the curre nt time resolution of the apparatus.