| Molecular dynamics is now routinely studied on femtosecond time scales using various spectroscopies. However, direct structural information of all nuclear coordinates involved in such dynamical processes requires resolution in time by x-ray or electron diffraction. The focus of this laboratory has been the development of ultrafast electron diffraction (UED) for recording structures in motion, which exploits the six-orders-of-magnitude higher scattering cross section of electrons compared with x-rays. In UED, a ferntosecond (fs) laser pulse is used to initiate a reaction, but unlike other ultrafast spectroscopies, the subsequent laser pulses normally used to probe the progress of the reaction are replaced with ultrashort pulses of electrons. Time-resolved diffraction patterns are then recorded at fixed time delays relative to the zero-of-time. This directly reflects the changing internuclear distances in the species under study.; A number of significant experimental challenges to the realization of UED have been surmounted over the last decade, and UED now approaches the combined spatial and temporal resolution necessary for tracking all nuclear coordinates during the making and breaking of chemical bonds, thereby permitting the direct observation of molecular structural dynamics in real time. In addition, the diffraction-difference method—which employs the subtraction of a reference diffraction signal from the signals recorded over the course of the reaction—can be used to select the contributions resulting only from changes in structure in the species under study, thereby enhancing the sensitivity of UED to chemical change. Contributions only from the products can be also isolated by adding the appropriately scaled parent diffraction signal at negative time to the difference curves, thus canceling out the parent contribution in each curve.; A variety of chemical reactions have been studied by UED. The spatial and temporal resolution of UED approached ∼0.01 Å and ∼1 ps, respectively, and we were sensitive to ∼1% changes in the mole fractions of the various chemical species over the course of the reaction. The results presented here provide the new limit of improved detection sensitivity, versatility, and resolution of UED, as well as the potential for its diverse applications. |
