Ultrafast Electron Diffraction (UED) with atomic-scale, combined spatial and temporal resolution is a powerful tool for determination of structural dynamics. In order to spatially resolve a molecular structure, the wavelength required is sub-Å, which can be easily obtained using accelerated electrons: λ_{de Broglie} = 0.067 Å at 30 keV. The atoms in a specimen act as scattering centers for incident electrons, and each atom becomes a coherent source of an outgoing spherical wave. The electron diffraction experiment thus becomes a conceptual analog of the well-known multi-slit diffraction experiment, but at the molecular length scale. That is why electron diffraction is often used to elucidate the three-dimensional architecture of isolated molecules, amorphous materials, and (thin) crystals.

Because of the very large cross-section for scattering of electrons, as compared to that of X-ray light, it is possible to achive the ultrafast time resolution when studying molecular systems in the gas phase (a nontrivial task because of the lack of long-range order and low molecular density). The UED approach developed at Caltech has been successfully used to study chemical reactions, excited-sate structure dynamics, and nonequilibrium conformational changes on their native ultrafast time scales (see abstracts below). With UED, dark transient structures unobservable by spectroscopic methods can be determined and visualized. Ongoing UED research involves studies of complex chemical structures and biological chromophores. The construction of a fourth-generation instrument capable of investigating biomolecules in the gas phase is a significant part in this effort. The goal is to reveal, in the absence of a perturbing solvent, the intrinsic structural dynamics of the species, in the hope of identifying degrees of freedom critical to the understanding of their biological functionality.