Introducing the fourth (temporal) dimension to electron crystallography constitutes the realm of ultrafast electron crystallography (UEC), which allows for studies of transient nonequilibrium structures with unprecedented spatiotemporal resolution. Visualization of such structures is fundamental to understanding phase transitions and dynamics of surfaces, nano-scale materials, and molecular interfaces. In the UEC apparatus the sample is mounted on a computer-controlled goniometer. With the electron gun focusing system, both the reflection and transmission diffractions can be obtained, and the patterns are recorded by an intensified CCD camera assembly capable of single electron detection. The scattering chamber is augmented with a retractable gas dosing assembly, a residual gas analyzer and a cooling system which enables temperature-dependent experiments down to 10 K.

The first UEC experiments were conducted on crystalline silicon with different adsorbates: hydrogen, chlorine, or molecular trifluoroiodomethane, aligned by the surface. Subsequently, different crystalline or amorphous systems were studied, including those of water on hydrophilic and hydrophobic surfaces. For all of these studies (see abstracts below), clear knowledge of the dynamics of the underlying crystalline substrate is prerequisite to the investigation of interfacial phenomena in different systems. More recently, UEC was extended to study correlated systems exhibiting superconductivity, phase transitions, and ablations. Because UEC is capable of deciphering the nature of atomic motions associated with complex condensed-matter phenomena, this research is continuing on other novel materials and interfaces.