Of the four fundamental parameters that we use to perceive the physical world (energy, momentum, position, and time) three correspond to the three broad categories of synchrotron experimental measurement techniques: spectroscopy (energy), scattering (momentum), and imaging (position). The fourth parameter—time—can in principle be applied to all the techniques.
At the ALS, many experiments can be carried out in real time, with data being recorded from the same sample as it changes over time. Some time-resolved experiments take advantage of the pulsed nature of the ALS's synchrotron radiation, which, like a strobe light, can capture a series of "snapshots" of a process that, when viewed sequentially, show us how a given process evolves over time. Other experiments simply require two pulses: one to "pump" energy into the sample system and a second to probe the system's excited state.
To improve time resolution, a high-speed laser can be used to "slice" out tiny slivers from the circulating electron bunches in the storage ring to produce pulses of synchrotron radiation lasting just 300 femtoseconds (the vibrational period for atomic motion is about 100 femtoseconds). This "ultrafast" regime opens the possibility of exploring the making and breaking of chemical bonds and the rearrangement of atoms, processes that ultimately determine the course of phase transitions in solids, the kinetic pathways of chemical reactions, and even the efficiency and function of biological processes.
Finally, ultrafast experiments require detectors that are fast enough to resolve the details of the dynamical processes. Streak cameras are a type of detector that captures temporal information and displays it spatially, by spreading it out into a streak—in a sense magnifying the timing structure of the data.
Selected Time-Resolved Highlights