New insight into ever-smaller structures of matter and their ever-faster dynamics holds great promise for pushing the frontiers of many fields in science and technology. The speed of electronic processes is determined by the separation of the energetic states of electrons. In atoms, the binding energy of electrons is on the order of tens to hundreds of electron volts, which explains why electronic processes occur on an attosecond timescale. One attosecond
(10^–18 s) is the billionth part of a nanosecond, which is itself the billionth part of a second. Put more picturesquely, one attosecond compares to one second approximately as one second to the age of the universe.

In pump/probe experiments, one pulse (of light, of x-rays, of electrons, etc.) triggers a process, and another pulse with an adjustable delay probes the subsequent temporal evolution of the process. This has turned out to be the most direct approach to time-domain investigations of fast-evolving microscopic processes...

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