During the past six years the minimum duration of optical (XUV) pulses has fallen from 5 femtoseconds (5×10−15 sec) to about 100 attoseconds (~10−16 sec)—less than the classical period of a ground-state electron in a hydrogen atom. Lasers drove this revolution by forcing electron wave packets to tunnel from the atom or molecules, move under the force of the time dependent electric field and then re-collide with their parent ions. From the ion’s perspective, an attosecond electron wave packet re-collides. Attosecond XUV pulses are the byproduct of this collision.
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