When light is absorbed by atoms, the electrons become excited. If the light particles, so-called photons, carry sufficient energy, the electrons can be ejected from the atom. This effect is known as photoemission and was explained by Einstein more than hundred years ago. Until now, it has been assumed that the electron start moving out of the atom immediately after the impact of the photon. This point in time can be detected and has so far been considered as coincident with the arrival time of the light pulse, i.e. with "time zero" in the interaction of light with matter.

Their measurements revealed that electrons from different atomic orbitals, although excited simultaneously, leave the atom with a small but measurable time delay of about twenty attoseconds.

German scientists hit electrons with light and then measured how they soon they moved. The delay between the bombardment and the movement of those electrons is the shortest interval of time ever measured, which is 20 attoseconds. An attosecond is one quintillionth of a second.The scientists tested the assumption, and this is what happened: Link via Popular Science | Photo: Thorsten Naeser / Max-Planck-Institute of Quantum Optics