Circa 2016
Laser physicists in Munich have measured a photoionization — in which an electron exits a helium atom after excitation by light — for the first time with zeptosecond precision. A zeptosecond is a trillionth of a billionth of a second (10^−21 seconds). This is the greatest accuracy of time determination ever achieved, as well as the first absolute determination of the timescale of photoionization.
If light hits the two electrons of a helium atom, one must be incredibly fast to observe what occurs. Besides the ultra-short periods in which changes take place, quantum mechanics also comes into play. Laser physicists at the Max Planck Institute of Quantum Optics (MPQ), the Technical University of Munich (TUM) and the Ludwig Maximilians University (LMU) Munich have now measured such an event for the first time with zeptosecond precision.
Either the entire energy of a light particle (photon) can be absorbed by one of the electrons or a division can take place, if a photon hits the two electrons of a helium atom. Regardless of the energy transfer, one electron leaves the atom. This process is called photoemission, or photoelectric effect, and was explained by Albert Einstein at the beginning of last century.
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