16-11-2016 | | By Paul Whytock
Photoionisation has been accurately measured for the first time and with zeptosecond accuracy. To fully grasp just how precise this is a zeptosecond is a trillionth of a billionth of a second. Or put another way, 0.000000000000000000001 seconds.
Scientists have hailed this as the greatest accuracy of time determination of an event in the microcosm ever achieved, as well as the first absolute determination of the timescale of photoionization which may possibly help with future development of quantum electronics.
Photoionisation happens when light strikes electrons in atoms and their state rapidly changes.
Laser physicists at the Max Planck Institute of Quantum Optics, the Technical University of Munich and the Ludwig Maximilians University Munich have now measured such an event in zeptoseconds.
Either the entire energy of a light particle (photon) can be absorbed by one of the electrons or a division takes 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. (Pictured is the possible position of the remaining electron after photoemission of an electron from a helium atom.)
It takes between five and fifteen attoseconds from the time a photon interacts with the electrons to the time one of the electrons leaves the atom To put that in context an attosecond is one quintillionth of a second, or put another way an attosecond is to a second what a second is to 31billion years billion years.
With their improved measurement method laser physicists can accurately measure events at a rate of up to 850 zeptoseconds. The researchers shone an attosecond-long, extremely ultraviolet (XUV) light pulse onto a helium atom to excite the electrons.
At the same time they fired a second infrared laser pulse, lasting about four femtoseconds (one femtosecond is one millionth of one billionth of a second). The infrared laser pulse detected the electron as soon as it left the atom following excitation by XUV light.
Depending on the exact electromagnetic field of this pulse at the time of detection, the electron was accelerated or decelerated. Through this change in speed, the physicists were able to measure photoemission with zeptosecond precision.
The researchers were also able to determine how the energy of the incident photon is quantum-mechanically divided between the two electrons of the helium atom in a few attoseconds before the emission of one of the particles.
Helium is the only multi electron system that can be calculated completely quantum mechanically.