The photoelectric effect is one of the physical phenomena in the early twentieth^e century led to the formulation of quantum mechanics : when an atom exposed to radiation of sufficiently high frequency , part of the radiation energy is absorbed by an electron of the atom , electron s ‘ then escapes from the atom. The emission of the electron is so fast , so far, in the models, they considered instantaneous . An international team of physicists , including Max Planck Institute for Quantum Optics in Garching , Germany , has shown that the electron could be issued with a delay, and delay depend on the quantum state in which he was within the atom.

If a delay existed without being detected, it should be small , the order of several tens of attosecond ( one attosecond is 10^–18 second , or one billionth of a billionth of a second) . To highlight a delay of this order , the researchers used two synchronized ultrashort optical pulses . One , ultraviolet attosecond lasted a few decades , it will provide the electron sufficient energy to escape from . The other impulse, infrared flash is a little longer – a few femtoseconds ( one femtosecond is 10^–15 second ) – and more intense it is that which measures the delay of the electron. Here’s how.

When an electron is emitted after the absorption of ultraviolet radiation , it falls under the influence of the electromagnetic field associated with the infrared flash . This field slightly changes the speed of the electron. This change depends on the one hand, when the infrared pulse reaches the atom from the initial ultraviolet flash – when the researchers controlled with precision – and , secondly , the delay of the electron. Researchers measured the speed of the electron in the infrared field on the time between the two laser pulses .

They have thus observed in a gas of neon atoms , a temporal curves measured rate , as the electrons are ionized from a deep layer electronic (orbital 2s) Or superficial (orbital 2p) Of the atom . This discrepancy could be due to a delay between the excitation of the atom by ultraviolet pulse when the electron escapes the atom and effectively is affected by the IR field . This period would be longer by about 20 attoseconds for an electron emitted from the surface layer than that emitted from the deep layer.

However , the physics community remains cautious. In 2007 , the same team had found , using a similar device , but applied to a solid – a crystal of tungsten – a delay of about 100 attosecond between electrons emitted from the conduction band of the crystal and those from deeper layers . ” Today, on this effect in solids , there are as many interpretations as theoretical studies , “says Fabien Quéré , researcher the Institute of Matter and Radiation Saclay , CEA . « If these new studies show that the study is now possible on the scale of an atom , the reason for delays is still hotly debated . »

A proposed interpretation is: when it emits an electron , the atom becomes a positively charged ion . This change alters the energy levels of the remaining electrons , which adjust their energy to new levels by transferring their surplus to the electron departure. This transfer could be the cause of the delay of the electron . The determination of the delay, even relative , would then better understand the dynamic interactions of electrons in atoms .