For a long time, the jumps and changes of state of the electrons in the atomic shell were considered too fast to be observed. It was only through the pioneering achievements of this year's three Nobel Prize winners in physics that these movements were made visible and measured for the first time. Anne L'Huillier, Pierre Agostini and Ferenc Krausz developed the technology of attosecond laser pulses, which make the reactions of the electrons measurable like a kind of stroboscope. This made it possible for the award winners to measure the duration of Einstein's photoelectric effect for the first time and to research the behavior of electrons. This laid the basis for analysis techniques that are now indispensable in medicine, materials research and chemistry.
In 1905, Albert Einstein published the physical explanation for the photoelectric effect - a crucial aspect of the interaction between light and matter. Accordingly, when atoms are irradiated with photons, they absorb their energy and their electrons change to a higher energy state. These quantum jumps are specific depending on the element and molecule and depend on the energy absorbed. In extreme cases, the energy is so high that one or more electrons are catapulted completely out of their orbitals - the atom becomes an ion. But these changes in the state of the electrons happen so quickly that for a long time they were considered almost instantaneous. “In the world of electrons, changes occur in a few tenths of attoseconds. However, an attosecond is so short that as many of them fit into one second as the number of seconds that have passed since the beginning of the universe,” says the statement from the Nobel Prize Committee.
But for a long time there were no lasers that could produce light in a sufficiently short-wave spectral range. “For pulses with a duration of less than a femtosecond, you need extremely ultraviolet laser light,” explains award winner Ferenc Kraus from the Max Planck Institute for Quantum Optics in Garching. “To generate even shorter pulses, there is no way around X-ray light.” Until the achievements of the three Nobel Prize winners, it was therefore impossible to trace the movements of electrons - there was no tool or measuring instrument for this. What exactly happens during quantum jumps and ionization in the atomic shell and how this differs for different elements and molecules remained unclear.
(Video: Max Planck Society)
Laser bombardment of noble gas atoms
Anne L'Huillier from Lund University and her team took the first step toward solving this problem in 1987 in a laser experiment with noble gases. They found that when an intense infrared laser beam shines through a cloud of noble gas atoms, a variety of frequency “overtones” are created. The emission intensity of these overtones initially increased, but then remained the same over a relatively wide frequency range and then fell steeply. Through theoretical calculations and models, L'Huillier and her colleagues provided an explanation for these so-called "higher harmonics". According to this, the laser bombardment first causes ionization of the noble gas atoms - an electron is released. However, as soon as the phase of the laser field changes, the electron reverses and recombines with the atom. A short-wave, high-energy X-ray or extreme UV photon is released. Because this process repeats itself several times, these photons are released at short, regular intervals. For the first time, this opened up a way to generate regular, standardized light pulses at extremely short intervals - the basis for attosecond laser measurements.
The two prize winners, Pierre Agostini from the Ohio State University in Columbus and Ferenc Krausz from the Max Planck Institute for Quantum Optics in Garching, developed the technical requirements for the next step in parallel. Agostini and his team developed the so-called RABBIT technique (Reconstruction of Attosecond Beating by Interference of Two-Photon Transitions). This makes it possible not only to generate attosecond pulses by irradiating a noble gas cloud, but also to measure the duration of the pulses. In 2001, Agostini and his colleagues were able to use this RABBIT technique to generate for the first time a series of UV laser pulses, each only 250 attoseconds long. At the same time, Krausz and his team, then still at the Vienna University of Technology, were also working on attosecond lasers and used specially coated XUV mirrors and the so-called streaking technique to generate 650 attosecond long laser pulses.
First time measurements of the quantum jump
A little later, Krausz and his colleagues succeeded for the first time in measuring the time required for electron release during the ionization of neon atoms. They were able to prove that electron emission does not occur instantaneously, but with a time delay that varies depending on the electron orbital and element. The electron from the 2p orbital is 21 attoseconds slower than an electron emitted from the 2s orbital. L'Huillier and her team further refined the experiment and were able to correct a confounding effect that had previously led to discrepancies between the experiment and theoretical models. At the same time, based on their measurements, they also determined the binding energies of the electrons in both orbitals of the neon.
“What began as a rather narrowly focused field of multiphoton processes in atomic physics has now expanded and pushed the boundaries of what is possible in many fields such as molecular physics, physical chemistry, solid state physics and applied areas,” explains the Nobel Prize Committee . Today it is difficult to imagine these areas without attosecond spectroscopy and the measurements of electron movements in atoms and materials.