A Snapshot of the World of Electrons

In physics class, you learn that electron movement is unpredictable. However, there is a newly developed technique that can record electron movement—a previously impossible feat. On October 3, 2023, the Nobel Prize Committee announced that this year’s winners of the Nobel Prize in Physics are French professor Pierre Agostini, Hungarian physicist Ferenc Krausz, and French-Swedish physicist Anne L’Huillier, who used short pulses of light to take pictures of electron movement inside atoms. 

The inspiration behind this technique is the famous Heisenberg uncertainty principle, which states that it is impossible to derive both the exact location of a particle and its exact momentum. As a result, electron orbitals were viewed as regions where electrons are likely to be in a given time—not discrete and precisely defined locations. The odds of an electron being in a certain place change every attosecond (10^-18 of a second), so if there were laser pulses that could interact with electrons every attosecond, they could perhaps detect electron motion.

In the 1980s, L’Huillier demonstrated a technique using laser lights on gas atoms to generate high-energy “harmonics.” Harmonics refers to light waves whose frequencies, which are the number of waves that pass by a fixed point per unit of time, are multiples of the frequency of the original laser. L’Huillier and her colleagues observed that when an oscillating infrared laser was shined on certain gasses, their atoms became excited and released additional colors of light with frequencies greater than those from the original laser (this effect is known as “overtones”). Interestingly, they noted that some of the extra colors released appeared brighter than others. Using quantum mechanics, they calculated the intensities of the overtones and were able to predict which overtones would be emitted by atoms when a laser hit them. L’Huillier combined certain atoms to collectively emit overtones, which would merge into one big wave that oscillates on an attosecond scale, similar to how an orchestra produces music with the combined force of many instruments. 

Later, Krausz and Agostini generated the first set of ultra-pulses of light that capture “frames” every few attoseconds. This allowed the scientists to reconstruct the movements of the super-fast electrons within atoms. In 2001, Agostini and his colleagues developed a technique called RABBIT or RABBITT (Reconstruction of Attosecond Beating By Interference of Two-photon Transitions). With RABBIT, he was able to generate a string of laser pulses that could capture “frames” every 250 attoseconds. Shorter time frames were more desirable because they could detect subtle changes in electron motion more precisely. In the same year, Krausz used a different method called streaking to generate individual laser pulses.  Later, L’Huillier was able to cut the time down to 170 attoseconds. 

Krausz used this attosecond technology to make certain observations about electron behavior. For example, in 2010, he determined that an electron in a lower energy state flees its host neon atom 21 attoseconds faster than an electron in a higher-energy state. He also discovered that electrons escape faster from liquid water than water vapor by tens of attoseconds. These findings prompted further research on electron behavior that could lead to advancements in the future.

Before the use of attosecond technology, people used femtosecond pulses (10^-15 seconds) to record the motions of molecules during chemical reactions, but their bandwidth wasn’t large enough for comparable electron measurements. Bandwidth refers to a frequency range in which light pulses are combined so that their energy is concentrated in a narrow window of time. Thus, attosecond technology allowing for the production of pulses every few attoseconds is a promising new technique that can be implemented in other fields of study. The Nobel Prize winners’ findings could aid in the development of electronics by allowing for greater electron control in certain materials. Also, this advancement could potentially lead to the development of a new in-vitro diagnostic technique to detect molecular traces of disease in blood samples. Attosecond pulses could potentially be used to detect cancer in its early stages. 

L’Huillier, Krausz, and Agostini’s Nobel-winning work enables the scientific world to gain better insight into electron motion. Detecting electron motion can drive progress in the fields of electronics, medicine, and more. Additionally, a greater understanding of the subtleties of electron behavior could drive further discoveries. Shattering the femtosecond barrier has created endless possibilities for scientists in the future.