photo: Norbert Kowalczyk | Unsplash
The Nobel Prize in Physics (2023)
Pierre Agostini, Ferenc Krausz and Anne L’Huillier
On Tuesday, 3 October the three winners of the Nobel Prize in Physics were announced: Pierre Agostini (France), Ferenc Krausz (Hungary) and Anne L’Huillier (France). The prize was awarded “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”.
Paweł Zajdel, PhD DSc, Associate Professor from the Faculty of Science and Technology explains the works of the physicists and the methods they developed, and lists further research perspectives.
| Paweł Zajdel, PhD DSc, Associate Professor |
This year’s Nobel Prize in Physics was awarded in equal parts to Anne L’Huillier, Pierre Agostini and Ferenc Krausz “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”.
When announcing the results, the Nobel Committee called it the prize “for electrons and flashes of light”, to which one could also add “coherence and beats”. 1 attosecond is equal 10-18of second. This is the time scale in which the electron cloud moves during chemical reactions. Therefore, the research methods awarded this year are (and will be) increasingly important in the analysis of chemical reactions at the level of changes in the electronic structure of single molecules.
To understand the groundbreaking nature of the awarded research, we need to go back in time more than 70 years. In 1964, the Nobel Prize in Physics was awarded to Charles Townes, Nikolai Bas and Alexander Prokhorov for their fundamental work in the field of lasers and masers, without which we would not have the prize in 2023. Interestingly, in the first years, attempts were made to obtain radiation properties completely opposite to those awarded the Nobel Prize this year. Scientists tried to extend the time of laser pulses so that their radiation energy (wave frequency) was as narrow as possible. This led to further discoveries that Nobel Prizes in 1997, 2001, 2005, 2012 and 2018 followed. It is worth recalling that the 2018 Nobel Prize winner, prof. Donna Strickland, visited the University of Silesia last year.
Generating short light pulses
What do we need to remember to understand the problem of generating light pulses at a specific time? The basis here is the relationship between two widths: the extension of the pulse in the time domain (its duration) and its energy width (popularly speaking, the number of colours that make it up). The fewer colours there are in the light, the longer the individual light pulses are, like in the case of first lasers. If we want to create a shorter light pulse, the wider it must be in the energy domain (it must consist of more colours).
And here a problem arises – how to force a laser, which in its ideal and original version produces an energetically narrow line (one colour), to produce a rainbow or a series of frequencies? Well, it can be done in several ways.
The first one is the use of organic dyes (e.g. rhodamine)which produce a very broad spectrum of light when exposed to a classic laser. Thanks to them, we can reduce the pulse duration to approximately 1 picosecond, namely 10-12 s.
The second method is based on the ‘non-ideality’ of lasers, which was an obstacle in the first years of research. The essence of ‘non-ideality’ was the generation of several or a dozen frequencies (modes) by lasers, which had to be superimposed on each other in a strictly defined way, the so-called mode locking. This allowed the construction of lasers whose energy spectrum had the shape of a comb, achieving a pulse time of 1 femtosecond, namely 10-15 s. (In 2005 John L. Hall and Theodor Hänsch were awarded Nobel Prize for the frequency comb).
At our university femtosecond lasers are used, among others, for the study of short-lived molecular states. Unfortunately, going below femtoseconds seemed impossible due to the still too small energy width of the light pulses.
This is where this year’s Nobel Prize winner Anne L’Huillier enters the scene. In 1988 she published a paper describing how powerful infrared laser pulses affect the gas stream. The essence of her research was to determine the possibility of generating waves with higher energy (so-called Harmonic Generation), i.e. energies 2, 3 or more times greater than the basic energy. It turned out that using a laser with a wavelength of 1064 nm, an energy of about 1.1 electronvolts (eV) and a pulse time of 10 picoseconds, she obtained waves with energies as much as 21 times higher. At the same time, waves with energies multiplied by subsequent odd numbers 19, 17, 15, etc. also appeared. So it wasn’t a continuous energy spectrum, but a spectrum with a very wide energy range. This technique was called High Harmonic Generation (HHG) and it was for this discovery that part of the prize was awarded.
In 2001, the second laureate, Pierre Agostini, entered the scene. He presented a method based on the fact that the waves created in the HHG method are coherent with each other, that is, they can interfere (overlap constructively) along with the original radiation used to produce them. Why is such a complicated method needed? It turns out that the combination of at least two adjacent harmonics (e.g. the seventeenth and nineteenth) and the original light pulse shifted in time allows for the extraction of electrons from the argon atom, which would be impossible if one of the components was missing (the so-called modulated two-photon transition, three-colour ionisation).
Examining the dependence of the energy of ejected electrons on the time shift between the waves allowed us to reconstruct the time structure of the pulses, which were approximately 250 attoseconds wide. This is how the technique called Reconstruction of Attosecond Beating by Interference of Two-photon Transitions (RABBITT) was developed. The second part of the prize was awarded for this discovery.
At the same time, Ferenc Krausz’s group worked on applications of the HHG method. He developed a technique that used a single pulse, in which an electric field stretched a light wave to spread the momentum distribution of electrons in a molecule. The name of the method (streaking) comes from stretching the narrow distribution of momentum in the atom into a line (streak). (Please do not ‘google’ the term streaking, because you will get a description of a different phenmenon). This method is also based on superimposing an infrared wave pulse with attosecond pulses, which knock electrons out of the tested molecule or atom. In this way, we obtain information about the distribution of momentum (in popular words, speed) of electrons in a molecule or atom. This is the third discovery awarded this year.
A comment should be added here about the nature of the electron and how the idea of using femtosecond pulses is often explained. Popular representation of this topic leads to statements such as: “Scientists can see where the electrons are located in atoms. Is this possible, and what about Heisenberg’s uncertainty principle?” This question also appeared at the Nobel conference and was addressed to prof. L’Huillier. Contrary to the presentation shown during this year’s Nobel Prize in Physics gala electrons aren’t balls placed in circular orbits and moving around in circles. Quantum mechanics only allows us to determine the distribution of probabilty of finding an electron with a specific momentum, and this is what is measured by the method called streaking.
So where does the question about the position of electrons come from? Well, spectroscopy in the far ultraviolet (XUV) range and even soft X-ray radiation (500 eV) allows to compare the electron momentum distribution calculated using quantum mechanical methods with the measurement results. Thanks to this, we can theoretically calculate the possible paths of the chemical reaction in which electrons are transferred and then compare them with the experiment. This allows us to eliminate inappropriate theoretical models, or confirm their correctness.
Thanks to the research of Prof. Huillier we can also study the time structure of electron emission processes through measurements of delays on the attosecond scale with which electrons are ejected from specific electron shells of elements.
So what is stopping us from the widespread use of these methods to study electron cloud motion in any molecules? The excitation of electrons in specific atoms is related to ‘tuning’ to the characteristic energies of their electron shells e.g. for oxygen the energies will be 18.2 eV, 41.6 eV or 543 eV. Unfortunately, the use of infrared lasers (basic energy 1.1 eV) limits us to only a few elements whose excitation falls within the range of the generated harmonic waves.
Can this limitation be overcome? A continuous spectrum of high-intensity radiation is obtained in synchrotrons, for example in Polish Solaris. Unfortunately, synchrotron radiation isn’t coherent on the time scale necessary to generate attosecond pulses, which limits its use.
The best source of coherent radiation in this range are currently free electron lasers (X-Ray Free Electron Lasers, XFEL), which produce coherent light in the X-ray range. Accelerator centres such as SLAC at Stanford and the European XFEL centre in Hamburg are becoming leading global centres in the field of research on the electronic structure of molecules on a time scale of 10-18s. It is worth adding that the construction of the Polish XFEL centre is planned at the National Centre for Nuclear Research in Otwock.
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