Physicists from Ludwig-Maximilians-Universit?t Munich (LMU) and the Max-Planck Institute for Quantum Optics (MPQ) use ultrashort laser pulses to detect photoelectron emissions in tungsten crystals dynamics. Almost a century ago, Albert Einstein won the Nobel Prize in Physics for his explanation of the photoelectric effect. Einstein's theory, published in 1905, included the idea that light is composed of particles called photons.
When light strikes a substance, the electrons in the sample react to the input energy, and this interaction creates what is known as the photoelectric effect. Light quanta (photons) are absorbed by the material and excite bound electrons. Depending on the wavelength of the light source, this may result in the ejection of electrons. The electronic band structure of the material in question has a strong influence on the time scale of light emission.
Physicists at Ludwig-Maximilians-Universit?t Munich (LMU) and the Max Planck Institute for Quantum Optics (MPQ) have now carefully studied the phenomenon of light emission . They measured the effect of tungsten's ribbon structure on the dynamics of photoelectron emission and provided a theoretical explanation for their observations.
This is now possible thanks to the development and continuous improvement of Attosecond technology. An "Atosecond" is one billionth of a second. The ability to reproducibly generate sequences of laser pulses lasting hundreds of attoseconds allows researchers to track the progression of light emission through periodic "freezing activity" - similar to a stroboscope, but with better temporal resolution.
In a series of photoelectron spectroscopy experiments, the team used attosecond pulses of extreme ultraviolet light to probe the light emission dynamics of tungsten crystals. Each pulse contains hundreds of X-ray photons, each with enough energy to displace a photoelectron. With the help of a detector mounted in front of the crystal, the team was able to characterize the ejected electrons in terms of flight time and emission angle.
The results showed that the electrons interacting with the incoming photons needed a moment to react to the encounter. The discovery was made by using a new method to generate attosecond pulses. Thanks to the introduction of a passive cavity resonator with an enhancement factor of 35, the new device can now generate attosecond pulses at a rate of 18.4 million times per second, approximately 1,000 times higher than previously common in similar systems. Because the pulse repetition rate is so high, only a few photoelectrons per pulse are enough to provide a high average flux.
"As negatively charged photoelectrons repel each other, their kinetic energy changes rapidly. In order to characterize their dynamics, it is important to distribute them over as many attosecond pulses as possible, "explained co-first author Dr. Tobias Saule. The increased pulse rate means that the particles have little chance to interact because they are well distributed in time and space, so the maximum energy resolution is largely preserved. In this way, the team was able to show that, in terms of the dynamics of light emission, electrons in adjacent energy states in the valence band (i.e., the outermost orbits of atoms in the crystal) with different angular momentum respond to the incoming photon. The reaction time also varies by tens of atto seconds.
Remarkably, the arrangement of atoms within the crystal itself has a measurable effect on the delay between the arrival of a light pulse and the ejection of a photoelectron. "Crystals are made up of many atoms with positively charged nuclei. Each nucleus is a source of electromotive force, which attracts negatively charged electrons - just like a round hole acts as a potential well for marbles," Stephan said Dr. Heinrich, co-first author of the report. "When an electron is removed from a crystal, what happens is a bit like the progress of a marble on a table with dimples."
The dimples represent the positions of individual atoms in the crystal, and they There is a pattern. For example, the trajectory of marbles is directly affected by their presence, and it differs from that observed on smooth surfaces. "It has now been demonstrated how this periodic potential within the crystal affects the temporal behavior of the light emission - and we can explain it theoretically," explains Stephan Heinrich. The observed delay can be attributed to the complex nature of electron transport from the interior of the crystal to the surface, as well as to interactions between the interior of the crystal and the surface.