[ad_1]
For his important work in transforming our understanding of gravity and spacetime, Albert Einstein won his only Nobel Prize for explaining the photoelectric effect. In the early 20th century, physicists discovered that when light is shone on a metal, it emits a few electrons. Interestingly, they found that the kinetic energy of the emitted electrons depends on the frequency of the incoming rays, not the intensity.
In 1905, Einstein explained this effect by proposing that light is made up of particles called photons. When a photon has energy above a certain limit, it is capable of ejecting an electron from a metal.
This effect is at the core of solar power: solar cells are specially engineered materials whose electrons can be destroyed by photons in sunlight. The electrons are made to flow through a wire to produce an electric current.
Better understanding the photoelectric effect could help us create new, more efficient solar cells and shed more light on the physics that produces the effect. Because it involves the electronic properties of matter, a clear theoretical understanding of it means that physicists can use it to reveal subatomic features that are inaccessible to other probes. Inspired by these opportunities and advances in electronics and optics in the post-war era, physicists took their studies to new heights in the 20th century.
a fleeting light
An important tool for studying the photoelectric effect has been ultrashort light pulses. Just last year, three physicists received the Nobel Prize in Physics for their contributions to developing such pulses.
A simple analogy illustrates their usefulness. The quality of images captured by a camera depends, among other things, on how long the light-sensitive surface is exposed to light. If a camera is to capture an image of a flying bird’s wing, its exposure must be shorter than the time it takes for the wing to move a short distance. If the exposure is longer, the wings will look blurry.
Similarly, physicists attempt to generate very short waves of light that illuminate an atom or molecule when pointing a sensitive camera at it. The shorter the wave, the more short-lived phenomena the camera can capture. Physicists found that they could study the physics of some heavy atomic nuclei using femtosecond waves of light. A femtosecond is just 10-15 Seconds.
Last year’s Nobel laureates developed a way to generate attosecond pulses – each pulse lasting about 10 seconds.-18 seconds long — needed to study electrons, which move around rapidly.
Design of molecules
Over the past decade, researchers have used attosecond pulses to study the photoelectric effect on smaller and smaller time scales. One focus area has been the photoionization delay: the time lapse between a reference event and when an electron is ejected. As two German physicists write in an article 2016 review In Physics,
“The duration of the ionization delay provides important information about the electronic structure of matter. These delays arise from the interactions of electrons with their environment, usually in the form of a potential representing the electronic structure of the molecule. Measuring such delays can thus shed light on the details of the potential of the motion of electrons, which can help us develop and validate theoretical models for molecules. Such advances could eventually open the door to controlling matter at its most fundamental level, allowing scientists to design molecules with desired electronic behaviour.”
For example, in 2010Ferenc Krausz, one of the 2023 laureates, led a team that discovered a 20-attosecond delay between two electrons leaving two nearby energy levels in a neon atom, even though they do not leave at the same time as expected. Researchers from the Autonomous University of Madrid Reported on June 20 This year the assumption that the nucleus of an atom moves much slower than its electrons is not justified for nuclear effects on matter. Instead, they found that the motion of the nucleus in just a few attoseconds can “substantially increase” the photoionization delay of electrons leaving the atom in H2, Molecule.
Design of molecules
In a new study Published on 21 AugustResearchers at the California-based SLAC National Accelerator Laboratory reported an unexpectedly large delay in the photoemission of electrons from oxygen and nitrogen atoms in nitric oxide (NO) molecules. The team’s innovation involved creating a device that could produce photons with the energy needed to eliminate core electrons, i.e., non-valence electrons that do not participate in chemical reactions, in an attosecond-physics setup.
“Our work is the first measurement of photoemission delays in the X-ray regime. Previous pioneering experiments have measured photoemission delays in the ultraviolet regime, but not the X-ray regime. When X-rays interact with matter, the most likely outcome is the removal of a core-level electron,” SLAC physicists and three co-authors of the result, James Cryan, Agostino Marinelli and Taran Driver, wrote in an email. the hindu“On the other hand, ultraviolet light only has enough photon energy to free some weakly bound electrons.”
They found that the core electrons in oxygen are emitted 700 attoseconds after their counterparts in nitrogen, rather than being emitted at the same time. Their paper attributed this delay to “multiple contributions”, including an emitting electron being ‘trapped’ by a potential energy barrier in the molecule called the shape resonance, colliding with another electron ejected by the atom – called an Auger-Meitner electron – and “multi-electron scattering effects”.
Mountains on the way
The results echo those of 2016 study in which another research group investigated the photoionization delay in water and nitrous oxide (N2O) molecule. The researchers wrote in their paper: “In the case of N2o, our measurements … reveal a surprisingly large delay, reaching 160 attoseconds … In contrast, the delay measured at the same photon energy in H2O are all below 50 attoseconds in magnitude.” Based on complex modeling and analysis, they were able to attribute the delay to N2O figure for the constraint imposed by resonance.
The components of the nitric oxide or nitrous oxide molecule exert electric and magnetic fields depending on their charges. The electron ejected by the photoelectric effect has to pass through these fields before it can escape the molecule completely. However, sometimes the electron does not have enough potential energy to cross them and becomes trapped – like a tired traveler surrounded by mountains.
Size resonance occurs when the electron’s wavelength is comparable to the size over which the trapping potential extends. If their energies are also comparable, the electron is likely to remain trapped for a long time by resonating with the trapping potential. The electron can escape by gaining more energy to cross the mountains or in some other way when the trapping potential decays. Quantum physics also gives the electron a small but non-zero chance of tunneling through the barrier. In every case, the result is a delay in the photoionization of the molecule.
“The photoemission delay we see in the X-ray regime is much larger than this previous measurement,” the trio said of the 2016 paper. “This is the result of a few effects.” One is that they used nitric oxide whereas the older experiment used nitrous oxide, “and the photoemission delay is very sensitive to molecular structure.” Another reason is that “the electrons involved in X-ray photoionization are typically highly correlated, and we found that overall this results in a larger photoemission delay.”
The third reason is the Auger-Meitner effect. When a core-level electron is removed from an atom, a higher-energy electron can drop down and fill this vacancy. Its extra energy is transferred to a valence electron that moves out of the atom as an Auger-Meitner electron. When these electrons “meet the electrons whose delays we were measuring, they pulled the electrons back a little and further increased the photoemission delay.”
‘I couldn’t even imagine it’
According to Cryan, Marinelli and Driver, their new work “advances our fundamental understanding of X-ray-matter interactions, which is particularly interesting for a few reasons. One notable reason is that the core electrons released by X-ray photoionization have strong interactions with other electrons in the molecule.”
He added that these interactions are “relevant to many applications, including the imaging of proteins and viruses that takes place right here at SLAC and at synchrotrons and X-ray free-electron lasers around the world.” “In making these measurements, we are also developing new experimental methods to probe electron correlation in real-world systems. Electron correlation is critical to defining and tuning the fundamental properties of matter, and a better understanding of this ubiquitous phenomenon will ultimately help us gain a deeper understanding of important biochemical reactions and choose new materials for next-generation electronics.”
As the trio put it: “Most of the research we do is basic, ‘blue-sky’ science, driven by the belief – supported by ample historical evidence – that reliably studying the fundamental behaviour of the universe yields practical applications that we could not have imagined before we began our research.”
The authors thank Adhip Agrawal, Assistant Professor of Physics at IIT Kanpur, for his feedback.
[ad_2]
Source link