For his monumental work in transforming our understanding of gravity and spacetime, Albert Einstein won his sole Nobel Prize for explaining the photoelectric effect. In the early 20th century, physicists found that when a metal is irradiated with light, it emits some electrons. Curiously they found the emitted electrons’ kinetic energy depended on the incoming rays’ frequency, not intensity.

In 1905, Einstein explained the effect by proposing that light is made of particles called photons. When a photon possesses more energy than some threshold, it is able to kick an electron in the metal out.

This effect is at the heart of solar power: solar cells are specially engineered materials whose electrons can be knocked out by the photons in sunlight. The electrons are made to flow through a wire to produce an electric current.

Understanding the photoelectric effect better could help us make new, more efficient solar cells and shed more light on the physics that produces the effect. Because it involves the material’s electronic properties, its clear theoretical understanding means physicists can use it to reveal subatomic features that are inaccessible to other probes. Motivated by these opportunities and advances in electronics and optics in the post-war era, physicists took their studies to new highs in the 20th century.

A fleeting light

One important tool to study the photoelectric effect has been the ultrashort light pulse. Just last year, three physicists won the physics Nobel Prize 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 the amount of time for which a photosensitive surface is exposed to light. If the camera has to capture an image of the wings of a bird in flight, its exposure needs to be shorter than the time taken for a wing to move by a short distance. If the exposure is longer, the wings will look blurred.

Similarly, physicists try to produce very short pulses of light that illuminate an atom or a molecule while a sensitive camera is pointed at it. The shorter the pulse, the more short-lived event the camera can capture. Physicists found they could study the physics of some heavy atomic nuclei using femtosecond pulses of light. One femtosecond is just 10^-15 seconds.

Last year’s Nobel laureates developed a way to generate attosecond pulses — each pulse is around 10^-18 seconds long — required to study electrons, which move around faster.

Designing molecules

In the last decade, researchers have used attosecond pulses to study the photoelectric effect at shorter and shorter timescales. One focus area has been the photoionisation delay: the time lapsed between some reference event and when an electron is knocked out. As two physicists from Germany wrote in a 2016 review in Physics:

“The length of ionisation delays provides important information on the electronic structure of matter. These delays arise from the interactions of electrons with their environment, typically in the form of a potential representing the molecule’s electronic structure. Measuring such delays can thus shed light on the details of the potentials in which electrons move, which can help us develop and validate theoretical models for molecules. Such advances could ultimately open the door to controlling matter at its most fundamental level, enabling scientists to design molecules with desired electronic behavior.”

For example, in 2010, Ferenc Krausz — one of the 2023 laureates — led a team that discovered a 20-attosecond delay between two electrons leaving two close energy levels in a neon atom, rather than leaving at the same time as expected. Researchers from the Autonomous University of Madrid reported on June 20 this year that the assumption that an atom’s nucleus is too slow-moving compared to its electrons for nuclear effects to matter is not well-founded. Instead, they found the nucleus’s motion in just a few attoseconds could “substantially increase” the photoionisation delay of electrons leaving an atom in an H₂⁺ molecule.

Designing molecules

In a newer study published on August 21, researchers from the SLAC National Accelerator Laboratory, California, reported an unexpectedly large delay in the photoemission of electrons from oxygen and nitrogen atoms in nitric oxide (NO) molecules. The team’s innovation included building a device that could produce photons with the energy required to knock off core electrons, i.e. non-valence electrons that don’t participate in chemical reactions, in an attosecond-physics setup.

“Our work is the first measurement of the photoemission delay in the X-ray regime. Previous pioneering experiments have measured the photoemission delay 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 of the result’s coauthors James Cryan, Agostino Marinelli, and Taran Driver wrote in an email to The Hindu. “Ultraviolet light, on the other hand, only has enough photon energy to release the less weakly bound electrons.”

They found core electrons in oxygen were emitted up to 700 attoseconds after their counterparts in nitrogen, rather than emerging at the same time. Their paper attributed this delay to “several contributions”, including a leaving electron being ‘trapped’ by a potential energy barrier in the molecule called a shape resonance, by colliding with another electron ejected by the atom — called the Auger-Meitner electron —, and “multi-electron scattering effects”.

Mountains in the way

The results echo those of a 2016 study in which another research group examined photoionisation delays in water and nitrous oxide (N₂O) molecules. The researchers wrote in their paper: “In the case of N₂O, our measurements … reveal surprisingly large delays reaching up to 160 attoseconds… In contrast, delays measured at the same photon energies in H₂O all lie below 50 attoseconds in magnitude.” Based on complicated modelling and analysis, they were able to attribute the delay in N₂O to a barrier imposed by the shape resonance.

The constituents of a molecule of nitric oxide or nitrous oxide exert electric and magnetic fields depending on their charges. An electron knocked out by the photoelectric effect needs to pass through these fields before it can completely exit the molecule. Sometimes, however, the electron may not have enough potential energy to overcome them and becomes trapped — like a tired hiker being surrounded by mountains.

A shape resonance occurs when the electron’s wavelength is comparable to the size over which the trapping potential is spread. If their energies are comparable as well, the electron is likely to be trapped for longer, resonating with the trapping potential. The electron can escape by acquiring more energy to surmount the mountains or if the trapping potential decays by some other means. Quantum physics also allows the electron a small but non-zero chance of tunnelling through the barrier. In every case, the result is a delay in the molecule’s photoionisation.

“The photoemission delays we observe in the X-ray regime are significantly larger than [in] this previous measurement,” the trio said of the 2016 paper. “This is a 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 is because “the electrons involved in X-ray photoionisation are particularly highly correlated, and we have found that overall this results in larger photoemission delays.”

A third reason is the Auger-Meitner effect. When a core-level electron is removed from an atom, a higher energy electron may drop down and fill this vacancy. Its excess energy is transferred to a valence electron that exits the atom as the Auger-Meitner electron. When these electrons “caught up with the electrons whose delay we were measuring, they dragged the electrons back a little and increased the photoemission delay some more.”

‘Could not have imagined’

According to Cryan, Marinelli, and Driver, their new work “furthers our fundamental understanding of X-ray-matter interactions, which are particularly interesting for a few reasons. One notable reason is that the core electrons released by X-ray photoionisation have strong interactions with the other electrons in the molecule.”

These interactions “are relevant in many applications, including the imaging of proteins and viruses that takes place right here at SLAC, and around the world at synchrotrons and X-ray free-electron lasers,” they added. “In making these measurements, we are also developing new experimental methods to probe electron correlation in real-world systems. Electron correlation is critical for 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: “So much of the research we perform is basic, ‘blue-sky’ science, powered by the conviction — which is backed up by ample historical evidence — that studying the fundamental behaviour of the universe reliably produces practical applications, which we could not have imagined before beginning the research.”

The author thanks Adhip Agarwala, assistant professor of physics at IIT Kanpur, for his feedback.

Scientists at the world’s largest physics experiment have reported the most precise measurement yet of the most massive subatomic particle we know. The finding sounds esoteric but it wouldn’t be an understatement to say it has implications for the whole universe.

The Greek philosopher Empedocles surmised 2,400 years ago that matter could be broken up into smaller and smaller pieces until we’re left with air, earth, fire, and water. Since the early 20th century, physicists have broken up matter into smaller and smaller pieces to find many different subatomic particles instead — as many as to fill a zoo.

The top quark

Rather than a ‘smaller’ particle, contemporary particle physicists are concerned with elusive particles.

More energetic particles often break down into ones with less energy. The greater the difference in energy between that of a particle and the products of its decay, the less time the particle exists in its original form and more quickly it breaks down. By the mass-energy equivalence, a more massive particle is also a more energetic particle. And the most massive particle scientists have found to date is the top quark.

It is 10-times heavier than a water molecule, about three-times as much as a copper atom, and 95% as much as a full caffeine molecule.

As a result, the top quark is so unstable that it could break up into lighter, more stable particles in less than 10⁻²⁵ seconds.

The top quark’s mass is very important in physics. A particle’s mass is equal to the sum of masses contributed from multiple sources. An important source for all elementary particles is the Higgs field, which pervades the entire universe. A ‘field’ is like a sea of energy and excitations in the field are called particles. This way, for example, an excitation of the Higgs field is called the Higgs boson just as an electron can be considered to be an excitation of an ‘electron field’.

All these fields engage with each other in specific ways. When the ‘electron field’ interacts with the Higgs field at energies much less than 100 GeV, for example, the electron particle will acquire some mass. The same thing goes for other elementary particles. (GeV, or giga-electron-volt, is a unit of energy used in the context of subatomic particles: 1 joule = 6.24 billion GeV.) Elucidating this mechanism won François Englert and Peter Higgs the 2013 physics Nobel Prize.

If the top quark is the most massive subatomic particle, it is because Higgs bosons interact most strongly with it. By measuring the top quark’s mass as precisely as possible, then, physicists can learn a lot about the Higgs boson as well.

“Physicists are intrigued by the top quark mass as there is something peculiar about it,” Nirmal Raj, particle theorist and assistant professor at the Indian Institute of Science, Bengaluru, told The Hindu. “On the one hand, it is the one closest to the Higgs boson’s mass, which is what one would ‘naturally’ expect before measuring it. On the other, all other [particles like it] are much, much lighter, making one wonder if the top quark is actually an oddball, not a ‘natural’ species.”

The universe as we know it

But the rabbit hole goes deeper.

Physicists are keen to study the Higgs boson also because of its own mass, which it acquires by interacting with other Higgs bosons. Importantly, the Higgs boson is more massive than expected — which is to say the Higgs field is more energy-laden than expected. And because it pervades the universe, the universe can be said to be more energetic than expected. This ‘expectation’ comes from calculations physicists have performed and they don’t have reason to believe they are wrong. Why does the Higgs field have so much energy?

Physicists also have a theory as to how the Higgs field originally formed (at the birth of the universe). If they are right, there is a small yet non-zero chance that one day in future, the field could go through a sort of self-adjustment that reduces its energy and modifies the universe in drastic ways.

They know the field has some potential energy today and there is a way it could shed some of it to have less and become more stable. There are two ways to get to this stable state. One is for the field to gain some energy first before losing it and more, like climbing one side of a mountain to get into a deeper valley on the other side. The other is if an event called quantum tunnelling happens, whereby the field’s potential energy would ‘tunnel’ through the mountain instead of having to climb over it and drop into the valley yonder.

This is why Stephen Hawking said in 2016 the Higgs boson could spell the “end of the universe” as we know it. Even if the Higgs field is slightly stronger than it is now, the atoms of most chemical elements will be destroyed, taking stars, galaxies, and earthlife with them. But while Hawking was technically correct, other physicists quickly said the frequency of the tunnelling event was 1 in 10¹⁰⁰ years.

The Higgs boson’s mass — 126 GeV/c² (a unit used for subatomic particles) — is also just about enough to keep the universe in its current state; anything else and the “end” would happen. Such a finely tuned value is obviously curious and physicists would like to know which natural processes contribute to it. The top quark is part of this picture by virtue of being the most massive particle, in a sense the Higgs boson’s closest friend.

“Measuring the top quark mass precisely has implications for whether our universe will tunnel out of existence,” Dr. Raj said.

Finding the top quark

Physicists discovered the top quark in 1995 at a particle accelerator in the US called the Tevatron, measuring its mass to be 151-197 GeV/c². The Tevatron was shut down in 2011; physicists continued to analyse data it had collected and updated the value three years later to 174.98 GeV/c². Other experiments and research groups yielded more precise values over time. On June 27, physicists at the Large Hadron Collider (LHC) in Europe reported the most precise figure yet: 172.52 GeV/c².

Measuring a top quark’s mass is difficult when its lifetime is around 10^-25 seconds. Typically, a particle-smasher will produce an ultra-hot soup of particles. If a top quark is present in this soup, it will quickly decay into specific groups of lighter particles. Detectors look out for these events, and when they happen track and record their properties. Finally, computers collect this data and physicists analyse them to reconstruct the physical properties of the top quark.

Scientists learn what to expect at each point of this process based on sophisticated mathematical models and must contend with many uncertainties. Many of the devices used in these machines also incorporate state of the art technologies; when engineers improve them further, the physicists results also improve that much.

Now researchers will incorporate the top quark’s mass measurement into calculations that inform our understanding of our universe’s particles. Some of them will use it to also quest for an even more precise value. According to Dr. Raj, precisely measuring the top quark’s mass is also key to knowing whether some other particle with mass close to that of the top quark could be hiding in the data.