Cutting-Edge Physics and Chemistry Unfold One Quintillionth of a Second at a Time

Scientific American

08-07-2025

Just about anybody who played hide-and-seek as a kid remembers counting, with eyes (presumably) covered, in units of one-one-thousand. 'One-one-thousand. Two-one-thousand. Three-one-thousand.' It's one way to develop a feel for the duration of a second. If you live to be 80 years old, you will experience 2,522,880,000 seconds, not any one of which feels like a long time. When you think about time, it's usually in many-second durations, like minutes, days and years. Unless you become a world-class athlete where differences measured in tenths, hundredths and maybe even thousandths of seconds can mean winning or losing Olympic gold, you might not think intervals shorter than a second are worth a second thought.
But what if you allow yourself to imagine what happens in the world at ever shorter time intervals? What if you had a temporal microscope for zooming in on time the way optical, electron and scanning tunneling microscopes let you zero in on ever finer spatial dimensions, even down to the atomic scale?
Welcome to the world of a cadre of scientists, some of them Nobel Prize winners, who live in the fastest science lane possible right now — the realm of attoseconds. By leveraging the evolution of laser science and technology, they have trained their attention on molecular, atomic and electronic behavior of ever finer temporal durations — from millionths (micro) to billionths (nano) to trillionths (pico) to quadrillionths (femto) to quintillionths (atto) of seconds.
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It's in the attosecond-by-attosecond time frame that lots of the sausage of physics and chemistry is made and can be probed. It is where light and electrons do much of the blindingly fast negotiation by which the energy they have to give and take redistributes as they interact. These are temporal realms that set the stage for many chemistry antics: things like electrons shifting between excited higher-energy states and lower-energy states and molecules morphing from reactants into products. In these instants, a chemical ring might open, an electron might fly away leaving a positively charged ion behind, or a photon might beam outward carrying spectroscopic intel that helps scientists figure out what just happened. These are the hidden micromatters that contribute to everything from photosynthesis in leaves to the photophysical basis of vision and the bond-making-and-breaking that underlies the multi-trillion-dollar chemical industry.
To those who wield state-of-the-art laser systems and light detectors to capture glimpses of the exquisitely fast happenings in these tiny contexts, even a microsecond or nanosecond can seem like an awfully long time. When you can watch molecules and reactions in attosecond time frames, 'there's this vast other space that is open to you,' says Stephen Leone, a physical chemist at the University of California, Berkeley, who recently chronicled his lifelong research adventure as an 'attosecond chemist' in an autobiographical essay in the Annual Review of Physical Chemistry. With short-enough pulses, he says, you can begin to observe the very movements of electrons that underlie the breaking or making of a chemical bond.
Here is what one attosecond looks like when you write it out: 0.000000000000000001 s. That's a billionth of a billionth of a second. An oh-wow factoid that attosecond aficionados sometimes roll out is that there are as many attoseconds in one second as there have been seconds ticking since the Big Bang. One tick on your kitchen clock amounts to an eternity of attoseconds. Here's another head-shaking attosecond fact: In one attosecond, light — which moves at the incomprehensible sprint of 186,000 miles per second — travels the span of a single atom.
Attoseconds are a natural time frame for atoms and their electrons, says John Gillaspy, a research physicist at the National Institute of Standards and Technology and former program director of atomic, molecular and optical experimental physics at the National Science Foundation. 'When you think about an electron orbiting a nucleus like a little planet moving around the Sun,' he says, 'the time scale for the orbit is about 1 to 1,000 attoseconds.' (He concedes that he often defers to this early 20th century metaphor for atoms because, he says in a spirit of commiseration, 'if you try to envision them quantum mechanically, you're liable to get quite confused and disturbed.')
To do attosecond science, you might start with a top-line femtosecond laser that produces millionths-of-billionths-of-a-second infrared pulses. Then, to produce even shorter-wavelength attosecond laser pulses, you likely will need a pulse-shortening technique, called high harmonic generation (HHG), which won some of its developers the 2023 Nobel Prize in physics.
Leone has put such tools and techniques to use in what are called pump-probestudies. These have two main parts. First, he and his team might vent a gas of, say, krypton atoms or methane molecules into the pathway of laser pulses. These pulses carry the photons that will interact with electrons in the sample particles. Then the scientists direct attosecond laser pulses at the sample at different delay times after the initial pulse, taking pains to measure the electromagnetic signals or electrons that emerge. The attosecond-precise monitoring of these signals can amount to a stop-motion movie of electrons, atoms or molecules.
In deep chemistry speak, Leone lists some of the attosecond- and femtosecond-fast shifts in electronic energy states and behavior that such techniques have opened to observations in unprecedented detail: chemical bond breaking, yes, but also more subtle yet influential energetic happenings that can thwart reactions or nudge molecules to change shape. These are phenomena in which theory has long outpaced experimental data. These subtler actions include 'curve crossings' and 'conical intersections,' which are terms reflective of the mathematical and geometric depictions of the energy-constrained behavioral 'choices' electrons have to make in atoms and molecules. Does this or that electron hold on to enough energy to cause a bond to break? Or does it vent that energy within the molecule or material more gently to elicit, say, a vibration between bonded atoms, or morph the molecule's shape from one isomer to another?
These secret, on-the-fly choices made by electrons leave their traces all over in our biology and could have practical applications — such as repairing broken chromosomes, detecting diseases from chemical hints in the molecular brew of our blood, or engineering laser pulses to produce never-before-seen molecules. 'We didn't understand any of this detail previously and now, I think, it has come into much greater clarity,' Leone says. It suggests ways to elicit specific electronic motions that one needs to break this or that bond or to cause a desired reaction, he adds.
The hushed, darkened labs of these laser-wielding experimentalists have an otherworldly feel. A typical centerpiece is a vibration-suppression table with surfaces as still as any place on Earth. Painstakingly aligned there are miniature Stonehenges of lenses and crystal elements that shift, split and recombine laser beams, compress or expand light pulses, and impart tiny delays into when pulses reach samples and detectors. Feeding into these optical pathways are the ultrashort laser pulses and, downstream, the sample atoms and molecules (supplied from nozzles attached to gas tanks or from heated crystals). Much of these setups must reside in steampunk-esque vacuum chambers so that air molecules don't sop up the precious data-bearing light or electron signals before they can make it to detectors and spectrometers.
'It's all a very complicated camera to produce some of the shortest events in time that humans can produce,' says theoretical chemist Daniel Keefer of the Max Planck Institute for Polymer Research in Mainz, Germany, coauthor of a 2023 article in the Annual Review of Physical Chemistry on the applications of ultrafast X-ray and HHG for probing molecules.
Keefer's primary tasks include calculating for experimentalists the laser-pulse energies and other conditions most suitable for the studies they plan to do, or helping them infer the electronic behavior in molecules hidden in the spectroscopic data they collect in the lab. But as elementary as these studies can be, some of the phenomena he has studied are as relevant to everyone as keeping their genes intact and functioning.
'It's all a very complicated camera to produce some of the shortest events in time that humans can produce.' —Daniel Keefer
Consider that the combination of ultrafast laser pulses and spectroscopic observation empowered him and colleagues to better understand how some of the celebrity molecules of biology, RNA and DNA, manage to quickly dissipate enough of the energy of incoming ultraviolet photons to prevent that energy from wreaking gene-wrecking, photochemical damage. It comes down to the way electrons within the molecules can benignly vent the UV energy by going back to their lowest-energy orbitals.
'This is one mechanism by which potential photodamage is prevented in living organisms exposed to sunlight,' Keefer says. These genetic molecules 'absorb UV light all the time and we're not having a lot of photodamage because they can just get rid of the energy almost instantaneously, and that greatly reduces the risk of your DNA breaking.'
Accelerating into the fastest lane
To generate attosecond laser pulses, scientists first ping a gas of atoms with an infrared laser. The laser beam gives a kick to every atom it passes, shaking the electrons back and forth in lockstep with its infrared light waves. This forces the electrons to emit new light waves. But they do so with overtones, the way a guitar string vibrates at not only a fundamental frequency but also a range of higher-frequency harmonic vibrations, or acoustic overtones. In the case of infrared laser light, the overtones are at much higher frequencies in the attosecond range, which correspond to ultraviolet or even X-ray wavelengths.
That's a huge bonus for attosecond scientists. When packed into supershort pulses, light of these wavelengths can carry sufficient energy to cause electrons to migrate within a molecule's framework. That influences how the molecule will react. Or the laser pulses can coerce electrons to leave the scene entirely, which is one of the ways atoms and molecules become ionized.
Gillaspy says that when he thinks of attosecond pulses of light, and yet-shorter pulses in the future (which would be measured in zeptoseconds), his science dreams diverge from spying on the private lives of electrons and toward what becomes possible by packing more energy into ever shorter pulses. Do this, Gillaspy says, and the power confined in the pulse can amplify, albeit ever so briefly, to astronomical levels. It's akin to the way a magnifying glass can concentrate a dull, palm-sized patch of sunlight into a pinpoint of brilliant sunlight that can ignite a piece of paper.
Concentrate enough laser power into a short-enough pulse, Gillaspy says, and you might gain access to the quantum vacuum, by which he means the lowest possible energy state that space can have. The quantum vacuum has only been indirectly measured and it sports a generous share of weirdness. Presumably, for example, the 'nothingness' of that vacuum actually seethes with 'virtual' matter-antimatter particle pairs that poof into and out of existence by the bazillions, in slices of time even faster than attoseconds.
'If you could get the laser intensity strong enough you might rip apart the virtual particles from each other in the quantum vacuum and make them real' — which is to say, observable, says Gillaspy. In other words, it could become possible to separate, detect and measure the members of those transient pairs of virtual particles before they annihilate each other and disappear back into the vacuum. 'This is where we could be ripe for fundamental discoveries,' Gillaspy says — although for now, he notes, the capability to produce the required laser intensities remains far off.
Jun Ye, a physicist at JILA, a joint research center of the University of Colorado and the National Institute of Standards and Technology, is deploying attosecond physics in pursuit of another believe-it-or-not goal. He intends to tap HHG to detect that mysterious cosmic stuff known as dark matter.
Despite never having directly detected dark matter in everyday life or in a laboratory, scientists presume its existence to make sense of the distribution and motions of matter on galactic scales. Without the presence of dark matter — in far more abundance than ordinary matter — and its cosmic-scale gravitational influences, the universe would literally look and behave differently. If the theory is true, a tantalizing consequence is that dark matter — whatever it is — should be abundantly present all around us here on Earth and so should be, in principle, detectable in a lab.
Ye is hoping to exploit HHG physics to develop a type of energy-measuring technique, called nuclear spectroscopy, that is especially suited to discern subtle energy shifts in the nuclei of atoms. In this context, it's the multitude of wavelengths of light that HHG naturally produces that make this spectroscopic technique so revealing. This, Ye says, could enable him to monitor minute variations in regular-matter atoms that might be caused by previously unknown interactions with dark matter.
At the heart of his plan is a new type of clock, a nuclear clock, that he and colleagues at JILA and elsewhere have been developing. The ticks of these clocks are based on nuclear oscillations (in the bundle of neutrons and protons in thorium-229 nuclei) rather than the electronic oscillations atomic clocks have been based on.
'If the dark matter out there interacts with regular matter, then potentially it will interact with neutrons and protons in atomic nuclei differently than with electrons,' Ye says. And if that is so, comparisons of spectroscopy data from the two types of clocks stand a chance of finally unveiling a dark matter influence on normal matter that has been in operation all along.
'This is how a lot of things start,' says Gillaspy. 'Breakthroughs can start with physicists and chemists just getting fascinated by some new thing, like attosecond phenomena, and then . . . you never know. You don't even imagine what kind of capabilities are going to arise from that.'