For all its possibilities, nature tends to replay one particular scene over and over again: the confrontation between matter and light.
It stages the scene in a practically infinite number of ways, but in the most familiar versions, light kick-starts a physical process that begins when a photon hits an atom or molecule. In photosynthesis, photons from the sun strike chlorophyll molecules in a plant to knock electrons loose, setting off the chemical conversion of carbon dioxide and water into sugar and oxygen. When you get a sunburn, photons of ultraviolet light strike and damage DNA molecules in your skin. You’ll find the process in technology, too, such as in solar panels, where silicon atoms arranged in a crystal convert photons from the sun into a flow of electrons that generate electric power.
But physicists still don’t know the details of what happens when photons meet atoms and molecules. The play-by-play occurs over attoseconds, which are quintillionths of a second (or 10-18 of a second). It takes a special laser that fires attoseconds-long pulses to study such ephemeral phenomena. You can think of the length of a laser pulse a bit like the shutter speed of a camera. The shorter the pulse, the more clearly you can capture an electron in motion. By studying these moments, physicists gain more understanding of a fundamental process ubiquitous in nature.
Last month, physicists at multiple academic institutions in China published results in Physical Review Letters showing that they measured the time it took an electron to leave a two-atom molecule after it had been illuminated with an extremely bright and short infrared laser pulse. While a two-atom molecule is relatively simple, their experimental technique “opens up a new avenue” to study how light interacts with electrons in more complex molecules, the authors wrote in the paper. (They did not agree to an interview with WIRED.)
In the experiment, the researchers measured how long it took for the electron to depart the molecule after the photons from the laser hit it. Specifically, they discovered that the electron reverberated back and forth between the two atoms for 3,500 attoseconds before it took off. To put that into perspective, that is a quadrillion times faster than the blink of an eye, which takes a third of a second.
To keep time in this experiment, the researchers tracked a property of the light known as its polarization, says physicist Alexandra Landsman of the Ohio State University, who was not involved in the study. Polarization is a property of many types of waves, and it describes the direction that they oscillate. You can think about polarization by imagining an ocean wave. The direction in which the wave crests and dips is its polarization direction—it is both perpendicular to the surface of the water and perpendicular to the direction in which the wave travels.
A light wave is an oscillation in the electromagnetic field, or the force field that permeates all space and pushes or pulls on electric charges. When light travels through a space, it oscillates this field, causing the strength of the force field to go up and down perpendicular to its direction of travel, like the ocean wave. The light’s polarization describes the direction that the field oscillates. When light polarized in a particular direction hits an electron, it will toggle that electron back and forth in parallel with that direction.
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In this experiment, the researchers engineered the laser light’s polarization to rotate steadily, as though the crests and dips of the electromagnetic field were a corkscrew spiraling through space. That rotation could also track time, like the second hand of a clock. They assumed that, as the laser pulse illuminated the molecule, the electron started to leave it when the pulse peaked in brightness. At that peak intensity, the light would be polarized in a particular direction, according to the sweep of the wave as it rotated. By comparing the angle of the polarized beam to the angle at which the electron was ejected from the molecule, they could measure how long it took for an electron to leave the molecule. Physicists refer to this laser timing technique as the “attoclock” method, as it is capable of measuring durations on the attosecond scale.
The attoclock not only kept time during the experiment, but it also supplied the photons that knocked the electron loose from the molecule. Roughly speaking, you can think of the electron in orbit around an atomic nucleus as being similar to the moon in orbit around Earth. Earth pulls the moon around using gravitational attraction, while the positively charged nucleus pulls the negatively charged electron around because of electrical attraction. If a powerful enough object hits the moon, it can knock it onto a different path, or out of Earth’s orbit completely. Similarly, if photons hit an electron, they could knock that electron into a different orbit—or out of orbit altogether.
But unlike Earth and the moon, electrons and photons obey the rules of quantum mechanics. According to these rules, an electron can only travel along specified trajectories, known as orbitals, which are spaced at discrete distances. In theory, you could nudge the moon to orbit Earth from any number of possible distances, giving you a continuous range of options. But you can’t do that with an electron. You have to hit it with enough energy to knock it into one of the allowed trajectories. Hit the electron with anything less, and it stays put in its original orbital.
This time, the researchers used a molecule consisting of an argon and a krypton atom. This is an unlikely pairing in nature, as argon and krypton don’t like to bond to other atoms. “The krypton and argon are only very loosely attached to each other,” says physicist Joachim Burgdörfer of the Vienna University of Technology in Austria, who was not involved with the work. But this made aspects of the experiment easier, says Burgdörfer. Because they were loosely bonded, they were relatively far apart, which made it easier to pinpoint which atom the electron is associated with at a given moment.
The researchers first knocked an electron from the krypton atom, so the molecule was positively charged. Then, for the actual measurement, they timed the departure of an electron that was originally orbiting the argon atom. After the electron encountered the laser pulse, it moved in a figure-8-like orbit around the argon and krypton atoms.
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In that way, it built on quantum mechanics principles first discovered in the 20th century, because the path of the electron around the argon-krypton molecule shares similarities with a crude model that pioneering researcher Wolfgang Pauli developed for a positively charged hydrogen molecule with one electron, says Burgdörfer. Pauli’s model predicted that the electron should trace a figure-8 pattern around the two atoms, as the electron does in this experiment.
The experiment also adds to the growing body of knowledge about the interaction between light and matter that physicists have collected over the last decade and a half. One pioneering experiment in Germany in 2010 used the attoclock technique to compare how quickly an electron could depart a neon atom from two different orbitals. They found that the electron departed the atom 20 attoseconds later from one orbital than the other. Prior to the invention of the attosecond laser, physicists had no stopwatch precise enough to discern the difference, so many had assumed that the neon atom ejected the electron instantaneously, regardless of the orbital. Since then, physicists have timed attosecond-scale processes of photons impinging on a single helium atom, for example, or a piece of solid nickel.
By studying these super fast processes, physicists hope to eventually be able to control them—and potentially exploit them—for new technologies. In the future, this research could help scientists control chemical reactions to design new types of synthetic molecules or to develop faster electronics technology, says Landsman. But first, it may help us better understand how the same fundamental building blocks give rise to the complex universe before us.