How to Use a Laser to Kick an Electron out of a Molecule


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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