Hydrocarbon molecule hit with laser pulses.

A hydrocarbon molecule’s reaction to laser pulses is captured for posterity. Credit: Matz Liebel

Optics and photonics

How to spy on a single molecule

Short laser bursts help to provide a real-time view of a molecule’s spectrum.

Observing the light that molecules emit and absorb, called spectra, is key to understanding chemical reactions. Now, physicists have found a way to observe spectral changes in an individual molecule at a scale of quadrillionths of a second.

To probe the spectrum of a single molecule, scientists usually rely on observing the photons that it emits, known as fluorescence. But fluorescence occurs too slowly for reactions to be monitored over time.

Matz Liebel and Niek van Hulst at the Institute of Photonic Sciences in Barcelona, Spain, and their colleagues used short laser pulses to interrupt a chemical reaction in molecules of the hydrocarbon dibenzoterrylene. By manipulating characteristics of the pulse, the researchers were able to observe the molecule’s spectrum through its fluorescence. They also varied the timing of the pulse to observe the reaction at many different points — and so build up a picture of the changing spectrum.