An ordinary comb parts hair into even strands. A laser 'comb' does the same to light: it emits not one color but thousands, spaced at exactly equal frequency steps. Such a tool has already helped measure time to billionths of a second and peer inside atoms.

But the comb has a limit: it struggles in the deep ultraviolet — where light is so energetic it knocks electrons out of molecules. And it's precisely there that many substances (ammonia, acetylene, water) have their key 'fingerprints.' To reach that region, scientists placed the comb directly inside a laser cavity, a chamber where light bounces between mirrors and gets amplified. Inside the cavity, the light passes through a gas and generates harmonics — waves with frequencies several times higher than the original. Thus, a single infrared comb became a whole fan of ultraviolet combs.

Using two such combs shining simultaneously, the authors measured how room-temperature samples of acetylene and ammonia absorb light at 210 and 149 nanometers. The resulting spectra revealed fine structure that is normally smeared out by the thermal motion of molecules (the Doppler effect — the same one that changes the pitch of a passing ambulance siren). The measurement accuracy was absolute in frequency: each line's position is known to better than one part per billion.

Now spectroscopists have a reliable tool for the most stubborn molecules. We'll be able to study more precisely how chemical reactions proceed in planetary atmospheres, in flames, or in interstellar clouds — places where conventional methods fail in the deep ultraviolet.