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.
as written by the authors arXiv:2602.13907
The optical frequency comb has made a significant impact in precision spectroscopy and on our ability to probe atomic, molecular and, recently, nuclear transitions to further our understanding of their fundamental properties and how their dynamics and complex interactions affect the observed world. To expand the energy scales and types of systems that can be studied, frequency comb sources from terahertz to vacuum ultraviolet frequencies and beyond have been pursued. Dual-comb spectroscopy, enabled by the development of these frequency comb sources, allows broadband absorption measurements of complicated spectra, exceeding the limitations of direct, single-comb spectroscopy. To date, however, the dual-comb approach has not been able to directly access many important transitions that lie at challenging vacuum ultraviolet wavelengths. Here, we demonstrate dual-comb spectroscopy in the vacuum ultraviolet utilizing intracavity high harmonic generation. This multi-harmonic source is used to measure molecular absorbance spectra at $$\lambda=210$$~nm and $$\lambda=149$$~nm from room-temperature samples of acetylene and ammonia, respectively. These measurements resolve the Doppler broadened structure of congested molecular spectra with absolute frequency accuracy. Noise contributions to the vacuum ultraviolet dual-comb spectroscopy measurements are characterized, guiding future efforts and technological development in this region.
Acetylene is the gas that burns in welding torches at over 3000 °C. Its molecule absorbs ultraviolet at 210 nanometers, and now this 'fingerprint' can be captured with precision to billionths of a part.
In 'Star Trek,' a tricorder identifies any substance in seconds. Frequency combs in the ultraviolet are a step toward such devices: they already distinguish molecules by their unique absorption lines.