Cavity-Trapping for Precision Molecular Spectroscopy
High-precision molecular spectroscopy is used in measurements of fundamental constants and in searches for new physics. The technique’s precision requires overcoming Doppler effects—spectral distortions caused by the motion of the molecules being probed. Researchers have now added a new method to the precision spectroscopy toolbox by realizing a decades-old proposal, based on trapping molecules in the standing wave created by a laser field in a cavity [1]. This confinement suppresses molecular motion along the cavity axis, dramatically mitigating Doppler-related broadening.
The experiment by Wim Ubachs at the Vrije Universiteit Amsterdam and collaborators was enabled by the recent progress of cavity-enhanced spectroscopic methods, whose exceptional sensitivity arises from the repeated interaction between molecules and laser light circulating multiple times within the cavity. The team further improved sensitivity by using novel laser-frequency modulation techniques that can filter out white noise in the measurements.
The researchers placed a gas of molecular hydrogen in a laser-illuminated cavity, probing a weak, infrared transition of the molecules. They observed exceptionally narrow absorption features, which were 4 orders of magnitude narrower than the expected Doppler broadening. Such features indicated that molecules were being trapped at the maxima of the intracavity light field, resulting in near-zero motion along the cavity axis. The analysis revealed that a detectable signal could be recorded even though only a small fraction of the molecules (one in 30,000) occupied trapped states.
The experiment was tailored to probe the spectrum of the hydrogen molecule—which offers an important benchmark of quantum electrodynamics calculations. The researchers note, however, that the technique could be extended to more complex molecules, including water, potentially becoming a broadly applicable tool for molecular spectroscopy.
–Matteo Rini
Matteo Rini is the Editor of Physics Magazine.
References
- W. Ubachs et al., “Letokhov-Chebotayev intracavity trapping spectroscopy of H2,” Phys. Rev. Lett. 135, 223201 (2025).