# Absolute frequency determination of molecular transition in the Doppler regime at kHz level of accuracy

## TL;DR

This paper reports a highly precise measurement of a molecular transition frequency in oxygen using Doppler-limited spectroscopy, achieving accuracy comparable to Doppler-free methods by referencing an optical atomic clock.

## Contribution

It demonstrates the first absolute frequency measurement of the P7 P7 O₂ B-band transition at kHz accuracy using a cavity ring-down spectrometer and optical frequency comb.

## Key findings

- Achieved relative uncertainty of 2×10⁻¹¹ in frequency measurement.
- Demonstrated Doppler-limited spectroscopy at accuracy levels typical for Doppler-free techniques.
- Referenced measurements to an optical atomic clock for high precision.

## Abstract

We present absolute frequency measurement of the unperturbed P7 P7 O$_2$ B-band transition with relative standard uncertainty of $2\times10^{-11}$. We reached the level of accuracy typical for Doppler-free techniques, with Doppler-limited spectroscopy. The Doppler-limited shapes of the P7 P7 spectral line were measured with a frequency-stabilized cavity ring-down spectrometer referenced to an $^{88}$Sr optical atomic clock by an optical frequency comb.