We demonstrate Doppler broadening thermometry (DBT) with all-frequency-domain measurements. Using the R(10) transition of CO at 1567 nm in a high-finesse optical cavity (mode width 0.6 kHz), we resolve Doppler profiles with high signal-to-noise ratios across 2–17 Pa pressures. A global Voigt-profile analysis yields temperatures deviating by only $-2.0\pm 3.6\mathrm{mK}$ from calibrated thermometers, with systematic errors suppressed below 9 ppm. The results show negligible dependence on line-shape models when accounting for pressure effects, resolving a long-standing challenge in DBT. This approach establishes a new paradigm for quantum-based thermometry and provides a precision platform for testing molecular collision physics.

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• Chemical Physics & Physical Chemistry
• Metrology
• Molecular spectra
• Thermodynamics
• Molecules
• Cavity resonators
• Optical absorption spectroscopy