Precise determination of ro-vibrational transition frequencies of molecules are interested in various studies such as molecular dynamics, metrology, astronomy, and fundamental physics. However, the accuracy of most line positions in the near infrared is limited to 10^-3 - 10^-4 cm^-1 (~ 1 MHz) due to broadening (Doppler and collision induced) and/or the weakness of overtone transitions. Here we introduce cavity-enhanced spectroscopy methods we developed for precision spectroscopy of molecules. Using high-finesse cavities, we demonstrate a detection sensitivity (noise-equivalent absorption coefficient) of 10^-12/cm and a frequency accuracy of 1 kHz [1]. Two-color double resonance (DR) spectroscopy using continuous-wave diode lasers [2] is also demonstrated, which allows us to probe highly-excited states of molecules with unprecedented accuracy. A few application examples will be given, including kHz-accuracy measurements of the (30012) and (60025) bands of CO₂ [3], and the unexpected “distorted” line profile of the Lamb-dip spectrum of HD [4,5].

[1] Wang et al., Communication: Molecular near-infrared transitions determined with sub-kHz accuracy. J Chem Phys 2017, 147, 091103.

[2] Hu et al., Comb-locked cavity-assisted double resonance spectroscopy based on diode lasers, Review of Scientific Instruments, 2021, 92, 073003.

[3] Hu et al., Optical-optical double-resonance absorption spectroscopy of molecules with kilohertz accuracy, Journal of Physical Chemistry Letters, 2020, 11, 7843.

[4] Tao et al., Toward a determination of the proton-electron mass ratio from the Lamb-dip measurement of HD, Phys Rev Lett, 2018, 120, 153001.

[5] Hua et al., Dispersion-like lineshape observed in cavity-enhanced saturation spectroscopy of HD at 1.4 um, Optics Letters, 2020, 45, 4863.