Cavity-Enhanced Comb-Based Double-Resonance Spectroscopy of High Rotational Energy Levels in the 9070–9370 cm⁻¹ Range of Methane

Accurate assignments of highly excited molecular ro-vibrational states are needed for the verification of theoretical predictions of high-temperature spectra observed e.g. in astrophysics. Optical-optical double-resonance (OODR) spectroscopy using a continuous wave (CW) pump and a cavity-enhanced comb probe is a new tool for broadband, sensitive and selective detection and assignment of sub-Doppler hot-band molecular transitions [1]. It allows determination of term values and rotational assignment of highly excited molecular states, providing unique reference data for verification of theoretical predictions. We have previously used this methods for detection and assignment of transitions in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3\mathrm{v}_{3}\leftarrow \mathrm{v}_{3}$</tex> range of methane, reaching levels with rotational quantum numbers, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$J$</tex>) up to 4 in the underexplored 9000 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> range of the P6 (triacontad) polyad [1]. Testing the predictions for higher rotational levels is important, because i) these states dominate the spectra at high temperatures, and ii) variational calculations, on which the state-of-the-art databases for astrophysical applications are based, may suffer from a lack of convergence for these levels.