Measurement and assignment of J ≥ 10 rotational energy levels in the 9510 to 9810 cm−1 and 6590 to 6900 cm−1 ranges of methane using optical frequency comb double-resonance spectroscopy

Accurate models of hot-band methane spectra are needed in astrophysics. Previous measurements of methane hot-band transitions in the P 6← P 2 polyad range have been limited to final rotational numbers of J ≤ 9, with theoretical predictions at higher J remaining unvalidated. We use optical-optical double resonance spectroscopy (OODR) with a 3.3 µm narrow linewidth pump to excite the ν 3 P(12, A 1 (2) ) transition ( P 2← P 0) and a cavity-enhanced frequency comb centered around 1.68 µm to probe sub-Doppler ladder-type ( P 6← P 2) and V-type ( P 4← P 0) transitions, as well as Doppler-broadened collision-induced four-level transitions ( P 6← P 2). 49 ladder-type transitions with final rotational states J = 10-12 in the range of 9510 to 9810 cm −1 (i.e., P 6 polyad) were assigned to effective Hamiltonian predictions and the ExoMol database, of which 6 reached vibrational states that had not been observed experimentally before. 19 sub-Doppler V-type transitions with final states J = 11-13 in the range of 6590 to 6900 cm −1 (i.e., P 4 polyad) were observed and assigned to the Hamiltonian and ExoMol, while only 2 of these V-type transitions could be unambiguously assigned to the WKLMC and HITRAN line lists. 170 Doppler-broadened four-level double-resonance (4LDR) lines were observed, 7 of which were newly observed lines compared with our previous work when pumping transitions starting from the J = 7 level in the ground state [Lehmann et al ., J. Chem. Phys. 163 , 144304 (2025)]. We could not assign these lines as they did not form combination differences with other observed 4LDR transitions.