Measurement and assignment of $J$ $\ge$ 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

Abstract

Accurate models of high temperature methane spectra are needed in astrophysics. Previous measurements of methane hot-band transitions in the $P$6 $\leftarrow$ $P$2 polyad range have been limited to final rotational numbers of $J$ $\le$ 9, with theoretical predictions at higher $J$s remaining unvalidated. Here, we use optical-optical double resonance spectroscopy (OODR) with a 3.3 $\mu$m narrow linewidth pump to excite the $\nu$${}_3$ P(12, A${}_1$${}^{(2)}$) methane transition ($P$2 $\leftarrow$ $P$0) and a cavity-enhanced frequency comb centered around 1.68 $\mu$m to probe the sub-Doppler ladder-type ($P$6 $\leftarrow$ $P$2) and V-type ($P$4 $\leftarrow$ $P$0) transitions, as well as Doppler-broadened collision-induced four-level transitions ($P$6 $\leftarrow$ $P$2). 49 ladder-type transitions with final rotational states $J$ = 10-12 in the range of 9510 to 9810 cm${}^{-1}$ (i.e., the $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., the $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 WKLMC and HITRAN line lists. 170 Doppler-broadened four-level double-resonance (4LDR) lines were observed, 7 of which were newly observed 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.