Assignment of collision-induced four-level double-resonance transitions in the 3$ν$${_3}$ ${\Leftarrow}$ $ν$${_3}$ spectral region of methane

Abstract: Optical-optical double-resonance (OODR) spectroscopy using a narrow-linewidth pump and a frequency comb probe has previously been used to measure and assign sub-Doppler transitions in the 3${\nu}$${_3}$ ${\Leftarrow}$ ${\nu}$${_3}$ spectral region [J. Chem. Phys. 161, 124311 (2024)] when pumping from the J = (7, A${_2}$) ground state. Doppler-broadened double-resonance transitions were also observed in those OODR spectra. In this paper, 68 of these Doppler-broadened transitions are assigned to four-level double-resonance transitions involving collisional transfer from the pumped A${_1}$ symmetry state to other A${_1}$ and A${_2}$ symmetry (I = 2 meta nuclear spin) levels of the ${\nu}$${_3}$ fundamental state. Assignments are made using combination differences and comparison with the term values and intensities of lines predicted by a new effective Hamiltonian, the accuracy of which has been validated by the sub-Doppler transitions.