New assignments, line intensities, and HITRAN database for CH ₃OH at 10 μm

The Fourier transform spectrum of CH₃OH in the 10μm region has been re-examined at higher pressure and path length than heretofore, as part of a program to provide comprehensive CH₃OH spectral data for astrophysical and atmospheric applications. With the increase in spectral sensitivity, it has been possible to assign new torsionally excited ν ₁₂ = 1 and ν₁₂ = 2 subbands plus further high-K, ν₁₂ = 0 subbands of the ν₈ CO-stretching band. Upper-state term values have been determined, and have been fitted to J(J+1) power-series expansions in order to obtain the excited ν₈ substate origins. A variety of weaker subbands from other modes has also been identified in the 10μm spectrum including ν₁₂ = 0, ν ₁₂ = 1, and ν₁₂ = 0←1 torsional subbands of the ν₇ in-plane CH₃ rock, ν₁₂ = 0←1 and ν₁₂ = 0←2 torsional combination subbands of the ν ₆ OH bend, and ν₁₂ = 0←2 subbands of the ν ₅ symmetric CH₃ bend. Line intensities have been retrieved line-by-line from the spectra. A large set of "unperturbed" ν₈ transitions has been modeled using the same type of multi-parameter effective Hamiltonian employed successfully for the ground state, with inclusion of the intensities of a subset of the stronger ν ₈ spectral lines in the fitting in order to obtain appropriate transition dipole terms. Together, a 10μm methanol database in HITRAN format has been generated.