The CH3D Absorption Spectrum Near 1.58 μm: Extended Line Lists and Rovibrational Assignments

Monodeuterated methane (CH₃D) contributes greatly to absorption in the 1.58 μm methane transparency window. The spectrum is dominated by the 3ν₂ band near 6430 cm^-1, which is observed in natural methane and used for a number of planetary applications, such as the determination of the D/H ratio. In this work, we analyze the CH₃D spectrum recorded by high-sensitivity differential absorption spectroscopy in the 6099-6530 cm^-1 region, both at room temperature and at 81 K. Following a first contribution to this topic by Lu et al., the room-temperature line list is elaborated (11,189 lines) and combined with the previous 81 K list (8962 lines) in order to derive about 4800 empirical lower-state energy values from the ratio of the line intensities measured at 81 K and 294 K (2<i>T</i>-method). Relying on the position and intensity agreements with the TheoReTS variational line list, about 2890 transitions are rovibrationally assigned to twenty bands, with fifteen of them being newly reported. Variational positions deviate from measurements by up to 2 cm^-1, and the band intensities are found to be in good agreement with measurements. All the reported assignments are confirmed by Ground-State Combination Difference (GSCD) relations; i.e., all the upper-state energies (about 1370 in total) have coinciding determinations through several transitions (up to 8). The energy values, determined with a typical uncertainty of 10^-3 cm^-1, are compared to their empirical and variational counterparts. The intensity sum of the transitions assigned between 6190 and 6530 cm^-1 represents 76.9 and 90.0% of the total experimental intensities at 294 K and 81 K, respectively.