Reinvestigation of the ν2 + 2ν3subband in the overtone icosad of12CH4using cavity ring-down (CRD) spectroscopy of a supersonic jet expansion

We report a detailed reinvestigation of the ν2 + 2ν3 combination band of methane 12CH4 centred at (7510.3378 ± 0.003) cm−1 ((225.154263 ± 0.0001) THz) within the icosad of the overtone absorption. A new experimental setup is described, allowing us to carry out cw-laser cavity ring-down spectroscopy (cw-CRDS) at instrumental resolution in the MHz range in seeded supersonic jet expansions down to rotational temperature of 7 K compared to previous cw-CRDS measurements in our group achieving about 50 K in expansions of neat CH4. We provide a careful re-analysis on the basis of our new experimental results for the Q and R branch transitions including data obtained between about 7 and 300 K under various conditions. We resolve previously observed discrepancies of assignments and are able to present a definitive assignment for lines involving angular momentum quantum numbers up to J = 4. The analysis of relative intensities in spectra taken at rotational and effective translational temperatures between about 50 K and less than 10 K indicate conservation of nuclear spin symmetry upon supersonic jet expansion, in agreement with previous results using other techniques and covering other spectral ranges.