Abstract

We have investigated the ν₂ + 2ν₃ combination band of methane ¹³CH₄ centered at 7493.15918 cm^–1 within the icosad of the overtone absorption. The jet-CRD setup combining supersonic jet expansions and cavity ring-down spectroscopy which was already used for the reinvestigation of the same spectral region for the main isotope of methane (¹²CH₄) has been used to record spectra of the Q and R branches at room temperature as well as at very low temperatures (down to 4 K). Based on our previous temperature-dependent investigations and the present results, we provide a careful analysis and the assignment for lines involving angular momentum quantum numbers up to J = 4. The analysis of the relative intensities in spectra taken at various rotational and effective translational temperatures demonstrate conservation of nuclear spin symmetry for ¹³CH₄ under the conditions of a supersonic jet expansion, similar to our previous results regarding ¹²CH₄ and also to further results using other techniques and covering other spectral ranges. This is in agreement with theoretical expectation regarding very slow nuclear spin symmetry relaxation under these conditions in supersonic jet expansions.