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.
Substantial help from and discussions with Sieghard Albert and Irina Bolotova are gratefully acknowledged. We also enjoyed useful exchange on the methane spectra with Hans Martin Niederer, Sigurd Bauerecker, Vincent Boudon, Tucker Carrington, Michael Rey, Elena Bekhthereva, Olga Gromova and Oleg Ulenikov. Andreas Schneider and André Laso provided important technical help. Our work is supported financially by ETH Zürich, the Schweizerischer Nationalfonds für Förderung der wissenschaftlichen Forschung and the European Research Council by an Advanced Grant. Our work is also to be seen in the context of the COST project MOLIM. The research leading to these results has also received funding from the European Research Council under the European Union's Seventh Framework Program (FP7, 2007–2013) ERC Grant No 290925.
This paper is dedicated to Jürgen Troe on the occasion of his 75th birthday, remembering numerous discussions on methane kinetics and dynamics during more than four decades.
©2015 Walter de Gruyter Berlin/Boston