Reinvestigation of the ν₂ + 2ν₃ subband in the overtone icosad of ¹²CH₄ using cavity ring-down (CRD) spectroscopy of a supersonic jet expansion

Abstract

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.

Keywords:

• high-resolution spectroscopy
• cavity ring-down spectroscopy
• supersonic jet expansions
• infrared
• methane CH₄
• nuclear spin symmetry

Acknowledgement

Andrés Laso, Edi Peyer and Andreas Schneider provided essential help with building the new experimental setup. Martin Suter contributed towards early experiments in the present work 40. We are grateful for discussions and exchange of data with Sieghard Albert, Sigurd Bauerecker, Hans-Martin Niederer and other members of our group. We also enjoyed discussions and correspondence on theory with Tucker Carrington and Xiao-Gang Wang and on experiments as well as the earlier results with Michael Hippler. Our joint projects with the Dijon groups of Vincent Boudon and the Tomsk groups of Oleg Ulenikov provided important motivation for the present work. Alain Campargue informed us about recent remeasurements of the 80 K spectra up to the icosad of methane by laser spectroscopy 39, previously measured and published using FTIR spectroscopy 22. Our work is supported financially by the Schweizerischer Nationalfonds and ETH Zürich.