Spectroscopic isotope ratio measurement of doubly-substituted methane

We have determined the isotope ratios of 13CH3D /12CH3D as well as 13CH3D /12CH4 with an uncertainty of ∼20‰ using a sensitive difference-frequency-generation spectrometer in the 3.4 μm region. Cooling the absorption cell drastically reduces overlap of the 13CH3D lines with the other abundant isotopic lines, and the extensive tunability of the spectrometer enables us to access transition pairs appropriate for isotope ratio measurements. We measure the intensity of the 13CH3D ν4RP (7, 0) line, the 12CH3D 2ν5RQ (9, 0) line, and the 12CH4ν2+ν4 R(10) F2 line at 2953 cm−1 as well as the 13CH3D ν4PP (6, 3) line and the 12CH4ν3 P (6) E line at 2955 cm−1 because they have similar transition frequencies, absorption strengths, and low state energies.

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► Isotope ratio of 13CH3D to 12CH3D and 12CH4 with uncertainty of ∼20‰.
► Cooling of absorption cell for reduction of overlap among absorption lines.
► The widely tunable spectrometer enables us to access appropriate transitions.