Spectroscopic isotope ratio measurement of doubly-substituted methane

Abstract

We have determined the isotope ratios of (13)CH(3)D /(12)CH(3)D as well as (13)CH(3)D /(12)CH(4) 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 (13)CH(3)D 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 (13)CH(3)D ν(4)(R)P (7, 0) line, the (12)CH(3)D 2ν(5)(R)Q (9, 0) line, and the (12)CH(4)ν(2)+ν(4) R(10) F(2) line at 2953 cm(-1) as well as the (13)CH(3)D ν(4)(P)P (6, 3) line and the (12)CH(4)ν(3) P (6) E line at 2955 cm(-1) because they have similar transition frequencies, absorption strengths, and low state energies.