Methane diode laser overtone spectroscopy at 840 nm

Abstract Overtone absorption lines of 12 CH 4 have been examined by using a tunable diode laser (TDL) spectrometer in the region around 11 900 cm - 1 (840 nm) where the combination overtone band ν 1 + 3 ν 3 lies. The spectrometer sources are commercially available heterostructure GaAlAs TDLs operating in the “free-running” mode, which allowed the detection of the line positions within 0.01 cm - 1 . The wavelength modulation spectroscopy (WMS) and the second harmonic detection technique permitted the measurements of minimum absorbances of the order of ≃ 5 × 10 - 6 . This allowed to observe the weakest lines of the band with absorption cross-sections of the order of ≃ 2 × 10 - 25 cm 2 /molecule or ≃ 0.5 km - 1 /amagat. For some of them self-, air-, He- and H 2 -broadening coefficients have been obtained at room temperature.