Methane diode laser overtone spectroscopy at 840 nm

Overtone absorption lines of 12CH_4 have been examined by using a tunable diode laser (TDL) spectrometer in the region around 11900 cm^-1 (840 nm) where the combination overtone band v1+3v3 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 2nd harmonic detection technique permitted the measurements of minimum absorbances of the order of ~5x10^-6. This allowed to observe the weakest lines of the band with absorption cross sections of the order of ~2x10^-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.