Methane diode laser overtone spectroscopy at 840 nm

Overtone absorption lines of 12CH4 have been examined by using a tunable diode laser (TDL) spectrometer in the region around 11900cm-1 (840 nm) where the combination overtone band ν1+3ν3 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 0.01cm-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 cm2/molecule or ≃0.5km-1/amagat. For some of them self-, air-, He- and H2-broadening coefficients have been obtained at room temperature.