Diode laser spectroscopy of carbon dioxide at 780 nm and methane at 840 nm

Overtone absorption lines of carbon dioxide and methane have been examined by using a diode laser (DL) spectrometer in the region around 12800 and 11900 cm^-1 respectively. The spectrometer uses commercially available double heterostructure InGaAlAs and AlGaAs DLs sources operating in "free-running" mode. In order to detect these very weak "overtone" features the sensitivity of the apparatus is increased by the use of the wavelength modulation spectroscopy (WMS) with the phase detection technique and a long optical path length obtained by using a Herriott type measurement cell. By this way the minimum detected absorption coefficient was 1.4x10^-8 cm^-1. These techniques permitted also the measurement of some collisional broadening and shifting coefficients by different buffer gases at room temperature. For this purpose a fit procedure that takes into account the instrumental effects has been adopted. The resolving power (~10^7) of the spectrometer allowed the detection of the absorption resonance positions with a maximum error of 0.01cm^-1. This let us obtain the CO_2 band parameters with a good precision.