Diode laser spectroscopy of ammonia and ethylene overtones

Some overtone absorption lines of ammonia and ethylene have been examined by using a tunable diode laser (TDL) spectrometer in the region around 12650 and 11800 cm^-1 respectively. The spectrometer sources are commercially available double heterostructure InGaAlAs and AlGaAs TDLs operating in the "free-running" mode. The high resolving power (~10^7) of the spectrometer permitted the detection and the study of the line positions of such molecules with a precision better than 0.01 cm^-1. In order to maximize the signal to noise ratio and to extract the necessary informations either on the line width and on the line position for the detected molecular resonances, the wavelength modulation spectroscopy (WMS) along with the 2nd harmonic detection techniques have been applied. For this purpose the fitting procedure took into account the instrumental effects and the amplitude modulation (AM) always associated with the frequency modulation (FM) of these type of sources. This technique permitted also the measurement of the collisional broadening and shifting coefficients by different buffer gases at room temperature.