Abstract.Overtone absorption lines of acetylene in the regions around 12700 and 11800cm-1 have been examined by the use of tunable diode lasers in free-running mode. The diode laser emission wavelength was scanned around the gas resonances by simply sweeping its injection current, permitting a direct observation of the absorption line-shapes. Weak overtone absorption lines have been detected by using the wavelength modulation (WM) spectroscopy with 2nd harmonic detection technique and the collisional broadening and shift coefficients have been obtained. The high resolving power and accuracy of the spectrometer permitted a wavenumber error of less than 0.01cm-1. The correct interpretation of the absorption signals when detecting the second harmonic in the presence of a sloping background is discussed.
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