Diode laser overtone spectroscopy of methane at 780 nm

A tunable diode laser spectrometer has been employed to examine the overtone absorption lines of $^{12}$CH$_4$ in the region around 12780 cm$^{-1}$ (780 nm) at room temperature. The spectrometer sources are commercially available tunable diode lasers operating in "free-running" mode. The measurement of the line positions within 0.01 cm$^{-1}$ has been possible by the use of the wavelength modulation spectroscopy and the second harmonic detection techniques that permitted the observation of minimum optical absorbances of the order of $\approx 10^{-6}$. The weakest observed lines of the band have absorption cross sections of the order of $1 \times 10^{-25}$ cm$^2$/molecule, that is $\simeq 0.3$ km$^{-1}$/amagat. For some of them self-, air-, He- and H$_2$-broadening coefficients have been obtained.