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$^{-1}$\mathrm{/}amagat\mathrm{.}Forsomeofthemself-,air-,He-andH$_2$-broadening coefficients have been obtained.