Diode laser overtone spectroscopy of methane at 780 nm

TL;DR

This study uses a tunable diode laser spectrometer to precisely measure overtone absorption lines of methane at 780 nm, revealing weak lines and broadening effects relevant for atmospheric and spectroscopic applications.

Contribution

First application of diode laser spectroscopy to methane overtone lines at 780 nm with high precision and detection sensitivity.

Findings

Line positions measured within 0.01 cm$^{-1}$

Weakest lines have absorption cross sections around 10$^{-25}$ cm$^2$/molecule

Broadening coefficients for various gases obtained

Abstract

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.