The intensity and pressure broadening of the 681.884 nm methane absorption line at low temperatures determined by intracavity laser spectroscopy

Abstract

The temperature dependence of the intensity and of the pressure broadened width of the 681.884 nm ro-vibrational absorption line in the weak 681.5 nm band of methane is studied experimentally using intracavity laser spectroscopy (ILS). These measurements are performed in the temperature range between 77 and 295 K using equivalent absorption pathlengths between 3.6 and 36 km. The collision-broadening data are measured for cases in which CH 4, N 2, H 2 and He are the collision partners at total pressures ranging between 100 and 500 torr. The intensity results obtained here are the first such data to be reported for the 681.884 nm line. An analysis of the line intensity data based upon their dependence on temperature suggests that the rotational quantum number J assignable to this line is 2. The 681.884 nm line examined here is found to be approximately 7 times weaker at ambient temperature than the 619.68 nm line studied previously using the ILS technique. The work presented here, therefore, extends the limit of detection sensitivity for quantitative spectroscopic applications of ILS in congested spectral regions by almost an order of magnitude. The significance of the results with respect to observational spectroscopy of planetary atmospheres is also discussed.