Absorption coefficients for the 727 nm band of methane at 77 K determined by intracavity laser spectroscopy

Abstract

Methane spectral features in the visible to near-IR region are prominent in the spectra of the outer planets but laboratory data for the appropriate methane conditions are required to interpret the observational data. By use of the intracavity laser spectroscopy technique, a moderately high resolution (≥500,000) absorption spectrum of the 727 nm band of methane at 77 K is obtained. The methane absorption bands in the visible to near-IR region are very weak, but intracavity laser spectroscopy provides sufficient sensitivity to perform the measurements and to extract quantitative data for methane at low temperatures. Absorption coefficients are determined and are reported as averages at one A intervals throughout the region 7127–7420 A. By integrating over the band, an intensity of 753 cm−1 km−1 am−1 is obtained. The results compare well with previous low resolution measurements on methane at room temperature, with gas phase results calculated using the absorption spectrum of liquid methane, and with absorption coefficients derived from methane features observed in the spectra of the outer planets and Titan.