High sensitivity CW-cavity ring down spectroscopy of CH 4 in the 1.55 μm transparency window

Abstract

The high resolution absorption spectrum of methane has been recorded at room temperature by high sensitivity CW-cavity ring down spectroscopy between 1.50 and 1.60 μm (6250–6680 cm −1). This spectral region has a low opacity and is then of particular interest for the spectral analysis of the outer planets and Titan. The recordings were performed with a sensitivity corresponding to a noise equivalent absorption of α min ∼ 5 × 10 −10 cm −1 and allowed for the detection of lines with an intensity smaller than 10 −28 cm/molecule far beyond the weakest lines provided by the HITRAN database. The spectrum exhibits a highly complex and congested structure with a kind of ‘fractal’ character and a density of transitions of a few tens per cm −1. The impact of the weak newly observed transitions on the total absorption in the region is discussed.