High sensitivity CW-cavity ring down spectroscopy of CH4 in the 1.55 μm transparency window

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. cordings were performed with a sensitivity corresponding to a noise equivalent absorption of αmin ∼ 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.