Bending modes metrology beyond 12 μm

Bending vibrational modes are responsible for a relevant fraction of molecular absorption spectra in the mid-infrared region at wavelengths beyond 12 μm. To date, the potential of this region for molecular fingerprinting, high-resolution spectroscopy and frequency metrology has been severely hampered by the lack of widely tunable single-mode lasers, with salt-diode lasers almost disappeared, nonlinear laser sources barely delivering optical powers in excess of few hundreds nanowatt [1] and commercial quantum-cascade-lasers (QCLs) hardly emitting beyond 12 μm in cw mode [2] . Recently, direct frequency comb spectroscopy has conquered such region and showed impressive capabilities to acquire ultrabroad high-resolution spectra till 16.7 μm [3] . Nevertheless, spectroscopic data of high metrological quality have not been reported yet, neither spectra with high quality factor in terms of signal-to-noise ratio per spectral point and number of spectral points per spectral feature.