  We report molecular detection using dual frequency-comb spectroscopy with
highly coherent broadband (6.6-11.4 {\m}m) long-wavelength infrared (LWIR)
combs. The combs were produced via intra-pulse difference frequency generation
(IDFG) in ZGP crystals using sub-three-cycle (20 fs) driving pulses from
mode-locked Cr:ZnS lasers at the central wavelength of 2.4 {\m}m). Real-time
and up to video rate (0.1-12 s per spectrum) acquisition of molecular spectra
with some 240,000 comb-mode-resolved data points spaced by 80 MHz and
referenced to a Rb clock with a signal-to-noise ratio (SNR) >300 has been
demonstrated. The key to achieving such a high rate of massive spectral data
acquisition is a low phase and intensity noise of the LWIR combs and excellent
mutual coherence. The high SNR was also facilitated by the high (7.5%) IDFG
conversion efficiency resulting in an average LWIR comb power of 300 mW per
channel.
