  Historically, there has been a trade-off in spectroscopic measurements
between high spectral resolution, broadband coverage, and acquisition time.
Optical frequency combs, initially envisioned for precision spectroscopy of the
hydrogen atom in the ultraviolet region, are now commonly used for probing
molecular rotational-vibrational transitions throughout broad spectral bands in
the mid-infrared with superior accuracy, resolution and speed. Here we
demonstrate acquisition of 2.5 million spectral points over the continuous
wavelength range of 3.2 to 5.1 microns (frequency span 1200 cm^{-1}, resolution
<100 kHz) via interleaving comb-tooth-resolved spectra acquired with a
highly-coherent broadband dual-frequency-comb system based on optical
subharmonic generation. With the original comb-line spacing of 115 MHz,
overlaying spectra with gradually shifted comb lines we fully resolved
amplitude and phase spectra of molecules with narrow Doppler-broadened lines,
such as {\nu}1+{\nu}3 band of carbon disulfide (CS2) and its isotopologues.
Also, we were able to observe the chemical oxidation reaction of CS2 by
detecting traces of carbonyl sulfide (OCS) via its asymmetric {\nu}3 stretch
band.
