  Improving spectral acquisition rate of broadband mid-infrared spectroscopy
promises further advancements of molecular science and technology. Unlike the
pump-probe spectroscopy that requires repeated measurements with different
pump-probe delays, continuous spectroscopy running at a high spectral
acquisition rate enables transient measurements of rapidly changing
non-repeating phenomena or statistical analysis of a large amount of spectral
data acquired within a short time. Recently, Fourier-transform infrared
spectrometers (FT-IR) with rapid delay scan mechanisms including dual-comb
spectrometers have significantly improved the measurement rate up to ~1
MSpectra/s that is fundamentally limited by the signal-to-noise ratio. Here, we
overcome the limit and demonstrate the fastest continuous broadband vibrational
spectrometer running at 80 MSpectra/s by implementing wavelength-swept
time-stretch spectroscopy technique in the mid-infrared region. Our
proof-of-concept experiment of the time-stretch infrared spectroscopy (TS-IR)
demonstrates broadband absorption spectroscopy of phenylacetylene from 4.4 to
4.9 {\mu}m (2040-2270 cm-1) at a resolution of 15 nm (7.7 cm-1) with a superior
signal-to-noise ratio of 85 without averaging and a shot-to-shot fluctuation of
1.3%.
