  This manuscript describes the development of an ultrafast (i.e.,femtosecond),
mid-infrared, laser-absorption diagnostic and its initial application to
measuring temperature, CO, and CH$_4$ in flames. The diagnostic employs a
Ti:Sapphire oscillator emitting 55-fs pulses near 800 nm which were amplified
and converted into the mid-infrared (mid-IR) though optical parametric
amplification (OPA) at a repetition rate of 5 kHz. The pulses were directed
through the test gas and into a high-speed mid-infrared spectrograph to image
spectra across a $\approx$30 nm bandwidth with a spectral resolution of
$\approx$0.3 nm. Gas properties were determined by least-squares fitting a
spectroscopic model to measured single-shot absorbance spectra. The diagnostic
was validated with measurements of temperature, CO, and CH$_4$ in a static-gas
cell with an accuracy of 0.7% to 1.8% of known values. Single-shot, 5 kHz
measurements of temperature and CO were acquired near 4.9 $\mu$m in a
laser-ignited HMX (i.e., 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane) flame and
exhibited a 1-$\sigma$ precision of 0.4% at $\approx$2700 K. Further, CH$_4$
and temperature measurements were acquired near 3.3 $\mu$m in a partially
premixed CH$_4$-air flame produced by a Hencken burner and exhibited a
precision of 0.3% at $\approx$1000 K.
