  Frequency combs have revolutionized the field of optical spectroscopy,
enabling researchers to probe molecular systems with a multitude of accurate
and precise optical frequencies. While there have been tremendous strides in
direct frequency comb spectroscopy, these approaches have been unable to record
high resolution spectra on the nanosecond timescale characteristic of many
physiochemical processes. Here we demonstrate a new approach to optical
frequency comb generation in which a pair of electro-optic combs is produced in
the near-infrared and subsequently transferred with high mutual coherence and
efficiency into the mid-infrared within a single optical parametric oscillator.
The high power, mutual coherence, and agile repetition rates of these combs as
well as the large mid-infrared absorption of many molecular species enable
fully resolved spectral transitions to be recorded in timescales as short as 20
ns. We have applied this approach to study the rapid dynamics occurring within
a supersonic pulsed jet, however we note that this method is widely applicable
to fields such as chemical and quantum physics, atmospheric chemistry,
combustion science, and biology.
