  Cavity-enhanced frequency comb spectroscopy for molecule detection in the
mid-infrared powerfully combines high resolution, high sensitivity, and broad
spectral coverage. However, this technique, and essentially all spectroscopic
methods, is limited in application to relatively small, simple molecules. Here
we integrate comb spectroscopy with continuous, cold samples of molecules
produced via buffer gas cooling, thus enabling the study of significantly more
complex molecules. We report simultaneous gains in resolution, sensitivity, and
bandwidth and demonstrate this combined capability with the first rotationally
resolved direct absorption spectra in the CH stretch region of several complex
molecules. These include nitromethane (CH$_3$NO$_2$), a model system that
presents challenging questions to the understanding of large amplitude
vibrational motion, as well as several large organic molecules with fundamental
spectroscopic and astrochemical relevance, including naphthalene
(C$_{10}$H$_8$), adamantane (C$_{10}$H$_{16}$), and hexamethylenetetramine
(C$_{6}$N$_4$H$_{12}$). This general spectroscopic tool has the potential to
significantly impact the field of molecular spectroscopy, simultaneously
improving efficiency, spectral resolution, and specificity by orders of
magnitude. This realization could open up new molecular species and new
kinetics for precise investigations, including the study of complex molecules,
weakly bound clusters, and cold chemistry.
