Infrared spectroscopy is an essential tool for probing molecular structure and dynamics. Over the last decade, cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) has emerged as a powerful technique for molecular spectroscopy in the mid-infrared region (3-15 um). CE-DFCS combines the broad spectral bandwidth and high frequency resolution of frequency combs with the improved detection sensivity provided by high-finesse optical cavities. At room temperature, however, all but the simplest molecules suffer from severe spectral congestion, which obscures the detailed dynamics encoded in spectroscopic fine structure. To overcome this challenge, we demonstrate the integration of CE-DFCS with cryogenic buffer gas cooling, which provides continuous, cold samples of gas-phase molecules at rotational and translational temperatures as low as 10 K. By significantly reducing internal partition functions and Doppler broadening, we take full advantage of CE-DFCS to reveal the intricate rovibrational structure of complex polyatomic molecules.
The systems investigated in this thesis range from extremely anharmonic molecules to unprecedentedly large carbon cages, illustrating the diverse ways in which a molecule can be complex. One case study is free internal rotation in nitromethane, a model system for large amplitude nuclear motion. This work has motivated our development of new theoretical tools to predict the high resolution spectra of nitromethane and several other floppy molecules. We have also focused on large molecules outside the domain of traditional high resolution spectroscopy. The culmination of these efforts is the rotationally resolved spectrum of the C60 fullerene. C60 is now both the largest molecule and the first example of icosahedral symmetry for which rovibrational quantum state resolution has been achieved. In addition to CE-DFCS, we have constructed a cw-QCL-based spectrometer to probe and manipulate the quantum states of C60. This work opens new avenues for fullerene research and for exploiting large molecules as platforms for quantum science.