Precision spectroscopy of complex molecules

High-resolution spectroscopy is a powerful tool to reveal the rich internal structure of small molecules in their electronic, vibrational, rotational, and spin degrees of freedom. Extending this level of characterization to systems of transitional nanoscale size, on the brink of treatment with standard molecular tools, is becoming possible using novel optical and cold molecule technologies. We plan to use cavity-enhanced frequency comb spectroscopy and buffer gas cooling for quantum-state-resolved characterization of nanoscale systems in order to bridge our understanding of small molecules with that of emergent phenomena in extended materials.

Achieving state-resolved spectroscopy in large molecules is subject to demanding experimental requirements. The need for a sensitive, high resolution, and broadband spectroscopic tools can be met using frequency combs, stabilized mode-locked lasers whose frequency spectra consist of thousands of evenly spaced, narrow “comb teeth.” Cavity-enhanced frequency comb spectroscopy (CE-FCS) matches a comb’s spectral lines to the modes of an optical cavity containing the absorber of interest. CE-FCS combines excellent frequency resolution limited only by the comb tooth linewidth, high sensitivity via cavity enhancement, and broadband detection across the comb spectrum. The Weichman Lab will build up capabilities for direct CE-FCS at near- and mid-infrared wavelengths.

In order to cleanly probe large systems in the gas phase, it is also essential to create a cold molecular sample of sufficient density. Large molecules can occupy millions of vibrational and rotational states at room temperature, yielding congested spectra that are difficult to resolve, interpret, and assign. By cooling isolated molecules to cryogenic temperatures, one can build significant ground state population and measure meaningful structure without obfuscation from hot populations. Collisions with a cold buffer gas inside a cryogenic buffer gas cell (CBGC) are a nearly universal method to achieve cooling, and can thermalize translational and rotational temperatures to 10-20 K. In recent work, the carbon-60 fullerene was cooled to 150 K, enabling a new record in spectroscopic quantum state resolution of a molecule of such large size and high symmetry. The Weichman Lab will develop a next-generation cryogenic cell coupled with a laser desorption source to achieve dense samples of large molecules at even lower temperatures.

Cavity-enhanced comb spectroscopy and cryogenic buffer gas cells are recently developed technologies whose combination will facilitate many avenues of future work. In the near term, we are interested in probing molecular endofullerenes as model systems for quantum confinement and guest-host interactions. We are also excited about a variety of future directions including state preparation of large molecules, laboratory astrochemistry, and aerosol chemistry.