Ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D spectroscopy, are widely used to study molecular dynamics. However, these techniques are typically restricted to optically thick samples, such as solids and liquid solutions. In this talk, I will present our work using cavity-enhanced frequency combs to perform all-optical ultrafast spectroscopy in dilute gasses, with detection limits orders of magnitude lower than conventional methods. My talk will have three parts.

First, I will describe our experimental work on UV/visible transient absorption spectroscopy in dilute molecular beams. Using a custom optical frequency comb and dispersion-managed cavity mirrors, we have achieved a sensitivity of $\Delta OD < 10^{-9}$ across nearly the entire visible spectral range. In the molecular beam we can vary the molecule’s temperature between a few 10’s of Kelvin to 450 K, and also simulate solvation and caging effects via forming clusters. I will present results on several molecules undergoing excited-state intramolecular proton transfer (ESIPT) and also preliminary results on internal conversion in uracil and 2-thiouracil in isolated gas-phase and Ar cluster environments.

Second, I will describe methods we have invented for using multiple frequency combs for performing multidimensional spectroscopy, and also cavity-enhancing the signals using higher-order cavity modes. This can enable cavity-enhanced ultrafast 2DIR spectroscopy of elementary hydrogen-bond networks such as small water clusters, and also trace-gas analysis in complex mixtures via high-resolution cavity-enhanced 2DIR.

Finally, I will discuss our work on high-resolution rotationally-resolved 2DIR spectra of freely rotating molecules and also control these spectra via polarization. Theoretically, we found new polarization conditions unique to suppressing whole branches of the rotationally-resolved spectra, and these theoretical predictions have recently been confirmed in experiments in gas-phase carbon dioxide. An accompanying suite of open-source software enables rapid calculation of rotationally-resolved 2DIR spectra under a wide range of conditions, using spectroscopic constants from standard reference databases (e.g. HITRAN) as input.