Tomography of a supersonically cooled molecular jet using cavity-enhanced direct frequency comb spectroscopy

Cavity-enhanced direct frequency comb spectroscopy is used to obtain simultaneously high resolution and broad spectral-bandwidth measurements of a supersonically cooled jet of acetylene molecules. We demonstrate a complete spatial mapping of density, velocity, and internal state distributions of a cold molecular jet via tomographic reconstructions. These measurements reveal interesting and never-before-observed features of the pulsed jet expansion, including anomalous C2H2 absorption lineshapes and anomalous spatial variations of the rotational temperature distribution. The demonstrated capabilities establish this technique as a powerful new tool for studies of cold molecules.