Cavity-Enhanced Optical-Optical Double-Resonance Polarization Spectroscopy Using a Frequency Comb Probe

Unambiguous quantum number assignment for observed states is a key aspect of spectroscopic analysis, but it becomes especially challenging for the crowded spectra of hot-band molecular transitions. These transitions can be resolved with sub-MHz precision over a wide bandwidth using optical-optical double-resonance (OODR) spectroscopy based on a continuous wave pump and an optical frequency comb probe [1]. In OODR, the change in total angular momentum quantum numbers, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta J$</tex>, for probe transitions can be assigned from the ratios of line intensities from two consecutive measurements with the pump linearly polarized parallel and perpendicular to the probe's linear polarization [1], [2]. This is because at the OODR transitions the molecular response is different along and perpendicular to the pump polarization axis, and depends on the J-values of the three states involved in the OODR transition [3].