Abstract

Accurate assignments of highly-excited molecular vibrational states are needed for the verification of theoretical predictions of high-temperature spectra observed in astrophysics and combustion environments. We previously demonstrated that optical-optical double resonance spectroscopy with a continuous wave (CW) pump and a frequency comb probe is a powerful tool for this purpose; it allows simultaneous measurement of multiple hot-band transitions from a selectively populated state with sub-Doppler resolution [1]. Using a 3.3 μm CW laser as a pump and a 1.67 μm comb as a probe, we detected previously unobserved transitions in the 3ν₃ ← ν₃ range of methane [2]. The sample was contained in a single-pass cell, and the comb probe spectra were measured using a Fourier transform spectrometer with comb-mode-limited resolution. Recently, we implemented an enhancement cavity for the comb probe that improved the absorption sensitivity by more than two orders of magnitude [3]. Here we use this cavity-enhanced system to measure 3ν₃ ← ν₃ methane transitions with much improved frequency and relative intensity accuracy, which allows unambiguous assignments of the final states using two independent methods.

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