Abstract
Optical-optical double-resonance spectroscopy with a continuous wave pump and frequency comb probe allows measurement of sub-Doppler transitions to highly excited molecular states over a wide spectral range with high frequency accuracy. We report on assessment and characterization of sub-Doppler double-resonance transitions in methane measured using a 3.3-$\ensuremath{\mu}\mathrm{m}$ continuous wave optical parametric oscillator as a pump and a 1.67-$\ensuremath{\mu}\mathrm{m}$ frequency comb as a probe. The comb spectra were recorded using a Fourier transform spectrometer with comb-mode-limited resolution. With the pump tuned to nine different transitions in the ${\ensuremath{\nu}}_{3}$ fundamental band, we detected 36 ladder-type transitions to the $3{\ensuremath{\nu}}_{3}$ overtone band region, and 18 V-type transitions to the $2{\ensuremath{\nu}}_{3}$ overtone band. We describe in detail the experimental approach and the pump stabilization scheme, which currently limits the frequency accuracy of the measurement. We present the data analysis procedure used to extract the frequencies and intensities of the probe transitions for parallel and perpendicular relative pump-probe polarization. We compare the center frequencies and relative intensities of the ladder-type transitions to theoretical predictions from the TheoReTS and ExoMol line lists, demonstrating good agreement with TheoReTS.
Highlights
Methane has long been a molecule of intense scientific interest [1,2]
We report here on the detection and assignment of 36 subDoppler resolution double-resonance transitions in methane reaching 32 final states in the 3ν3 band region
The transition center frequency accuracy was in the 1.3-3 MHz range, limited by the pump frequency stability
Summary
Methane has long been a molecule of intense scientific interest [1,2]. It is an increasingly important fossil fuel and greenhouse contributor [3], and it is produced or consumed by many anaerobic organisms [4]. Ghysels et al used a diode laser spectrometer to record spectra at 1000 K in the Tetradecad region (5693– 6257 cm−1, dominated by the 2ν3 band) [14] and Wong et al recorded FTIR spectra in the 5400–9000-cm−1 range (up to the Triacontad range) at temperatures up to 1000 K [13] In both cases, the high-temperature spectra were compared to predictions from the TheoReTS line list and very good agreement was found up to the Icosad polyad (7700 cm−1). We compare the frequencies and intensities of the ladder-type probe transitions to theoretical predictions from the TheoReTS [11,39] and Exomol [40,41] databases
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