Measurement and assignment of double-resonance transitions to the 8900–9100- cm−1 levels of methane

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-μm continuous wave optical parametric oscillator as a pump and a 1.67-μm 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 ν 3 fundamental band, we detected 36 ladder-type transitions to the 3ν 3 overtone band region, and 18 V-type transitions to the 2ν 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.

Sub-Doppler Double-Resonance Spectroscopy of Methane Using a Frequency Comb Probe

“…In total, for the nine different pump transitions we detected V-type resonances in 18 different overtone transitions, listed in Table II in Ref. [26]. The center frequencies of five of these V-type resonances, obtained from a fit of a Lorentzian function, can be compared to the center frequencies of the corresponding 2ν 3 overtone transitions measured in the Doppler-limited regime by Zolot et al [25] using dual comb spectroscopy (DCS).…”

“…Next, the idler frequency was stabilized to the center of the addressed transition using a frequency modulated Lambdip error signal from a reference cell filled with pure CH 4 (see Ref. [26], for details of the stabilization scheme). This lock provided long-term frequency stability of the idler of AE1 MHz, limited by a drift of the offset of the error signal.…”

“…The comb was rf-stabilized to a GPS-referenced Rb oscillator, as described in Ref. [26]. The carrier envelope offset frequency was fixed at 20 MHz, while the repetition rate, f rep , was locked to a tunable direct digital synthesiser that allowed stepping of the comb mode frequencies during spectral acquisition.…”

“…A fit to the Dopplerbroadened spectrum, described in detail in Ref. [26], revealed that the sample temperature was 111(4) K, i.e., significantly above 77 K. We attribute this to inefficient cooling of the cell and contributions from the warmer gas in the uncooled part of the cell (temperature retrieved from a spectrum with pump beam off was found to be 100 K), as well as to the heating by the pump beam (predicted to yield 7 K temperature increase).…”

“…Moreover, the sub-Doppler V-type resonances allow determination of line center positions with the same accuracy even in the case of overlapping Dopplerbroadened lines [e.g., the Pð3; F 2 Þ triplet, see Ref. [26] ], which is not possible with Doppler-limited spectroscopy.…”