Discovery of New Lines in the $R9$ Multiplet of the $2v_3$ Band of ${{}^{12}\mathrm{C}\mathrm{H}}_4$

We present the first results for resolving methane (${\mathrm{CH}}_4$) line transition frequencies down to the kilohertz level for overlapping lines using comb-linked cavity ring-down spectroscopy, while most available laboratory measurements, having resolution at the megahertz level, cannot separate merged lines. To demonstrate the technique, Lamb-dip spectra and linear-absorption spectra were used to identify overlapped lines of vibration-rotation spectra in the $R9$ multiplet of the $2v_3$ band. Three new weak lines were found for the first time. The experimental methods are extensible to other important bands of ${\mathrm{CH}}_4$ and many other gas-phase molecules, and should provide a more detailed understanding of molecular structure and line parameters for future high-precision studies.

Figure 1

Experimental apparatus. Extended cavity diode laser (ECDL), booster optical amplifier (BOA), digital gate and delay generator for ring-down signal (DGDG), voltage-controlled oscillator (VCO), high reflectivity mirror (HM), photodetector for measuring ring-down signal (PD1), photodetector for cavity length stabilization (PD2), dichroic mirror (DM), piezo-electric transducer (PZT), Polarization beam splitter (PBS), acoustic-optic modulator (AOM).

Figure 2

The two clearly observed Lamb dips of the $10F2\leftarrow 9F1$ 1(a) and $10F1\leftarrow 9F2$ 1(b) transitions. We assume the frequency of the $10F1\leftarrow 9F2$ 1 line is greater than that of $10F2\leftarrow 9F1$ 1.

Figure 3

The fitting comparison of the linear spectrum with different methods: (a) all line positions floated; (b) strong line positions fixed using Lamb-dip results, weak lines floated; (c) like (b) but with an extra weak line added—flatter residuals suggest the existence of a previously unreported line. The symbol $\sigma$ denotes the standard deviation of the fitting residuals.