Absolute frequency list of the ν_3-band transitions of methane at a relative uncertainty level of 10^−11

Abstract

We determine the absolute frequencies of 56 rotation-vibration transitions of the ν(3) band of CH(4) from 88.2 to 90.5 THz with a typical uncertainty of 2 kHz corresponding to a relative uncertainty of 2.2 × 10(-11) over an average time of a few hundred seconds. Saturated absorption lines are observed using a difference-frequency-generation source and a cavity-enhanced absorption cell, and the transition frequencies are measured with a fiber-laser-based optical frequency comb referenced to a rubidium atomic clock linked to the international atomic time. The determined value of the P(7) F(2)((2)) line is consistent with the International Committee for Weights and Measures recommendation within the uncertainty.

Accurate transition frequency list of the ν_3 band of methane from sub-Doppler resolution comb-referenced spectroscopy

“…The experimental apparatus for frequency measurements of the allowed transitions are almost identical with those of the previous paper [18]. Briefly, the pump and signal sources for DFG are a 1.06 μm Nd:YAG laser and a 1.5 μm ECLD.…”

“…Briefly, the pump and signal sources for DFG are a 1.06 μm Nd:YAG laser and a 1.5 μm ECLD. Three PPLN modules are used as nonlinear optical devices: two are identical to that used in the previous work [18], and the other, which has a high conversion efficiency of 40%∕W, is newly introduced for measurements from 91.7 to 93.1 THz. The pump frequency is offset-locked to a certain mode of an Er fiber comb with a repetition rate of about 67 MHz.…”

“…Subsequently, the DFG spectrometer was combined with an optical frequency comb for absolute frequency measurement [17]. Later, an enhanced-cavity absorption cell (ECAC) was employed as a frequency reference for reducing the linewidth of a signal source, a widely tunable extended-cavity laser diode (ECLD), as well as enhancing the effective absorption length and optical electric field strength [18]. We reported a transition frequency list that covers 54 transitions from the P1 to P7 transitions and from the Q1 to Q7 transitions of 12 CH 4 in the frequency range of 88.2-90.5 THz and a typical uncertainty of 2 kHz [18].…”

“…We have measured transition frequencies of the P8 to P12 transitions and of the Q8 to Q14 transitions in addition to the previous work [18] with a typical uncertainty of 3 kHz. Furthermore, the tunable range of the DFG spectrometer increases from 86.7 to 93.1 THz by introducing a PPLN module and an L-band fiber amplifier.…”

Atomic and molecular spectroscopy with optical-frequency-comb-referenced IR coherent sources

“…), absolute frequency metrology, environmental applications (indeed, after water and carbon dioxide, CH 4 is the third major greenhouse gas in the Earth's atmosphere), and combustion processes. In particular, the strong ν 3 (stretching) fundamental band (at 3019 cm −1 ) has always been the object of accurate investigations and, in this regard, among all the recently developed IR laser sources, DFGs in periodically-poled non-linear crystals have proved to be the most reliable ones for high-resolution spectroscopy [42]. Figure 4 shows the DFG spectrometer implemented by the Naples group [43].…”