Wavelength of the 3.39-μm laser-saturated absorption line of methane

The wavelength of the 3.39-μm line of methane has been measured with respect to the Kr⁸⁶ 6057-Å standard by using a frequency-controlled Fabry-Perot interferometer. We have exhaustively studied systematic offsets inherent in the experiment, including effects due to asymmetry of the Kr standard line. Lacking a convention relating the defined Kr wavelength 6057.802 105 Å to observables of the krypton line (e.g., center of gravity or fringe maximum intensity point), we report two methane wavelengths: λ_{max I}=33 922.314 04 Å and λ_cg=33 922.313 76 Å. Both results have an uncertainty of δλ=±1.2×10^-4 Å or δλ/λ=±3.5×10^-9. Multiplication by the frequency measurement of the preceding letter gives the speed of light, c_cg=299 792 456.2(1.1) m/sec.