Accurate transition frequency list of the ν₃ band of methane from sub-Doppler resolution comb-referenced spectroscopy

We have carried out sub-Doppler resolution spectroscopy of the ν₃ band of ¹²CH₄ using a difference-frequency-generation source, an enhanced cavity absorption cell, and an optical frequency comb, and we have added 150 lines to the transition frequency list published earlier [Opt. Express 19, 23878 (2011)]. It now covers 204 lines from the P(12) to R(8) transitions, involving 21 weak forbidden transitions, with a typical uncertainty of 3 kHz for allowed transitions and 12 kHz for forbidden transitions. The combination differences between the allowed and forbidden transitions enable us to determine a scalar rotational constant and three scalar distortion constants of the ground vibrational state with greatly reduced uncertainties.