Extended sub-Doppler resolution spectroscopy of the ν3 band of methane

We have performed sub-Doppler resolution rovibrational spectroscopy of the ν3 band of methane (12CH4) in the mid-infrared using an optical parametric oscillator in a double-pass configuration for saturated absorption. An optical frequency comb was used for absolute frequency calibration. We have measured 22 methane line centers ranging from the P(13) to R(10) transitions with a typical uncertainty of 0.40 MHz. These measurements improve previously known transition frequencies by at least an order of magnitude.