with counter-propagating laser beams aligned in the geometry of a Sagnac interferometer [@Hannemann2007] to avoid a residual Doppler shift. The laser beams crossed the 99% $^{13}$C-enriched CO molecular beam, perpendicularly. A second pulsed laser at 207 nm, based on a frequency-tripled dye laser, delayed by around 10 ns from the probe laser to minimize the AC-Stark effects, is employed for ionizing the [A$^1\Pi$]{} excited state population. This method of $2+1'$ resonant-enhanced multiphoton ionization (REMPI) yielded better results, than one-color $2+2$ REMPI measurements, in terms of signal-to-noise ratio and suppression of the AC-Stark effect.

In order to reduce the AC-Stark effect induced by the probe laser, the resonant CO lines were measured for different laser intensities and their transition frequencies extrapolated to a zero-intensity AC-Stark-free frequency. An example of such an extrapolation, for the S(0) line, is included in Fig. \[fig:stark\]. Using part of the cw-seed radiation, an etalon spectrum and a saturated iodine spectrum are recorded simultaneously with the $^{13}$C$^{16}$O two-photon spectrum to interpolate and linearize the scan and to calibrate the absolute frequency of the resonances, respectively. The frequency chirp between the PDA output and cw seed laser is measured on-line, yielding an average chirp offset that is included 