Cross sections of photoionization and ac Stark shift measured from Doppler-free B ← X(0, 0) excitation spectra of CO

Using high-resolution, Doppler-free spectroscopy, we recorded through resonance-enhanced multiphoton ionization the response of two-photon rotational lines of the Q-branch of CO B←X(0, 0) to an independent photoionizing optical field. In direct proportion to the photoionization intensity, transitions Q(0)–Q(12) were lifetime broadened and shifted toward positive frequency, from which we extracted absolute cross sections of photoionization (σi) and Stark shift (σs), respectively. Cross sections of σi=10±3×10-18 cm2 and σs=7±2×10-18 cm2 were determined for the case of photoionizing light at 230 nm, tuned in near resonance with the two-photon transition. Photoionization by 345-nm light resulted in similar cross sections, of σi=6±2×10-18 cm2 and σs=8±2×10-18 cm2. We observed a variation of σi and σs with rotational level for photoionization at 230 nm.