Using high-resolution, Doppler-free spectroscopy, we recorded through resonance-enhanced multiphoton ionization the response of two-photon rotational lines of the <i>Q</i>-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 (σ<sub>i</sub>) and Stark shift (σ<sub>s</sub>), respectively. Cross sections of σ<sub>i</sub>=10±3×10<sup>−18</sup> cm<sup>2</sup> and σ<sub>s</sub>= 7±2×10<sup>−18</sup> cm<sup>2</sup> 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 σ<sub>i</sub>= 6±2×10<sup>−18</sup> cm<sup>2</sup> and σ<sub>s</sub>=8±2×10<sup>−18</sup> cm<sup>2</sup>. We observed a variation of σ<sub>i</sub> and σ<sub>s</sub> with rotational level for photoionization at 230 nm.
© 1999 Optical Society of America
(020.6580) Atomic and molecular physics : Stark effect
(260.5210) Physical optics : Photoionization
(300.6320) Spectroscopy : Spectroscopy, high-resolution
(300.6410) Spectroscopy : Spectroscopy, multiphoton
(300.6540) Spectroscopy : Spectroscopy, ultraviolet
M. D. Di Rosa and R. L. Farrow, "Cross sections of photoionization and ac Stark shift measured from Doppler-free B ← X(0, 0) excitation spectra of CO," J. Opt. Soc. Am. B 16, 861-870 (1999)