Abstract

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 $B\leftarrow X(0, 0)$ 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 $({\sigma }_i)$ and Stark shift $({\sigma }_s),$ respectively. Cross sections of ${\sigma }_i=10\pm 3\times {10}^{-18} {\mathrm{cm}}^2$ and ${\sigma }_s=7\pm 2\times {10}^{-18} {\mathrm{cm}}^2$ 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 ${\sigma }_i=6\pm 2\times {10}^{-18} {\mathrm{cm}}^2$ and ${\sigma }_s=8\pm 2\times {10}^{-18} {\mathrm{cm}}^2.$ We observed a variation of ${\sigma }_i$ and ${\sigma }_s$ with rotational level for photoionization at 230 nm.

© 1999 Optical Society of America

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