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 reson ance-enhanced multiphoton ionization the response of two-photon rotational lines of the Q-branch of CO B <-- X(0, 0) to an independent photoionizing o ptical field. In direct proportion to the photoionization intensity, transi tions Q(0)-Q(12) were lifetime broadened and shifted toward positive freque ncy, from which we extracted absolute cross sections of photoionization (si gma(i)) and Stark shift (sigma(s)), respectively. Cross sections of sigma(i ) = 10 +/- 3 x 10(-18) cm(2) and sigma(s) = 7 +/- 2 x 10(-18) cm(2) were de termined for the case of photoionizing light at 230 nm, tuned in near reson ance with the two-photon transition. Photoionization by 345-nm light result ed in similar cross sections, of sigma(i) = 6 +/- 2 x 10-18 cm(2) and sigma (s) = 8 +/- 2 x 10(-18) cm(2). We observed a variation of sigma(i) and sigm a(s) with rotational level for photoionization at 230 nm. (C) 1999 Optical Society of America [S0740-3224(99)01805-6].