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 {ital Q}-branch of CO B{l_arrow}X(0, 0) to an independent photoionizing optical field. In direct proportion to the photoionization intensity, transitions Q(0){endash}Q(12) were lifetime broadened and shifted toward positive frequency, from which we extracted absolute cross sections of photoionization ({sigma}{sub i}) and Stark shift ({sigma}{sub s}), respectively. Cross sections of {sigma}{sub i}=10{plus_minus}3{times}10{sup {minus}18} cm{sup 2} and {sigma}{sub s}= 7{plus_minus}2{times}10{sup {minus}18} cm{sup 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}{sub i}= 6{plus_minus}2{times}10{sup {minus}18} cm{sup 2} and {sigma}{sub s}=8{plus_minus}2{times}10{sup {minus}18} cm{sup 2}. We observed a variation of {sigma}{sub i} and {sigma}{sub s} with rotational level for photoionization at 230 nm. {copyright} 1999 Optical Society of America