Single-frequency laser measurements of two-photon cross sections and Doppler-free spectra for atomic oxygen

Absolute bound-bound atomic two-photon cross sections have been measured using a single-frequency laser for the first time, eliminating the usual uncertainties about unresolved temporal fluctuations. For the 3p³P_J^'←2p³P₂ transition in atomic oxygen at 226 nm the integrated cross section is ${Σ}{{J}^{{'}}}^{}{{σ}}₀⁽²⁾({J}^{{'}}{→}2)=1.87±{}0.60×{}{10}^{{-}35}$ ${cm}⁴$. Doppler-free spectra have been used to measure relative fine-structure cross sections and energy spacings. Absolute and relative two-photon cross sections agree well with perturbation theory, validating this approach.