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 $3p{}_{}{}^3{}_{}{}^{}P_{J^{\prime }}^{}{}_{}{}^{}\leftarrow 2p{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ transition in atomic oxygen at 226 nm the integrated cross section is $\Sigma {J^{\prime }}^{}{\sigma }_0^{\mathrm{}\left(2\right)\mathrm{}}(J^{\prime }\to 2)=1.87\mathrm{}\pm 0.60\times {10}^{-35\mathrm{}}$ ${\mathrm{cm}}^4$. 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.