Measurement of the isotope shift of the $2{{\rm{p}}}^{4}{}^{3}{{P}}_{2} \rightarrow 2{{\rm{p}}}^{3}3{\rm{p}}{}^{3}{{P}}_{2}$ two-photon transition of O I and a revision of the triplet energy levels of atomic oxygen

Two-photon induced fluorescence of atomic oxygen, which is routinely used for plasma and flame diagnostics, is implemented with two counterpropagating laser beams of sufficient monochromaticity to permit Doppler-free spectroscopy. A single-mode injection-seeded pulsed Ti:sapphire laser has been frequency-doubled twice to produce narrow-band pulsed radiation at wavelengths around 225.6 nm, suitable to excite the ground-state-to-2p33p 3P2 2-photon resonance line of oxygen. Accurate measurement of the injection-seeding wavelength provides new data on the excitation energy of the 3p 3P states. The experiment was done both sequentially and simultaneously on 16O and 18O, which makes it possible to test recent calculations of the isotope shift. Having an absolute measurement of the excitation wave-number from the ground level, which has been the case only twice in previous spectroscopic studies of O I, we can re-examine the energy levels of the subset of triplet states and present an updated set of optimized energy values.