Measurement of the isotope shift of the 2p⁴ 2p³3p ³ P₂two-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 ³P two-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 reexamine the energy levels of the subset of triplet states and present an updated set of optimized energy values.