Laser measurement of the (16)O-(18)O isotope shift at optical frequencies.

We demonstrate the full resolution of isotope shifts in atomic O by means of sub-Doppler laser optogalvanic spectroscopy of (18)O-enriched samples. The measurements are performed to within a few percent accuracy on four transitions ranging from 605 to 646 nm and involving excited states. Previously published values from conventional spectroscopy are either unavailable or are one order of magnitude less accurate and in marginal agreement with our data.