Field Isotope Shifts in the 3p sup 5 4p - 3p sup 5 5d 604. 3 nm Transition in Argon I.

We have measured the isotope shifts between sup 36,38,40 Ar I using Doppler-free intermodulated optogalvanic spectroscopy on the 4p-5d transition at 604.3 nm. A field shift which is sixty percent greater than that of a 4s-4p transition previously measured was observed. The large relative field shift in the case of an atom of such low Z is surprising and not yet explained by any calculation.