Field isotope shifts in the 3p54p-3p55d 604.3 nm transit argon I

The authors have measured the isotope shifts between 36,38,49 Ar I using Doppler-free intermodulated optogalvanic spectroscopy on the 4p-5d transition at 604.3 nm. A field shift which is 60% 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.