Field Isotope Shifts in the 3p sup 5 4p-3p sup 5 5d 604.3nm Transition in Argon I

The isotope shifts between sup 36, 38, 40 Ar on the 4p-5d transition at 604.3nm in Ar I have been measured using the Doppler-free intermodulated optogalvanic spectroscopy technique. Measurements were carried out using a tunable dye laser scanning a glow discharge at 250 mtorr. A field shift which is sixty percent greater than that of a 4s-4p transition at 667.7nm previously measured was observed. A nonzero value for a filed shift in a p-d transition can be explained by Hartree Fock calculations, configuration mixing and other second order effects. The large relative field shift in this case for an atom of such low Z, however, is surprising and not yet explained by any calculation.