Specific Mass Shift in Gd I and Dy I

High-resolution diode-laser spectroscopy has been performed on atomic beams of natural Gd and Dy. Isotope shifts of the even-mass isotopes have been measured for two transitions in Gd I and one transition in Dy I. Specific mass shifts, as well as field shifts, have been derived for transitions of 4f⁷5d6s²-4f⁷6s²6p in Gd I, and 4f¹⁰6s²-4f⁹5d6s² in Dy I: the specific mass shift is much larger than the normal mass shift. It has been found that the specific mass shift of the 4f¹⁰6s² 4f⁹5d6s² transition in Dy I is about one order of magnitude larger than that of the 4f⁷5d6s²-4f⁷6s²6p transition in Gd I. This shows that the specific mass shift, related to the correlation effect between electrons, strongly depends on the orbital angular momentum of electrons.