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.
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