Specific mass shift in GdI and DyI

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 m easured for two transitions in Gd I and one transition in Dy I. Specific ma ss shifts. as well as field shifts. have been derived for transitions of 4f (7)5d6s(2) -4f(7)6s(2)6p in GdI, and 4f(10)6s(2)-4f(9)5d6s(2) 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 4f(10)6s(2)-4f(9)5d6s(2) transiti on in Dy I is about one order of magnitude larger than that of the 4f(7)5d6 s(2)-4f(7)6s(2)6p transition in Gd I. This shows that the specific mass shi ft. related to the correlation effect between electrons, strongly depends o n the orbital angular momentum of electrons.