ANALYSIS OF THE EVEN-PARITY RYDBERG SERIES OF GD-I TO DETERMINE ITS IONIZATION-POTENTIAL AND ISOTOPE SHIFT

Three-step resonance ionization mass spectroscopy has been performed t o observe the even-parity Rydberg series converging to four states of Gd+ (4f(7)5d6s(D-10(5/2)o), 4f(7)5d6s(D-10(7/2)o), 4f(7)5d6s(D-10(9/2) o) and 4f(7)5d6s(8S(7/2)(o))). Using low-angular-momentum states as th e second-step intermediate levels, simple and unperturbed series struc tures have been obtained. From an analysis of these structures, the fi rst ionization potential of atomic Gd has been determined with improve d accuracy as 49 601.45 +/- 0.35 cm(-1). In addition, on the basis of the isotope shifts of the Rydberg states the isotope shift (Gd-160-Gd- 154) of the ionization potential of -77 +/- 20 mK has been found.