The atomic level structure of the element fermium was investigated for the first time using a sample of 2.7x10(10) atoms of the isotope 255Fm with a half-life of 20.1 h. The atoms were evaporated from a filament and stored in the argon buffer gas of an optical cell. Atomic levels were sought by the method of resonance ionization spectroscopy using an excimer-dye-laser combination. Two atomic levels were found at wave numbers (25 099.8+/-0.2) and (25 111.8+/-0.2) cm(-1). Partial transition rates to the 5f(12)7s(2) (3)H(e)(6) ground state have been determined from their saturation characteristics. Multiconfiguration Dirac-Fock calculations suggest that the leading orders of these levels could be the 5f(12)7s7p (5)I(o)(6) and 5f(12)7s7p (5)G(o)(5) terms.

Extensive relativistic configuration interaction calculations have been performed for the 4d104f and 4d105s states of the Ag-like isoelectronic ion using the GRASP2K package based on the multi-configuration Dirac–Fock method. The active space techniques have been employed to extend the configuration expansion systematically. The electron correlation effects, Breit interaction and quantum electrodynamics effects to the atomic state wavefunctions and the corresponding energies have been taken into account. Good agreement has been obtained in a general trend with the existing experimental and theoretical values. The change of the ground state configuration from 4d105s to 4d104f is predicted to occur at Z = 62. The transition energy and probability of the M1 transition between the 4d104f multiplets are, respectively, scaled to be 0.01(Z*)4 cm−1 and 1.95 × 10−17(Z*)11.97 s−1, where Z* = Z − 34.42 is the effective nuclear charge number for Ag-like ions with Z ⩾ 62. Our results show the need for more high-precision experiments for Z ⩾ 68 ions.

Ab initio calculations of dielectronic recombination rate coefficients of Rh-like gadolinium and tungsten have been performed. Energy levels, radiative transition probabilities, and autoionization rates of Pd-like gadolinium and tungsten for [Zn]4p 6 4d 8 4fnl, [Zn]4p 6 4d 8 5l nl, [Zn]4p 6 4d 8 6l nl and [Zn]4p 5 4d 10 nl, [Zn]4p 5 4d 9 4fnl, [Zn]4p 5 4d 9 5l nl, [Zn]4p 5 4d 9 6l nl (n 18) complexes were calculated using the flexible atomic code. The contributions from resonant and nonresonant radiative stabilizing transitions to the total rate coefficients are discussed. Results show that the contributions from nonresonant radiative stabilizing transitions are significantly enchanced for W when compared with Gd as a result of lowering of energy levels relative to the ionization limit. In addition, the widely used Burgess-Merts semiempirical formula may underestimate the dielectronic recombination rate coefficients in the temperature regions of interest. The present calculated rate coefficients are fitted to a semiemperical formula. The data obtained are expected to be useful for modelling plasmas both for extreme ultraviolet lithography source development and for fusion applications.

A B S T R A C TTransition energies, probabilities and branching ratios for electric dipole allowed (E1) and forbidden (M1, E2, M2) lines have been calculated for the 3s 2 3p 5 , 3s3p 6 and 3s 2 3p 4 3d configurations of Fe x. From the transition probabilities, lifetimes of all 31 levels of these low-lying configurations are also derived, and compared with experiment. By applying systematically enlarged multiconfiguration Dirac±Fock wavefunctions, most important effects of relativity, correlation and the rearrangement of the electron density are treated within the same (computational) model.