No abstract

Optical spectroscopy of a primordial isotope has traditionally formed the basis for understanding the atomic structure of an element. Such studies have been conducted for most elements and theoretical modelling can be performed to high precision, taking into account relativistic effects that scale approximately as the square of the atomic number. However, for the transfermium elements (those with atomic numbers greater than 100), the atomic structure is experimentally unknown. These radioactive elements are produced in nuclear fusion reactions at rates of only a few atoms per second at most and must be studied immediately following their production, which has so far precluded their optical spectroscopy. Here we report laser resonance ionization spectroscopy of nobelium (No; atomic number 102) in single-atom-at-a-time quantities, in which we identify the ground-state transition SP. By combining this result with data from an observed Rydberg series, we obtain an upper limit for the ionization potential of nobelium. These accurate results from direct laser excitations of outer-shell electrons cannot be achieved using state-of-the-art relativistic many-body calculations that include quantum electrodynamic effects, owing to large uncertainties in the modelled transition energies of the complex systems under consideration. Our work opens the door to high-precision measurements of various atomic and nuclear properties of elements heavier than nobelium, and motivates future theoretical work.

The shape of exotic even-mass [182][183][184][185][186][187][188][189][190] Pb isotopes was probed by measurement of optical isotope shifts providing mean square charge radii (hr 2 i). The experiment was carried out at the ISOLDE (CERN) on-line mass separator, using in-source laser spectroscopy. Small deviations from the spherical droplet model are observed, but when compared to model calculations, those are explained by high sensitivity of hr 2 i to beyond mean-field correlations and small admixtures of intruder configurations in the ground state. The data support the predominantly spherical shape of the ground state of the proton-magic Z 82 lead isotopes near neutron midshell (N 104). The subtle interplay between individual and collective behavior of a finite number of strongly interacting fermions leads to aspects of mesoscopic systems that can only be studied in atomic nuclei [1]. For neutron-deficient nuclides around the closed proton shell at Z 82, this interplay leads to the appearance of states with different shapes at low excitation energy. These so-called shape coexisting states can be interpreted as particle-hole excitations across the closed proton shell gap [2] whereby the interaction of the valence proton particles and holes with the neutrons drives the nucleus into deformation. The phenomenon of shape-coexistence is subject to intensive experimental and theoretical studies [3,4]. Alpha-decay experiments have revealed a triplet of low-lying 0 states in the 186 Pb nucleus, which is located at neutron midshell between N 82 and 126 [1]. Excited bands built on top of the 0 states were observed in [182][183][184][185][186][187][188][189][190], and recent lifetime measurements confirmed the deformed character of the bands [11]. For 186 188 Pb, it was concluded that the ground state and the 2 1 state have a very different structure, the 0 ground state of predominantly spherical and the 2 1 state of predominantly prolate character. Monopole transition strengths between the 0 states were used to estimate the mixing between the normal and intruder configuration [12] and revealed limited configuration mixing in the 190;192;194 Pb ground-state wave function [13,14]. But as the excited 0 states become lower in energy when approaching N 104 ( 186 Pb), the mixing could increase substantially.Several theoretical models have been applied to describe the structure of the neutron-deficient lead isotopes with their coexisting and mixed spherical, prolate, and oblate states, such as phenomenological shape mixing calculations [13,14], symmetry guided shell model and interacting boson model truncations [15,16], and beyond mean-field approaches [4,17,18]. All models that provide a consistent picture of the available data suggest that the ground state of lead isotopes is dominated by spherical configurations, even when the prolate and oblate rotational bands come down very low in energy around N 104, and the barrier that separates the corresponding structures in the total energy surface is very small. But all models also ...

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.

Until recently, ground-state nuclear moments of the heaviest nuclei could only be inferred from nuclear spectroscopy, where model assumptions are required. Laser spectroscopy in combination with modern atomic structure calculations is now able to probe these moments directly, in a comprehensive and nuclear-model-independent way. Here we report on unique access to the differential mean-square charge radii of ^{252,253,254}No, and therefore to changes in nuclear size and shape. State-of-the-art nuclear density functional calculations describe well the changes in nuclear charge radii in the region of the heavy actinides, indicating an appreciable central depression in the deformed proton density distribution in ^{252,254}No isotopes. Finally, the hyperfine splitting of ^{253}No was evaluated, enabling a complementary measure of its (quadrupole) deformation, as well as an insight into the neutron single-particle wave function via the nuclear spin and magnetic moment.