Laser spectroscopy of nobelium isotopes

Abstract

Laser spectroscopy is a versatile tool to unveil fundamental atomic properties of an element and the ground state information of the atomic nucleus. The heaviest elements are of particular interest as the ordering of their shell electrons is strongly influenced by electron-electron correlations, quantum electrodynamics and relativistic effects leading to distinct chemical behaviour. The elements beyond fermium (Z > 100) are accessible in fusion evaporation reactions at minute quantities and at high energies, hampering their optical spectroscopy. Recently, the RAdiation Detected Resonance Ionization Spectroscopy (RADRIS) technique was employed to explore the electronic structure of the element nobelium (No, Z = 102). The 1S0->1P1 ground state transition of this element was identified. In this work, the pioneering experiment on laser spectroscopy of nobelium was extended to the isotopes of nobelium (252-254 No). These were produced in fusion-evaporation reactions by bombarding lead targets (206-208 Pb) with 48Ca projectiles. After separation from the primary beam by the velocity filter SHIP (Separator for Heavy Ion reaction Products), at GSI, the fusion products were stopped in 95 mbar high-purity argon gas and collected onto a thin tantalum filament. After a sufficient collection time, which depended on the half-life (T1/2) of the isotope under consideration, the primary beam was blocked in order to have a background free signal. During the beam-off period, the collected nobelium ions were re-evaporated as neutral atoms from the filament and were probed by two laser beams for ionization. The created photo-ions were detected by their characteristic alpha decay. With this technique the isotope shift of the transition was measured for the isotopes 252-254 No. A hyperfine splitting of the 1P1 level was resolved in 253No. These measurements in combination with state-ofthe-art atomic calculations provided a deep insight into the evolution of nuclear deformation of the investigated nobelium isotopes in the vicinity of the deformed shell closure at neutron number N = 152 along with an assesment of the magnetic moment, µ, and the spectroscopic quadrupole moment, Qs, for 253No. Moreover, several high-lying Rydberg states were measured for the first time in 254No. These Rydberg states, populated in different ways enabled establishing different Rydberg series and the extraction of the first ionization potential of the element plus an additional low-lying atomic state in 254No that is optically inaccessible from the ground state.