Resonance ionization spectrum of autoionization states of lutetium atom

Abstract

<sup>177</sup>Lu is an important medical isotope used in imaging-guided radiotherapy, and it can be produced by irradiating <sup>176</sup>Lu or <sup>176</sup>Yb with high abundance. With an increasing demand for medical isotopes, it is very essential to improve the supply capacity for <sup>177</sup>Lu. The multi-step multi-color photoionization method is an effective method to obtain isotopes, and the information of odd-parity autoionization levels is essential. Laser resonance ionization spectroscopy is one of a few spectroscopic experimental methods that can study autoionization levels. An experimental system is developed for the frontier spectroscopic research, and it consists of custom-made tunable lasers and a high-resolution time of flight mass spectrometer. The lifetime of the excited state 35274.5 cm<sup>–1</sup> is measured to be (31.6 ± 1.7) ns by the delayed photoionization method for the first time. A three-step three-color photoionization process is used to detect the autoionization levels, with a delay of 30 ns between <i>λ</i><sub>2</sub> – <i>λ</i><sub>1</sub> and <i>λ</i><sub>3</sub> – <i>λ</i><sub>2</sub> respectively, in order to avoid any unexpected transitions. Forty-seven odd-parity autoionization levels are obtained, of which 33 levels are discovered for the first time, and the <i>λ</i><sub>2</sub> and <i>λ</i><sub>1</sub> are blocked to exclude possible interference peaks, such as the <i>λ</i><sub>1</sub>+<i>λ</i><sub>3</sub>+<i>λ</i><sub>3</sub> transition. Several autoionization levels show asymmetrical peak shapes, and the Fano fitting is carried out for all the levels to determine the widths and relative transition strengths of the autoionizing transitions. This study provides critical data for the high-efficient photoionization of lutetium atoms in the visible range. The angular momenta of 21 odd-parity autoionization levels in an energy range of 50650–51650 cm<sup>–1</sup> are identified for the first time, which provides a reference for determining the forbidden state of electric dipole transitions from other excited states and ascertaining the electronic configuration.