Laser Spectroscopic Investigations of Praseodymium I Transitions: New Energy Levels

We report the discovery of about 140 new energy levels of the neutral praseodymium atom, found by means of laser-induced fluorescence spectroscopy. Their energy has been determined with an uncertainty of 0.010 cm？1 using a wave number calibrated Fourier-transform spectrum. 1. Introduction This work is a continuation of our systematic investigation of the praseodymium atomic spectrum (Pr I) with the goals of a more complete knowledge of its energy level scheme and to widen the classification of its spectral lines. Praseodymium belongs to the rare earth elements, which have in common an open 4f electron shell. In nature one finds only one stable isotope, 141Pr59, with the electronic ground state [Xe]4f3？？6s2, 4I°9/2 and nuclear spin quantum number I=5/2. Its nuclear magnetic dipole moment is [1], and its nuclear electric quadrupole moment Q=？0.0024b [2]. Due to the open 4f shell, Pr has a huge number of energy levels. This in turn leads to a very rich line spectrum. In 1978, the electronic levels discovered by several authors were collected and published, see [3]. Later, great progress was achieved by Ginibre [4–6]. She evaluated high resolution Fourier transform (FT) spectra, thereby discovering a large number of electronic levels of Pr I and Pr II and determined their parities, angular momenta, and hyperfine (hf) constants. Using an atomic-beam-magnetic resonance method, hf constants of many known low-lying metastable levels were determined with high accuracy by Childs and Goodman [7]. Investigation of the hf structure of Pr I lines was performed later by Kuwamoto et al. [8], Krzykowski et al. [9], and Furmann et al. [10]. Our group has been concerned with investigations of the hf structure of Pr I lines since 1999. First values of hf constants have been published in [11]. Later on we concentrated mainly on the discovery of unknown energy levels. Some of the results are published in [12–15]. The spectrum and level structure of the Pr atom are of astrophysical interest [16, 17] and of course are indispensable for a thorough theoretical description of the level scheme [11]. 2. Experimental Details Our investigations are based on a combination of laser-induced fluorescence (LIF) and Fourier-transform (FT) spectroscopy. Spectral lines of Pr were excited by laser light, generated by means of a tunable single-frequency dye laser system. Our source of free atoms was a dc hollow cathode discharge, which produced free Pr atoms by cathode sputtering. Due to the collision processes within the Ar plasma, also high-lying Pr levels are populated, thus the laser light