Ionization Energy of Promethium Measured—Almost 75 Years After Its Discovery

Ionization Energy of Promethium Measured—Almost 75 Years After Its Discovery


Promethium (Pm) is a rare element with no stable isotopes that was discovered in 1945. Despite its rarity, it has some applications, e.g., in luminous paint or atomic batteries. Some of its fundamental properties, however, have not been determined experimentally. The first ionization potential (IP), for example, had never been measured. This is due to the element’s complex electronic structure, which makes its atomic spectra difficult to interpret.

Dominik Studer, University of Mainz, Germany, and colleagues have experimentally determined the precise value of the first ionization potential of Pm. The team used resonance ionization spectroscopy (RIS), a method that works even with very small samples, to record atomic spectra of Pm. Their sample consisted of approximately 15 ng of purified 147Pm. The method uses a pulsed laser to ionize the Pm atoms. The resulting ion beam is sorted by mass and the desired ions are counted using a channel electron multiplier.

Using this approach, the team measured over 1,000 atomic transitions in the blue and near-infrared (NIR) parts of the spectrum, most of them between high excited energy levels. The first ionization potential was determined indirectly by measuring the ionization of weakly bound states within a static electric field, which allowed the researchers to extrapolate the ionization energy without an electric field. They found a value of 45020.8(3) cm−1. This is in good agreement with the trend of the IPs of all lanthanides. According to the researchers, the determined precise IP could be a valuable benchmark for ab initio atomic physics and quantum chemistry calculations.


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