New Study Expands Nickel Autoionization Spectra to Advance Laser Isotope Separation Technologies

A recent study examined a new method to identify nickel autoionization states, which has implications for laser isotope separation and photoionization research. This study, which was published in the journal Spectrochimica Acta Part B: Atomic Spectroscopy, presented a data set of nickel autoionization states that expands the initial states (1). Led by researcher Li Yun-Fei and his team at the National Key Laboratory of Particle Transport and Separation Technology and the Research Institute of Physical and Chemical Engineering of Nuclear Industry in China, the study introduces 170 autoionization states of nickel, with 150 of them being newly reported using advanced resonance ionization mass spectrometry (RIMS) techniques (1).

What is nickel?

Nickel is a naturally occurring metal on the periodic table often found in soil and volcanic emissions (2). It can be combined with other metals, such as iron, to produce alloys (2). Nickel is often used to create jewelry and coins (2). Many compounds formed by nickel dissolve in water (2).

Because nickel plays an important role in several industrial applications, researchers are seeking to learn more about its properties and ionization states. This study investigates that more in depth.

What was the experimental procedure?

In this study, the research team concentrated on studying the 3d⁸4s(⁴F)5s configuration of nickel, with atoms selectively excited to high-lying energy states within the 48,000–52,050 cm⁻¹ range using a three-color photoionization scheme (1). These excited atoms were subsequently scanned for autoionization in the 64,600–68,700 cm⁻¹ region. What the researchers discovered was that the scans revealed a rich structure of odd parity autoionization states, many of which had eluded detection in prior studies (1).

When it comes to laser isotope separation, laser frequencies need to be tuned accurately to ionize the selected isotopes while leaving the others neutral. As a result, it is important to understand the fine structure and characteristics of atomic autoionization states (1). In this context, the identification of new states with narrow peak widths and favorable angular momentum properties marks an important advancement (1).

The use of dye lasers was also important in the study. The researchers used them for autoionization scanning and excitation in the study. To ensure the authenticity of the observed spectral peaks, researchers utilized a rigorous experimental design that included cross-scanning of multiple excited states and λ₂-blocked control experiments (1). What these experiments did was remove the peaks caused by unintended transitions, refining the spectral data to improve accuracy (1).

How was Fano profile fitting used in the study?

Fano profile fitting analyzes and models spectral data that exhibits an asymmetrical shape (3). In this study, the researchers used it to characterize each spectral peak’s energy position, asymmetry, and full width at half maximum (FWHM). This analytical technique allowed them to quantify transition properties with high accuracy (1). Notably, the majority of the identified autoionization states exhibited narrow FWHM values of less than 1 cm⁻¹ and angular momentum quantum numbers (J) of 4 or 5 (1). These characteristics suggest the presence of low-n autoionizing Rydberg series, which are particularly advantageous for developing high-efficiency photoionization schemes (1).

Beyond practical applications in isotope separation, this data set provides critical insights for theoretical atomic physics. It offers an empirical benchmark for refining theoretical models such as the Multiconfiguration Dirac-Hartree-Fock (MCDHF) approach, which aims to predict atomic properties based on quantum mechanics (1).

What are the broader implications of this study?

This study is important because it extends to the development of laser ion sources, where efficient photoionization pathways can dramatically improve performance. Additionally, the work enhances our understanding of transition metal excitation mechanisms, particularly for elements like nickel, which play a pivotal role in both industrial and scientific applications (1).

References

1. Jun-Yao, Z.; Jing-Yi, X.; Hong-Run, Z.; et al. Resonance Ionization Mass Spectrometry of Nickel Odd-parity Autoionization States. Spectrochimica Acta Part B: At. Spectrosc. 2025, 231, 107249. DOI: 10.1016/j.sab.2025.107249
2. National Institute of Health, Nickel. NIH.gov. Available at: https://pubchem.ncbi.nlm.nih.gov/compound/Nickel (accessed 2025-07-16).
3. Zhang, W.; Craddock, T. J. A.; Li, Y.; et al. Fano Resonance Line Shapes in the Raman Spectra of Tubulin and Microtubules Reveal Quantum Effects. Biophys. Rep. 2022, 2 (1), 100043. DOI: 10.1016/j.bpr.2021.100043