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Researchers at Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz (HIM) have developed a novel method for investigating the internal structure of atoms and discovered previously unknown atomic transitions in samarium, a rare earth element. Their findings were in the journal Â鶹ÒùÔºical Review Applied.

The ability to describe the internal structure of atoms is important not only for understanding the composition of matter, but also for designing new experiments to explore fundamental physics. Specific experiments require samples of atoms or molecules with particular properties, which depend heavily on the phenomenon to be explored. However, the knowledge of the energy-level structure of many atoms remains incomplete, particularly in the case of the rare earth and actinide atoms.

Spectroscopy is one of the most widely used techniques for studying the structure of atoms. This technique is based on the principle that electrons absorb or emit energy when they move between energy levels in an atom. Each element has a unique set of wavelengths of light that are emitted or absorbed due to these transitions. This is known as the atomic spectrum.

"High-resolution, broadband spectroscopy is essential for precision measurements in atomic physics and the search for new fundamental interactions," explains Razmik Aramyan, Ph.D. student in the group of Prof. Dr. Dmitry Budker and main author of the paper. "But progress is often hindered by the difficulty of measuring complex atomic spectra, mainly due to two technical limitations: the difficulty of properly distinguishing the signals emitted by the sample and the limited range of wavelengths that instruments can detect."

To overcome those limitations, Aramyan and his collaborators have applied and further developed a method known as dual-comb spectroscopy (DCS), which allows to measure atomic spectra at a wide band of electromagnetic frequencies with high resolution and high sensitivity.

The DCS is based on the optical frequency comb technique, for which the Nobel Prize in Â鶹ÒùÔºics was awarded in 2005. Optical frequency combs are specialized lasers that measure exact frequencies of light. In DCS, two of these combs are used in coherent mode, enabling more accurate measurements of the sample's spectrum than conventional methods.

In order to detect weak signals with high precision—one of the challenges of DCS—the group also implemented multiple photodetectors to improve what is known as the signal-to-noise ratio. This combination made it possible to clearly read the experimental data and determine the different wavelengths of the spectrum.

"This study introduces an enhanced multichannel DCS approach that combines a photodetector array with a novel scheme for resolving frequency ambiguities, enabling ambiguity-free, high-signal-to-noise-ratio broadband measurements," summarizes Aramyan.

This is the first step toward implementing "Spectroscopy 2.0," an international project that aims to develop what is known as a "massively parallel spectroscopic tool"—one that can perform a large number of spectroscopic measurements simultaneously. This tool will be used to perform spectroscopy of dense atomic and molecular spectra under intense magnetic fields.

First successful application: The spectrum of samarium vapor

DCS is particularly well suited to filling gaps in atomic data, as the current publication confirms. Thanks to their innovative approach, Aramyan and colleagues were able to record the spectrum of samarium vapor at different temperatures and analyze the spectral behavior at different samarium concentrations. When comparing their results with existing data sets, they found spectroscopic lines that were previously unknown.

"We have discovered several previously undescribed samarium absorption lines. This illustrates the potential of our method to uncover previously unknown atomic properties. It opens up promising possibilities for massively parallel spectroscopy, for example for the spectroscopy of atoms in pulsed, ultra-high magnetic fields," concludes Aramyan.