Hyperfine Structure and Isotope Shifts of Xenon Explored with Doppler-Free Saturated Absorption Spectroscopy

In a pioneering study published in Spectrochimica Acta Part B: Atomic Spectroscopy, Alexandre Kolomenskii and coworkers from Texas A&M University in College Station, Texas, delves into the hyperfine structure (HFS) and isotope shifts (ISs) of xenon using Doppler-free saturated absorption spectroscopy. The research focuses on near-infrared (NIR) transitions in the 820–841 nm spectral range, utilizing commercial xenon gas samples with natural isotopic abundances.

Modern car xenon lamp headlight | Image Credit: © Scanrail - stock.adobe.com

Doppler-free saturated absorption spectroscopy is a technique used to achieve high-resolution spectroscopic measurements by eliminating the broadening effects caused by the Doppler effect. It involves employing two laser beams to interact with the atoms or molecules of interest. One laser beam excites the atoms to an excited state, while the other beam probes the absorption of the excited atoms. By carefully adjusting the frequencies of the two laser beams, the Doppler broadening caused by thermal motion of the atoms can be canceled out, allowing for precise observation of fine spectral features such as hyperfine structure and isotope shifts. This technique is particularly useful when studying transitions with small frequency variations, enabling detailed investigations of atomic or molecular properties with exceptional accuracy. Doppler-free saturated absorption spectroscopy has applications in various fields, including atomic physics, spectroscopy, precision measurements, and laser frequency stabilization.

To facilitate their investigation, the researchers employed a highly tunable narrow-line Ti:sapphire laser for precise measurements. Notably, the team successfully resolved the hyperfine structures of ¹²⁹Xe and ¹³¹Xe, isotopes that possess nonzero magnetic moments and electric quadrupoles, leading to significant HFS line splitting. Additionally, the study observed clear isotope shifts for even isotopes near wavelengths of 820.860 nm and 841.150 nm in vacuum.

A Ti:sapphire laser is a solid-state laser that uses a titanium-doped sapphire crystal as its gain medium. It emits intense, ultrashort pulses of laser light in the NIR to visible wavelength range. Ti:sapphire lasers are widely used in scientific research for applications such as ultrafast spectroscopy and microscopy, thanks to their tunability, high power, and femtosecond pulse duration.

Through comprehensive modeling of the observed absorption lines, the researchers achieved qualitative replication of the experimental spectral profiles. The absorption peaks corresponding to the hyperfine structure and isotope shifts of xenon isotopes serve as distinctive spectral fingerprints for their identification. The ultimate objective lies in the quantitative spectral analysis, enabling the determination of isotopic abundances in natural xenon gas samples, including both stable and radioactive isotopes.

The investigation of hyperfine structure and isotope shifts in atomic spectra offers valuable insights into nuclear spins, moments, and charge distribution. xenon, with its seven stable isotopes and various natural abundances, provides an intriguing subject for such studies. The odd isotopes ¹²⁹Xe and ¹³¹Xe, in particular, exhibit HFS because of their nonzero nuclear spin, while ¹³⁶Xe, characterized by closed shells and 82 neutrons, behaves as a magic nucleus.

The ability to accurately measure hyperfine structure and isotope shifts is vital for numerous applications. In the case of xenon gas, it holds potential for highly sensitive detection of underground or underwater nuclear explosions, astrophysical research including meteorite dating, atmospheric studies, and medical applications.

This work opens avenues for further investigations into the isotopic abundances of xenon gas, contributing to advancements in diverse fields ranging from nuclear security to atmospheric and astrophysical research. The findings of this study serve as a testament to the power of Doppler-free saturated absorption spectroscopy in unraveling the intricate characteristics of Xe's hyperfine structure and isotope shifts.

Reference

(1) Bounds, J.; Kolomenskii, A.; Trainham, R.; Manard, M.; Schuessler, H.Hyperfine structure and isotope shifts of xenon measured for near-infrared transitions with Doppler-free saturated absorption spectroscopy. Spectrochimica Acta Part B: At. Spectrosc. 2023, 202, 106635. DOI: 10.1016/j.sab.2023.106635