A research team from the Innovation Academy for Precision Measurement Science and Technology of the Chinese Academy of Sciences has achieved a remarkable milestone in the precise measurement of the vibrational-rotational spectra of hydrogen molecular ions, known as HD+. Their findings, published in Physical Review A on November 14, 2025, detail groundbreaking techniques that enhance the accuracy of these measurements.
The team successfully prepared a beryllium and HD+ two-component ion Coulomb crystal at a temperature of just 18 millikelvins (mK) within a linear ion trap. This temperature reduction was crucial for minimizing the effects of Doppler broadening on their measurements, allowing for unprecedented precision in the vibrational-rotational transition spectra of HD+ molecular ions.
To facilitate this, the researchers introduced an innovative method known as resonance-enhanced threshold photoionization (RETPI). This technique enabled them to prepare HD+ ions in the vibrational-rotational ground state with a remarkable population degree of 93%. This significant achievement surpasses traditional methods, such as cryogenic cooling or optical pumping, thereby establishing a strong foundation for subsequent high-resolution transition detection.
Innovative Techniques for Spectral Measurement
In their experimental setup, HD+ ions manifested as non-fluorescent “dark ions” within the ion crystal, presenting unique challenges for spectral measurement. To overcome this, the researchers developed a spatially resolved fluorescence collection technique, employing a high-sensitivity, electron-multiplying intensified CCD (EMICCD) camera. This advanced configuration allowed for real-time imaging of the ion crystal and the non-destructive measurement of HD+ ion numbers during resonant dissociation.
Utilizing these innovative techniques, the team measured, for the first time, the vibrational-rotational transition spectrum of HD+ ions, specifically the transition from states (v,N):(0,0) to (6,1). The transition frequency was determined to be 303,396,506.7(20) MHz, achieving a relative accuracy reaching parts per billion (ppb). Notably, this value aligns closely with the most precise theoretical predictions from quantum electrodynamics (QED).
The researchers further deduced the value of the proton-electron mass ratio, denoted as μpe, to be 1836.152648(45). This measurement is consistent with the 2022 recommended value from the International Council for Science’s Committee on Data for Science and Technology, underscoring the reliability of their findings.
The implications of this research extend beyond academic curiosity; precise measurements of fundamental constants can drive advancements in various fields, including particle physics and cosmology. The team’s work not only showcases the capabilities of modern experimental physics but also sets a new standard for future research involving molecular ions.
The study, led by Qian-Yu Zhang and his colleagues, marks a significant step in understanding the fundamental properties of matter. Their research could pave the way for further explorations in quantum mechanics and enhance our comprehension of the universe’s underlying principles.
For more information, please refer to the published work by Qian-Yu Zhang et al., titled “Rovibrational spectroscopy of state-selected HD + ions through spatially resolved fluorescence collection” in Physical Review A.
