Researchers at the Innovation Academy for Precision Measurement Science and Technology (APM) of the Chinese Academy of Sciences have achieved a breakthrough in high-resolution molecular spectroscopy. By cooling hydrogen molecular ions (HD⁺) to just 18 millikelvins (mK), they’ve obtained the most precise vibrational-rotational spectra ever recorded for this fundamental molecule. The findings, published in Physical Review A, validate quantum electrodynamics (QED) theory with unprecedented accuracy.
The Importance of HD⁺
HD⁺, composed of one proton, one deuteron, and one electron, serves as a crucial testing ground for fundamental physics. Its simple structure allows for highly accurate theoretical calculations, making it ideal for verifying QED and determining fundamental constants like the proton-electron mass ratio. Any discrepancies between experimental results and theory could signal new physics beyond the Standard Model.
Cooling to the Limit
The key to this precision was extreme cooling. To minimize measurement errors caused by atomic motion (Doppler broadening), the researchers used laser-cooled beryllium ions to chill the HD⁺ ions to near absolute zero. This process significantly reduces thermal noise, allowing for more accurate spectral readings.
Preparing the Ground State
A major challenge was the low population of HD⁺ molecules in their lowest energy state (v = 0, N = 0). To overcome this, the team employed a technique called resonance-enhanced threshold photoionization (RETPI). RETPI precisely prepares HD⁺ ions in the ground state with an initial population degree of 93%, dramatically increasing the signal strength for subsequent measurements. Compared to traditional methods, this approach offers a substantial improvement in ground-state preparation efficiency.
Detecting “Dark Ions”
HD⁺ ions, when trapped alongside fluorescent beryllium ions, appear as non-fluorescent “dark ions.” Monitoring the change in their number during resonant dissociation is crucial for spectral measurement. To address this, the researchers developed a spatially resolved fluorescence collection technique using a high-sensitivity, electron-multiplying intensified CCD (EMICCD) camera. This setup allows real-time imaging of the ion crystal and non-destructive measurement of HD⁺ ion numbers.
Unprecedented Accuracy
Using these innovative methods, the team measured the vibrational-rotational transition spectrum of HD⁺ ions (v,N):(0,0)→(6,1) with a frequency value of 303,396,506.7(20) MHz. The relative accuracy of this measurement reaches parts per billion (ppb), matching the most precise QED theoretical predictions. This validation further strengthens the Standard Model of particle physics.
“These measurements represent a significant step forward in high-precision spectroscopy and provide a stringent test of fundamental physical theories,” said Dr. [Researcher’s Name].
The ability to measure molecular spectra with such precision opens new avenues for exploring fundamental physics and refining our understanding of the universe
