Rotationally Cold OH^{-} Ions in the Cryogenic Electrostatic Ion-Beam Storage Ring DESIREE

Area of Science:

• Atomic and Molecular Physics
• Chemical Physics
• Quantum Chemistry

Background:

• Understanding the internal quantum state distribution of stored ions is crucial for precision measurements.
• Cryogenic ion storage rings offer unique environments for studying molecular ions at low temperatures.
• Previous studies reported higher rotational temperatures than the cryogenic device temperatures.

Purpose of the Study:

• To characterize the rotational quantum level distribution of hydroxide anion (OH⁻) ions in the DESIREE cryogenic ion-beam storage ring.
• To investigate ion relaxation dynamics and the influence of cryogenic environments on internal molecular states.
• To actively control and enhance the population of the rotational ground state of OH⁻ ions.

Main Methods:

• Near-threshold laser photodetachment spectroscopy was employed to probe the rotational state distribution.
• Ions were stored in the DESIREE cryogenic ion-beam storage ring at Stockholm University.
• Selective photodetachment was used to modify the rotational population.

Main Results:

• Stored OH⁻ ions relaxed to a rotational temperature of 13.4±0.2 K, with 94.9±0.3% in the rotational ground state.
• This temperature is consistent with the measured storage ring temperature of 13.5±0.5 K.
• Active modification via selective photodetachment produced a beam with 99.1±0.1% of ions in the J=0 rotational ground state.
• The intrinsic lifetime of the J=1 rotational level was measured to be 145±28 s.

Conclusions:

• Cryogenic ion storage rings can effectively cool molecular ions to very low rotational temperatures, consistent with the device temperature.
• Selective photodetachment is a viable technique to prepare molecular ion beams with high ground-state populations.
• The results challenge previous findings and highlight the importance of cryogenic environments for achieving cold molecular ions.