Laser Scheme for Doppler Cooling of the Hydroxyl Cation (OH$^+$)

TL;DR

This paper proposes a laser cooling scheme for trapped OH$^+$ ions using specific electronic transitions, enabling efficient cooling without co-trapped species, which could advance quantum and chemical research.

Contribution

The authors develop a novel laser cooling method for OH$^+$ ions that does not rely on near-diagonal Franck-Condon factors, expanding the range of molecules amenable to laser cooling.

Findings

Identified relevant transitions for photon cycling and repumping in OH$^+$

Coupling into other electronic states is strongly suppressed

Calculated photon scatterings needed for cooling to Raman sideband regime

Abstract

We report on a cycling scheme for Doppler cooling of trapped OH$^+$ ions using transitions between the electronic ground state $X^3\Sigma^-$ and the first excited triplet state $A^3\Pi$. We have identified relevant transitions for photon cycling and repumping, have found that coupling into other electronic states is strongly suppressed, and have calculated the number of photon scatterings required to cool OH$^+$ to a temperature where Raman sideband cooling can take over. In contrast to the standard approach, where molecular ions are sympathetically cooled, our scheme does not require co-trapping of another species and opens the door to the creation of pure samples of cold molecular ions with potential applications in quantum information, quantum chemistry, and astrochemistry. The laser cooling scheme identified for OH$^+$ is efficient despite the absence of near-diagonal Franck-Condon factors, suggesting that broader classes of molecules and molecular ions are amenable to laser cooling than commonly assumed.