Ultracold molecular hydrogen ions in a linear radiofrequency trap: Novel systems for molecular frequency metrology

We have measured the ro-vibrational infrared transition (v = 0, J = 2) → (v = 4, J = 1) in ultracold HD⁺ molecular ions embedded inside Be⁺ Coulomb crystals, by 1+1 REMPI spectroscopy. The molecular ions, trapped in a linear radiofrequency trap, were cooled to temperatures of few tens of mK, via sympathetic cooling using laser-cooled Be⁺ ions. For detection, we have used secular excitation of the HD⁺ ions to monitor their loss rate. The transition frequency agrees with a theoretical prediction.