State-Selective Ionization and Trapping of Single H$_2^+$ Ions with (2+1) Multiphoton Ionization

We report on efficient rovibrational state-selective loading of single H$_2^+$ molecular ions into a cryogenic linear Paul trap using (2+1) resonance-enhanced multi-photon ionization (REMPI). The H$_2^+$ ions are created by resonant two-photon excitation of H$_2$ molecules from the $X\;^1\Sigma_g^+$ state to the $E,F\;^1\Sigma_g^+$ state, followed by non-resonant one-photon ionization. The H$_2^+$ ions are produced from residual gas and sympathetically cooled by a co-trapped, laser-cooled $^9$Be$^+$ ion. By tuning the wavelength of the REMPI laser, we observe the loading of single H$_2^+$ ions via the ($\nu' = 0$, $L' = 0, 1, 2, 3$) rovibrational levels of the $E,F\;^1\Sigma_g^+$ intermediate state. We measure the success probability for the production of H$_2^+$ in the ($\nu^+ = 0$, $L^+ = 1$) state via the ($\nu' = 0$, $L' = 1$) level to be 85(6)% by quantum logic spectroscopy (QLS) of the hyperfine structure of this rovibrational state. Furthermore, we load an H$_2^+$ ion via the ($\nu' = 0$, $L' = 2$) level and confirm its rovibrational state to be ($\nu^+ = 0$, $L^+ = 2$) by QLS. We perform QLS probes on the ion over 19 h and observe no decay of the rotationally excited state. Our work demonstrates an efficient state-selective loading mechanism for single-ion, high-precision spectroscopy of hydrogen molecular ions.