We report on efficient rovibrational state-selective loading of single ${\mathrm{H}}_2{}^{+}$ molecular ions into a cryogenic linear Paul trap using ($2+1$) resonance-enhanced multiphoton ionization (REMPI). The ${\mathrm{H}}_2{}^{+}$ ions are created by resonant two-photon excitation of ${\mathrm{H}}_2$ molecules from the $X{}^1{\mathrm{\Sigma }}_g^{+}$ state to the $E,F{}^1{\mathrm{\Sigma }}_g^{+}$ state, followed by nonresonant one-photon ionization. The ions are produced from ${\mathrm{H}}_2$ residual gas and sympathetically cooled by a cotrapped, laser-cooled ${}^9{\mathrm{Be}}^{+}$ ion. By tuning the wavelength of the REMPI laser, we observe the loading of single ${\mathrm{H}}_2{}^{+}$ ions via the (${\nu }^{\prime }=0, L^{\prime }=0,1,2$) rovibrational levels of the $E,F{}^1{\mathrm{\Sigma }}_g^{+}$ intermediate state. We measure the success probability for the production of ${\mathrm{H}}_2{}^{+}$ in the (${\nu }^{+}=0, L^{+}=1$) state via the (${\nu }^{\prime }=0, L^{\prime }=1$) level to be $85\left(6\right)\%$ by quantum logic spectroscopy (QLS) of the hyperfine structure of this rovibrational state. Furthermore, we load an ${\mathrm{H}}_2{}^{+}$ ion via the (${\nu }^{\prime }=0, L^{\prime }=2$) level and confirm its rovibrational state is (${\nu }^{+}=0, L^{+}=2$) by QLS. We perform QLS probes on the ion over $19\mathrm{h}$ 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.

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• Molecular spectra
• Multiphoton or tunneling ionization & excitation
• Trapped ions
• Molecule trapping & guiding
• Photoionization of molecules