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

Abstract

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 (${\nu }^{\prime }=0$, $L^{\prime }=0,1,2,3$) 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 (${\nu }^{\prime }=0$, $L^{\prime }=1$) 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 (${\nu }^{\prime }=0$, $L^{\prime }=2$) 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.

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