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Molecular spectroscopy studies have been carried out on various diatomic molecular ions co-trapped with atomic ions in ion traps, including CaH$^+$, MgH$^+$, CaO$^+$, HD$^+$, N$_2^+$ and H$_2^+$, contributing to fundamental physics research and quantum information processing applications [1-6]. On a similar platform consisting of a single molecular ion co-trapped with one Ca$^+$ qubit ion, we aim to study polyatomic molecular ions with more complex internal structure than diatomic species. These ions are of particular interest due to, for instance, ultrafast intramolecular vibrational energy redistribution which plays an important role in chemical and biological processes. We plan to perform pump-probe experiments with femtosecond laser pulses on polyatomic molecular ions in our system [7]. The interaction between the pulses and the target molecule can be characterized as single photon absorption events which introduce photon recoils that affects the center-of-mass motion of the two-ion crystal. Making use of the Ca$^+$ qubit transition, we could probe the change of its motional state and thus detect the single photon absorption events on the molecule [8]. This method is tested on CaOH$^+$ where we perform vibrational spectroscopy on its O-H stretching mode which has relatively high transition energy and transition dipole moment. With such measurement, we also plan to investigate the possibility of utilizing such intramolecular vibrational mode for rotational state preparation of a polyatomic molecular ion like CaOH$^+$ with optical pumping via vibrational transitions, as has been demonstrated on diatomic molecular ions with smaller mass [9-10].
[1] F. Wolf et al., Nature 530(7591), 457–460 (2016)
[2] C. W. Chou et al., Nature 545(7653), 203–207 (2017)
[3] M. Sinhal et al., Science 367(6483), 1213–1218 (2020)
[4] S. Alighanbari et al., Nature 581(7807), 152–158 (2020)
[5] D. Holzapfel et al., arXiv:2409.06495 (2024)
[6] L. Qi et al., arXiv:2411.07137 (2024)
[7] P. Schindler, New J. Phys. 21(8), 083025 (2019)
[8] C. Hempel et al., Nature Photonics 7, 630–633 (2013)
[9] P.F. Staanum et al., Nature Phys 6(4), 271–274 (2010)
[10] T. Schneider et al., Nature Phys 6(4), 275–278 (2010)
