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Description
High resolution spectroscopy of molecular nitrogen ions is a prime candidate to measure potential temporal changes in the proton-to-electron mass ratio, $\mu$ [1].
Ion traps facilitate a high degree of localisation in a highly isolated and stable environment. In addition, the shared motional modes of ions co-trapped in the same potential enable techniques such as sympathetic cooling [2] and quantum logic spectroscopy [3]. These techniques allow cooling and read-out of the internal state of a molecular ion, provided a suitable auxiliary ion can be found.
In this experiment, a single $^{14}$N$_2^+$ ion will be co-trapped, in a linear Paul trap, with a $^{40}$Ca$^+$ ion which will act as a frequency reference and be used for the sympathetic cooling and state detection of the nitrogen ion. A vibrational Raman transition in the electronic ground state of $^{14}$N$_2^+$ will be compared to a quadrupole transition in the $^{40}$Ca$^+$ ion. After excitation, the state of the $^{14}$N$_2^+$ ion will be transferred to the $^{40}$Ca$^+$ ion via the shared motion of the ions in a quantum logic spectroscopy scheme.
Prerequisite to this are the preparation of $^{14}$N$_2^+$ into a specific rovibronic state and its non-destructive state detection. Recently, a 2+1’ resonance-enhanced multiphoton ionisation (REMPI) scheme was developed, using the a$^1\Sigma_\textrm{g}$(ν=6) $\leftarrow$ X$^1\Sigma_\textrm{g}^+$(ν=0) band in $^{14}$N$_2$ for the resonant excitation. This scheme was demonstrated to prepare $^{14}$N$_2^+$ in the rovibronic ground state with high purity [4].
References
[1] M. Kajita et al., Physical Review A 89, 032509 (2014).
[2] J. B. Wübbena et al., Physical Review A 85, 043412 (2012).
[3] P. O. Schmidt et al., Science 309, 5735 (2005).
[4] A. Gardner et al., Scientific Reports 9, 506 (2019).
Keywords: high resolution spectroscopy, REMPI, molecular ions
