Towards internal and external sympathetic cooling of CaH⁺

The cooling of molecular ions to their internal and external ground state is crucial for many applications of trapped molecular ions. We are implementing a general method for cooling the external and internal degrees of freedom of the molecular ions using laser-cooled atomic ions and neutral atoms, respectively [1]. Our plan is to demonstrate this method with calcium mono-hydride ions (CaH⁺) using co-trapped and laser cooled calcium (⁴⁰Ca⁺) ions and potassium (³⁹K) atoms for sympathetic cooling. In our ion-atom hybrid trap [2], we can stably hold the three species together and control the chemical reactions between them. We have previously measured a slow charge exchange between Ca⁺ and K and were able to minimize the rate coefficient to (9.7 ± 3.9) x 10^-11 cm³/s with 4% of Ca⁺ in the P_1/2 state [3]. Recently, we have measured that the molecular ions (CaH⁺) whose motion is sympathetically cooled by atomic ions (Ca⁺) are stable in the presence of neutral atoms (³⁹K). Our next step is to perform rotational spectroscopy of CaH⁺ using resonance enhanced multi-photon dissociation (REMPD) [4] to determine the internal molecular ion temperature. This will allow us to detect the cooling of the rotational motion of the molecular ions due to the interaction with the ultracold neutral atoms.



References:

[1] E. R. Hudson EPJ Techn. Instrum. 3, 8 (2016)

[2] S Jyothi et al. Rev. Sci. Instrum. 90, 103201 (2019)

[3] H. Li et al. Phys. Chem. Chem. Phys. 22, 10870 (2020)

[4] A. T. Calvin et al. J. Phys. Chem A 122, 3177 (2018)