Abstract
The direct laser cooling of neutral diatomic molecules in molecular beams suggests that trapped molecular ions can also be laser cooled. The long storage time and spatial localization of trapped molecular ions provides an opportunity for multi-step cooling strategies, but also requires careful consideration of rare molecular transitions. We briefly summarize the requirements that a diatomic molecule must meet for laser cooling, and we identify a few potential molecular ion candidates. We then carry out a detailed computational study of the candidates BH+ and AlH+, including improvedab initio calculations of the electronic state potential energy surfaces and transition rates for rare dissociation events. On the basis of an analysis of the population dynamics, we determine which transitions must be addressed for laser cooling, and compare experimental schemes using continuous-wave and pulsed lasers.
Highlights
The long-held notion that laser-cooling molecules is infeasible has been recently overturned by the transverse laser-cooling of SrF [1, 2]
Most intuitive of the types of transitions which might lead to diagonal Franck-Condon factors (FCFs) [40], are transitions which excite a single electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO)
A simple figure of merit (FOM) for the probabilistic FCF approach is calculated by comparing the total time required to Doppler cool the molecule to the vibrational decay rate, since all levels are approximately populated at steady-state
Summary
The long-held notion that laser-cooling molecules is infeasible has been recently overturned by the transverse laser-cooling of SrF [1, 2]. On the basis of spectroscopic data available in the literature (see the appendix), our molecular ion survey found BH+ [28, 29, 30, 31, 32, 33] and AlH+ [34, 29, 35, 36, 37] to be among the most promising candidates In this manuscript, we review all identified challenges of maintaining a closed excitation scheme for direct laser cooling of BH+ and AlH+ stored in ion traps. Unlike for the case of SrF, where slow vibrational decay relative to the interaction time allows a straightforward probabilistic approach to predict repump requirements [1], designing a laser cooling experiment for trapped ions requires careful modeling of vibrational decays within the ground state, since they result in diffusion of parity and rotational quantum numbers. We present a few additional classes of molecules that need to be studied in more detail to decide whether they are candidates for direct laser cooling experiments
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