Abstract
Trapped molecular ions in pure rovibronic states are desirable in experiments ranging from cold chemistry to searches for physics beyond the Standard Model. Resonance-enhanced multiphoton ionisation (REMPI) can be used to prepare molecular ions in specific internal states with high fidelities. However, in the presence of electric fields, ionisation spectra exhibit frequency shifts and the ionisation thresholds are broadened. For this reason, REMPI studies are normally conducted in low and highly homogeneous electric fields, whereas the operating principle of rf ion traps requires electric fields that vary in space and time. In order to investigate the impact of this on the state-selectivity of REMPI in ion traps, we have simulated the expected broadening of the ionisation threshold under various operating conditions of a typical linear Paul trap. In many cases, the width of the ionisation threshold exceeds the separation between rotational energy levels, preventing state-selective ionisation. Careful choice of the trapping and laser parameters during loading can reduce this broadening, enabling state-selective ionisation in some instances. Where this strategy is not sufficient, the broadening can be reduced further by rapidly switching the trapping voltages off and on again during loading. This has been demonstrated experimentally for a Coulomb crystal of ^{40}hbox {Ca}^+ ions without descrystallising it.
Highlights
Trapped molecular ions in pure rovibronic states are desirable in experiments ranging from cold chemistry to searches for physics beyond the Standard Model
In order to assess the influence of electric trapping fields on the state purity of molecular ions generated via Resonance-enhanced multiphoton ionisation (REMPI), we investigated the expected broadening of the ionisation threshold
This broadening is quantified by the full-width at half-maximum (FWHM) of the ionisation threshold
Summary
Resonance-enhanced multiphoton ionisation (REMPI) can be used to prepare molecular ions in specific internal states with high fidelities. Internal-state pumping can be employed to significantly increase the population of polar ions in the rovibronic ground s tate[24,25,26,27,28] This is only possible for a small number of specific molecules with suitable transitions. Molecular ions can be prepared in a low-temperature thermal distribution of their internal states through collisional thermalisation with a neutral laser-cooled s pecies[29,30]. While thermalisation with BBR in room-temperature traps limits this approach for polar molecular ions, they can be loaded state-selectively into cryogenic traps where thermal excitation of rotational levels is greatly reduced. In order to assess the influence of electric trapping fields on the state purity of molecular ions generated via REMPI, we investigated the expected broadening of the ionisation threshold. As a proof of concept, we demonstrate that the trap drive can be rapidly switched off and on again without decrystallising trapped ions
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