Rotational excitation in sympathetic cooling of diatomic molecular ions by laser-cooled atomic ions

TL;DR

This paper investigates how sympathetic cooling of diatomic molecular ions by laser-cooled atomic ions can induce rotational excitation, affecting molecular quantum states during the cooling process.

Contribution

It provides estimates of rotational state changes during sympathetic cooling and assesses cooling times in different experimental setups.

Findings

Rotational excitation accumulates over collisions during sympathetic cooling.

Cooling times vary depending on the experimental configuration.

Electric field-induced rotational transitions can impact molecular state purity.

Abstract

Sympathetic cooling of molecular ions through the Coulomb interaction with laser-cooled atomic ions is an efficient tool to prepare translationally cold molecules without, ideally, affecting the internal state of the molecular ions. However, the electric field due to the Coulomb interaction may induce rotational transitions that change the purity of initially quantum state prepared molecules. Here, we use estimates of rotational state changes in single collisions of diatomic ions with atomic ions [arXiv:1905.02130] to determine the overall rotational excitation accumulated over the sympathetic cooling. Considering two different experimental scenarios, that of a molecular ion co-trapped with a single atomic ion and a molecular ion immersed in a Coulomb crystal of atomic ions, we also estimate the cooling time.