Production of NaCa$^+$ molecular ions in the ground state from cold atom-ion mixtures by photoassociation via an intermediate state

TL;DR

This paper theoretically investigates optical pathways for forming cold NaCa$^+$ molecular ions from atom-ion mixtures, demonstrating that a two-photon photoassociation scheme can efficiently produce ground state ions.

Contribution

It introduces a new two-photon photoassociation method for creating ground state NaCa$^+$ ions from cold atom-ion mixtures, based on ab initio potential energy calculations.

Findings

Two-photon scheme yields significant ground state ion production.

Single-step process requires higher density or laser intensity.

Analysis of pathways identifies efficient formation routes.

Abstract

We present a theoretical analysis of optical pathways for formation of cold Ca($^1$S)Na$^+$($^1$S) molecular ions via an intermediate state. The formation schemes are based on ab initio potential energy curves and transition dipole moments calculated using effective-core-potential methods of quantum chemistry. In the proposed approach, starting from a mixture of cold trapped Ca$^+$ ions immersed into an ultracold gas of Na atoms, (NaCa)$^+$ molecular ions are photoassociated in the excited E$^{1}\Sigma^+$ electronic state and allowed to spontaneously decay either to the ground electronic state or an intermediate state from which the population is transferred to the ground state via an additional optical excitation. By analyzing all possible pathways, we find that the efficiency of a two-photon scheme, via either B$^{1}\Sigma^+$ or C$^{1}\Sigma^+$ potential, is sufficient to produce significant quantities of ground state (NaCa)$^+$ molecular ions. A single-step process results in lower formation rates that would require either a high density sample or a very intense photoassociation laser to be viable.