Laser-optical cycling of cryogenically cooled BN⁻ molecular anions

Abstract We investigate an optical cycling scheme for Doppler cooling cold trapped 11 B 14 N − ions using transitions between the X 2 Σ + ground state and the B 2 Σ + excited state, and analyze here the relevant transitions for photon cycling and repumping. Our results show that slow population decay via the first excited electronic state A 2 Π cannot be neglected. To improve the optical cycling efficiency, we consider additional transitions beyond what would be expected from the highly diagonal FranckCondon factor involving the B 2 Σ + ( v = 0) ← X 2 Σ + ( v = 0) transition. We estimate that the number of cycled photons alone is not likely to be sufficient to bring buffer-gas-cooled 11 B 14 N − to temperatures near the Doppler cooling limit. Hence, pre-cooling, e.g., using a combination of cryogenic buffer gas and photodetachment cooling, will be essential to maximize the optical cycling efficiency and to reach a regime where Coulomb crystallization occurs. To explore pre-cooling with He or Ar buffer gases, we therefore also performed extensive quantum calculations of potential energy curves, transition moments and radiative rate coefficients for the BN − –buffer gas systems, to be implemented in a later study. Our results provide key insights for generating cold negative ions. These anions have, in fact, promising applications in various fields, ranging from quantum science and technology to fundamental physics and chemistry.