Bichromatic slowing of MgF molecules in multilevel systems

We present a theoretical study of the bichromatic force exerted on magnesium monofluoride (MgF), in which we have considered the complex vibrational and rotational levels and the effects of small internal splittings and degeneracies, including fine and hyperfine structures and the magnetic quantum numbers. We calculate some parameters used in the MgF molecular transitions between X ²Σ⁺, A ²Π and B ²Σ⁺ states. It is the first time that the radiative lifetimes of B ²Σ⁺ →X ²Σ⁺ and B ²Σ⁺→A ²Π have been derived by ab initio calculations. The detailed numerical modeling of bichromatic forces by direct numerical solution for the time-dependent density matrix is presented. Here, we propose a simplified numerical model to study the dynamic process of MgF slowing by neglecting the effect of the high vibrational levels, in which a skewed magnetic field is applied to destabilize the dark state. We deduce the relation between per-bichromatic-component irradiance I and the total Rabi frequency amplitude. We also compare our proposed simplified model with two other theoretical ones (Aldridge et al 2016 Phys. Rev. A 93 013419). Monte Carlo simulations show that a buffer-gas-cooled MgF beam with a forward velocity of 120 m s^-1 can be decelerated nearly to several m/s within a distance of ∼0.5 cm. Comparing with the B →X transition, we find that the A →X transition in MgF is more suitable for the bichromatic force cooling and slowing.