Laser cooling of barium monofluoride

Abstract

In this work, the first implementation of direct laser cooling of barium monofluoride (BaF) molecules using sub-Doppler forces is presented. This species is a promising candidate for parity violation measurements, the search for the electron’s permanent electric dipole moment and ultracold chemistry. However, due to its large mass, comparatively narrow linewidth and potential branching losses through an intermediate electronic state, this molecular species is notoriously difficult to cool. To achieve laser cooling, first, spectroscopic measurements of the relevant optical transitions were performed. This allowed for an improvement of the molecular constants by one order of magnitude. Next, Doppler-free spectroscopy was conducted on the cooling transition, which revealed a resolved hyperfine splitting in the excited state. Previous molecular laser cooling experiments employed sinusoidal sideband modulation to address such hyperfine structure states. Here, serrodyne modulation was used to create optimized optical spectra, resulting in significantly improved laser cooling. Finally, a Raman cycling scheme was implemented to achieve background-free imaging of the resulting cold molecular beam. In conclusion, an intense and transversally cold molecular beam of 138BaF was prepared, which paves the way for precision tests of fundamental symmetries using BaF.