Theoretical investigation of a two-stage buffer gas cooled beam source

A novel two-stage helium buffer gas cooled beam source is introduced. The properties of the molecular beams produced from this source are investigated theoretically using the CaF as a test molecule. The gas-phase molecules are first produced inside a 3~K helium buffer gas cell by laser ablation and subsequently cooled down to 3~K by collisions with buffer gas atoms. The precooled molecules are then extracted into the 0.5~K helium buffer gas cell where they are cooled further down to 0.5~K by collisions with cold helium atoms. Finally, the cold molecules are extracted out into the high vacuum through the 0.5~K cell exit aperture and form a molecular beam. The mean forward velocity and the beam flux are calculated to be 45~m/s and 8$\times$10$^{12}$ molecules per pulse respectively when both cells are operated in the so-called hydrodynamic entrainment regime. Using this flux and Maxwell-Boltzmann probability density function at 0.5~K, the number of the molecules moving with speeds $\leq$~5~m/s is calculated to be 8$\times$10$^{9}$. These slow and intense beams of the cold molecules are beneficial for efficient magneto-optical trapping of the molecules, investigating sympathetic cooling of the molecules with ultracold atoms, and performing ultrahigh precision molecular spectroscopy.