Monte-Carlo simulations of the capture and cooling of alkali-metal atoms by a supersonic helium jet

We present three-dimensional Monte-Carlo simulations of the capture of 1000 K ^7Li or 500 K ^87Rb atoms by a continuous supersonic ^4He jet and show that intense alkali-metal beams form with narrow transverse and longitudinal velocity distributions. The nozzle creating the ^4He jet is held at approximately 4 K. These conditions are similar to those in the cold ^7Li source developed by some of us as described in [Phy. Rev. A 107, 013302 (2023)]. The simulations use differential cross-sections obtained from quantum scattering calculations of ^7Li or ^87Rb atoms with ^4He atoms for relative collision energies between k× 1 mK to k× 3000 K, where k is the Boltzmann constant. For collision energies larger than ≈ k× 4 K the collisions favor forward scattering, deflecting the ^7Li or ^87Rb atoms by no more than a few degrees. From the simulations, we find that about 1% of the lithium atoms are captured into the ^4He jet, resulting in a lithium beam with a most probable velocity of about 210 m/s and number densities on the order of 10^8 cm^-3. Simulations predict narrow yet asymmetric velocity distributions which are verified by comparing to fluorescence measurements of the seeded ^7Li atoms. We find agreement between simulated and experimentally measured seeded ^7Li densities to be better than 50% across a range of ^4He flow rates. We make predictions for capture efficiency and cooling of ^87Rb by a supersonic ^4He jet. The capture efficiency for ^87Rb is expected to be similar to ^7Li.