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, cold alkali-metal beams form. 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 and k×3000 K, where k is the Boltzmann constant. For collision energies higher than approximately 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 seeded into the ⁴He jet, resulting in a lithium beam with a most probable velocity of about 2100.28em0exm/s and number densities on the order of 10⁸4pt0excm^-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 seeding efficiency and cooling of ⁸⁷Rb by a supersonic ⁴He jet. The seeding efficiency for ⁸⁷Rb is expected to be similar to ⁷Li.