Preparation of individual magnetic sub-levels of He(2S) in a supersonic beam using laser optical pumping and magnetic hexapole focusing.

We compare two different experimental techniques for the magnetic-sub-level preparation of metastable He in the 2S level in a supersonic beam, namely, magnetic hexapole focusing and optical pumping by laser radiation. At a beam velocity of v = 830 m/s, we deduce from a comparison with a particle trajectory simulation that up to 99% of the metastable atoms are in the M = +1 sub-level after magnetic hexapole focusing. Using laser optical pumping via the 2P-2S transition, we achieve a maximum efficiency of 94% ± 3% for the population of the M = +1 sub-level.