Preparation of individual magnetic sub-levels of ⁴He(2³S₁) 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 4He in the 23S1 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 MJ″ = +1 sub-level after magnetic hexapole focusing. Using laser optical pumping via the 23P2–23S1 transition, we achieve a maximum efficiency of 94% ± 3% for the population of the MJ″ = +1 sub-level. For the first time, we show that laser optical pumping via the 23P1–23S1 transition can be used to selectively populate each of the three MJ″ sub-levels (MJ″ = −1, 0, +1). We also find that laser optical pumping leads to higher absolute atom numbers in specific MJ″ sub-levels than magnetic hexapole focusing