Preparation of individual magnetic sub-levels of ${}^4$He($2^3$S${}_1$) 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 ${}^4$He in the $2^3$S${}_1$ 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 $M_{J^{{}^{\mathrm{\prime }\mathrm{\prime }}}}=+1$ sub-level after magnetic hexapole focusing. Using laser optical pumping via the $2^3$P${}_2-2^3$S${}_1$ transition, we achieve a maximum efficiency of $94\pm 3$% for the population of the $M_{J^{{}^{\mathrm{\prime }\mathrm{\prime }}}}=+1$ sub-level. For the first time, we show that laser optical pumping via the $2^3$P${}_1-2^3$S${}_1$ transition can be used to selectively populate each of the three $M_{J^{{}^{\mathrm{\prime }\mathrm{\prime }}}}$ sub-levels ($M_{J^{{}^{\mathrm{\prime }\mathrm{\prime }}}}=$ -1, 0, +1). We also find that laser optical pumping leads to higher absolute atom numbers in specific $M_{J^{{}^{\mathrm{\prime }\mathrm{\prime }}}}$ sub-levels than magnetic hexapole focusing.