New Value for the Metastability Exchange Cross Section in Helium

We have investigated the angular aspect of metastability exchange collisions between ground-state and $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ metastable ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms theoretically and experimentally. Coupling between the orientations of the two $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$ hyperfine levels and partial coherence survival after a collision are shown to be important. We propose a different interpretation of previous experiments, and we suggest a new value of the metastability exchange cross section: $\sigma =(7.6\mathrm{}\pm 0.4)\times {10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$. The case of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures is also studied.