Developing a spin polarized helium ion source for surface electronic structure studies

Development of a low energy spin polarized He$\sp+$ source for application in studies of electron spin correlations in ion neutralization at surfaces is described. The source is based on an optically pumped helium discharge. An rf discharge is used to produce metastable helium atoms in a clean pyrex glass cell. A 300 mW Ti:sapphire laser operating at 1.083 $\mu$m is used to optically pump the He(2$\sp3$S) atoms on the $\rm 2\sp3S\sb1\leftrightarrow 2\sp3P\sb1\ (D\sb1)$ transition to preferentially populate the $\rm He(2\sp3S)\ M\sb{j}(M\sb{s}) = +1\ or\ -1$ magnetic sublevels. The electron spin polarization of the resulting atoms is measured using optical absorption. He(2$\sp3$S) polarizations of $\sim$60-70% can be routinely achieved. He$\sp+$ ions, which are produced in metastable-metastable collisions and therefore are expected to be polarized, are extracted from the discharge cell and are focused using a system of ion optics. The ion energy spread is large, $\sim$30-40 eV, but can be reduced to $\sim$3-4 eV by energy filtering. The resulting ion current, $\sim$0.5 nA, is sufficient to study spin dependences in a variety of ion-surface interactions