Development of a spin-polarized helium(+) ion source for surface studies

Development of a low-energy electron-spin-polarized He$\sp+$ ion source is described. The ion source uses an optically pumped rf discharge. Metastable He(2$\sp3$S) atoms are produced by electron impact in the discharge and then are optically pumped into the M$\sb{\rm j}$(M$\sb{\rm s}$) = +1 or $-$1 magnetic sublevels by circularly polarized radiation from a Ti:sapphire laser operating at 1.083 $\mu$m. He$\sp+$ ions are produced by Penning ionization in metastable-metastable collisions and should be spin-polarized because of spin conservation in such processes. A method for accurately determining the net polarization of He(2$\sp3$S) atoms in the discharge by measuring the absorption of light from a tunable diode laser is described. This method can be used to optimize source performance. Optically pumping the discharge on the D0, D1, or D2 transitions gave polarizations of 31.2%, 47.1%, and 18.2%, respectively. It is expected that optically pumping the discharge on the D1 transition will produce $\sim$80+% polarization of the metastable atoms after suitable optimization of operating parameters