In a refinement to MDS it is possible to spin polarise the He* beam to provide qualitative and some quantitative data on magentic properties of the surface. The net magnetic moment of the He* allows for it to be optically pumped within a region of applied magnetic field.
The magnetic projections of the He* atom's two electron's spins are unequal. This gives rise to each excited atom having a magnetic moment. By imposing an external field upon the He* beam we can define an axis of polarisation for the beam. By then optically pumping the He* beam with circularly polarised light the population of atoms with one magentic orientation is depleted in favour of the other. With the highly polarised He* any further optical manipulation of the beam will lead to a loss of polarisation.
The polarisation of the He* beam incident upon a surface produces a spin dependent component of the de-excitation probability. Where the surface has organised net spins the relative orientation of these spins to the spin direction of the incoming He* beam affects the available channels for de-excitation. By reversing the He* beam polarisation an asymmetry between the secondary electron yields can be observed.
The assymetry spectra can indicate the energy levels of spin polarised electrons in the surface structure. By varying the angle of beam polarisation to surface magnetisation direction the relative orientations of the surface spin states can be determined.