Spin-polarized ion neutralization spectroscopy (SPINS), in which a beam of electron-spin-polarized He+ ions is directed onto a surface and analysis is done of the electrons ejected from the surface as a result of ion neutralization, is a unique tool for surface studies that has unparalleled surface specificity. This technique has been used in studies described in this thesis of Mg surfaces and rare gas van der Waals solids. The possibility of plasmon excitation in He+ ion neutralization at Mg has been investigated, but this process is found to be unlikely. Instead, the mechanism for ion neutralization appears to be resonance neutralization followed by electron ejection through Auger deexcitation. Experiments at frozen Xe surfaces found that ion neutralization proceeds by an Auger neutralization-type process above the surface, while experiments at frozen Kr are explained by the formation of two types of collision complexes at the surface, one ejecting an electron by double ionization of a surface atom, the other by single ionization of two surface atoms.