Abstract
An apparatus for measurements of appearance potential spectra in the 20-2 000 eV and 10-10 torr range has been constructed and tested. The total resolution is found to be about 0.8 eV. Spectra for different core levels of Ca, Sr, and Ba as well as of their oxides have been measured. The data are discussed in terms of localized, unoccupied electronic states. Comparisons are made with other types of experimental data and with models involving the (local) density of states. For Ba complications arise due to the very localized 4f states, but the results can be explained if the whole two-electron-one-hole system is taken into account.

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