Measurement of Free-Atom K-Shell Binding Energies of Light Elements

K-shell binding energies in free atoms for Be, B, and C have been obtained to within a few tenths of an eV by applying Auger spectroscopy to fast Be, B, and C ion beams being core-excited in single collisions with CH4 and He. The present values, which correspond to the lowest possible ionization energy, i.e., for B, BE1 S = E(1s2s22p)3P0 - E(1s22s22p)2 P0, are considerably higher than previous values derived from measurements where solid or molecular samples have been used, while good agreement is found with recent free-atom theoretical data.