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 CH 4 and He. The present values, which correspond to the lowest possible ionization energy, i.e., for B, B E 1 S = E (1 s 2 s 2 2 p ) 3 P 0 - E (1 s 2 2 s 2 2 p ) 2 P 0 , 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.