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The spectra of X-ray photoelectron spectroscopy (XPS) were measured for Fe16N2(001) single crystal films epitaxially grown by molecular beam epitaxy (MBE). It was found that the binding energy of N-1s (E N 1S) of Fe16N2 is lower than that of Fe4N, indicating that the energy level of N-1s electron in Fe16N2 is higher due to the larger value of the electron-electron Coulomb energy within N atom. From this result, it is expected that more electrons transfer from N atoms to N atoms in Fe16N2 than in Fe4N. It is likely that the N atoms in Fe16N2 act as electron acceptors and this can lead to the nearly full spin polarization in each Fe atoms with a ferromagnetic coupling due to electron hopping.