The core-to-σ* excited state is repulsive for the bond elongation; on the other hand, the core ionized state and the core-to-π* state are bound and the core-to-Rydberg states are almost parallel to the potential energy curve of the core ionized state. Thus, the core-electron excitation to the unoccupied σ* orbital can be mixed with the one-electron Rydberg or continuum orbital, as dependent on the bond distance, and even with the unoccupied π* orbital in some cases. Within the framework of

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... lectron picture, we show σ* orbitals mixed with Rydberg character in the 1s excitation of O 2 and CH 3 F, and Rydberg orbitals mixed with valence character in the 1s excitation of CH 4 , CO 2 , and N 2 O.