Shake up in PES and EA

In the photoelectron spectrum of ethylene, shown in Sec.H.5., the strong band at 27.2 eV is a shake-up band. It is due to simultaneous ionization af one 2b1u electron at 19.0 eV and excitation of another electron from π (1b3n) to ft (1b2g). Since two-electron transitions are forbidden, the shake-up configuration has intensity = 0. However, the shake-up configuration has the same symmetry as the PES band at 23.5 eV. Interaction will therefore take place, and the shake-up configuration will steal intensity from the PES band.