Valence ionization spectra of 4π-electron molecules with low-lying satellites involving n-π* and π-π transitions

The valence ionization spectra up to 25-30 eV of the 4 pi-electron molecules, butadiene, acrolein, glyoxal, methylenecyclopropene and methylenecyclopropane were investigated by the SAC-CI method. Accurate theoretical assignments of the spectra were given and further the natures of the low-lying satellites were examined. Acrolein and glyoxal have the low-lying satellites of n(-1)pi(-1)pi* and n(-2)pi* states and the outermost satellites are lower than the pi(-2)pi* state of butadiene. However, their intensities are very small, since they do not effectively interact with the main peaks. The pi(-2)pi* state of methylenecyclopropene with constrained pi-conjugation was calculated to be much higher than that of butadiene, though the first IP is lower. In these spectra, some split peaks were calculated at 15-16 eV and the continuous shake-up states were obtained in the region higher than similar to 18 eV.