Ionization spectra of XONO2 (X=F, Cl, Br, I) studied by the SAC–CI method Original Research Article

Recently observed valence-shell ionization spectra of XONO2 (X=F, Cl, Br, I) are investigated by the SAC–CI (symmetry-adapted–cluster configuration-interaction) method. This is the first attempt to give the quantitative assignments for these molecules by the reliable ab initio calculations including electron correlations. Calculated spectra agree very well with the experimental ones, especially for FONO2 and ClONO2. New assignments are proposed for the primary peaks in the outer-valence region, which were characterized by the Koopmans theorem in the previous experimental work. It is also found that there are many shake-up states with considerable intensity in the inner-valence region, especially for BrONO2 and IONO2. The theoretical ionization spectrum of IONO2 is presented prior to the experimental observations.