High-resolution threshold photoelectron spectra of molecular oxygen in the 18–24 eV region

A high-resolution threshold photoelectron spectrum of molecular oxygen has been studied between 18 and 24 eV using the penetrating field technique and a synchrotron radiation source. Nine ionic states are observed, and three of them for the first time. Higher vibrational levels of the b 4Σg− state and the 2Φu state are also studied. Furthermore, the doublet structure of the 2Φu state is resolved for the first time. Two new states with quartet multiplicity are assigned to the 4Πg and the 4Πu states which dissociate to the limit, [O(3P)+O+(2D)], at 22.059 eV. Also we tentatively assign another newly observed band to the 2Πu state. The long vibrational progression of the 3 2Πu state is clearly observed in the region 22.3–23.8 eV with doublet splitting which increases with increasing of the vibrational quantum number.