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 ²$Φu$ state are also studied. Furthermore, the doublet structure of the ²$Φu$ state is resolved for the first time. Two new states with quartet multiplicity are assigned to the ⁴$Πg$ and the ⁴$Πu$ states which dissociate to the limit, $[O(3P)+O+(2D)],$ at 22.059 eV. Also we tentatively assign another newly observed band to the ²$Πu$ 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.