Vibrationally resolved oxygen K→Π∗ spectra of O2 and CO

Abstract A near-edge X-ray absorption fine-structure (NEXAFS) study of the O K→Π ∗ excitation in CO (534 eV) and O 2 (531 eV) molecules has been carried out. A Franck–Condon analysis of the transitions has been accomplished assuming a Morse potential energy curve for the core hole states. The vibrational constants of O 2 are in agreement with values expected from the equivalent core molecule OF. The values of the lifetime widths of the Π ∗ excited states are Γ O 2 =149±10 meV and Γ CO =158±8 meV. The measured lifetime width of O 2 is an upper limit to the true value as unresolved spin–orbit structure is believed to be present, which may cause an additional broadening of ∼5 meV. In spite of this, it is substantially smaller than the hitherto accepted value of 180 meV, necessitating a re-evaluation of most previously published data.