Vibrationally resolved oxygen K->pi* spectra of O and CO

A near-edge X-ray absorption fine-structure (NEXAFS) study of the O K->pi* excitation in CO (534 eV) and O2 (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 O2 are in agreement with values expected from the equivalent core molecule OF. The values of the lifetime widths of the ?* excited states are ?O2=149±10 meV and ?CO=158±8 meV. The measured lifetime width of O2 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.