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

A near-edge X-ray absorption fine-structure (NEXAFS) study of the O K --> Pi* 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 Pi* excited states are Gamma(O2) = 149 +/- 10 meV and Gamma(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 similar to 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. (C) 1999 Elsevier Science B.V. All rights reserved.