Vibrationally resolved O 1s core-excitation spectra of CO and NO

High-resolution photoabsorption spectra of CO and NO below the O is ionization threshold are presented. The vibrational fine structure of the O 1s-->pi* and O 1s(-1) Rydberg excitations could be resolved for both molecules, allowing a determination of the vibrational energies and intramolecular distances of the core-excitation states in CO and NO from Franck-Condon analyses. Ab initio calculations are performed for the O 1s--> pi* excitation in CO to give an independent confirmation of the spectroscopic parameters derived from the Franck-Condon analysis. The spectral features of the O 1s(-1) Rydberg region in CO are reassigned on the basis of the experimental results. The results obtained for the O 1s(-1)3s Rydberg state in NO support the idea of a weakening of the molecular bond upon an O 1s(-1) ionization process. [S1050-2947 (99)06205-8].