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

High-resolution photoabsorption spectra of CO and NO below the O is ionizat ion threshold are presented. The vibrational fine structure of the O 1s-->p i* and O 1s(-1) Rydberg excitations could be resolved for both molecules, a llowing a determination of the vibrational energies and intramolecular dist ances of the core-excitation states in CO and NO from Franck-Condon analyse s. Ab initio calculations are performed for the O 1s--> pi* excitation in C O to give an independent confirmation of the spectroscopic parameters deriv ed from the Franck-Condon analysis. The spectral features of the O 1s(-1) R ydberg 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 i dea of a weakening of the molecular bond upon an O 1s(-1) ionization proces s. [S1050-2947 (99)06205-8].