Vibrationally resolved O1score-excitation spectra of CO and NO

High-resolution photoabsorption spectra of CO and NO below the O $1s$ ionization threshold are presented. The vibrational fine structure of the O $1\overrightarrow{s}{\pi }^{*}$ 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 $1\overrightarrow{s}{\pi }^{*}$ 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.