The influence of valence excitations on spectral distribution of vibrational excitations associated with C 1s photoionization in the CO molecule

► A new mechanism of non-FC distributions in free molecules is predicted. ► The mechanism is originated by coupling of valence and electronic transitions. ► The work of the mechanism is examined by considering C 1s shell ionization in CO. ► The distribution fv' differs substantially from the FC-distribution for v′>4. ► The mechanism is general and works in photoemission from various chemical compounds.