Dissociative double photoionisation of CO below the CO⁺⁺ threshold

Dissociative double photoionisation of CO below the adiabatic double ionisation threshold has been studied by the time-of-flight photoelectron- photoelectron coincidence (TOF-PEPECO) and the velocity imaging photoionisation coincidence (VIPCO) methods. By combining the results from the two methods which give complementary information, the double photoionisation process is identified to CO+hv→CO⁺*+e^-→C ⁺(²P)+[O⁺(²D/²P)] nl+e^-→C⁺(²P)+O⁺( ⁴S)+2e^-. The high energy resolution of the TOF-PEPECO spectrometer enables us to resolve individual fragment [O⁺( ²D/²P)]nl states and also intermediate CO ⁺* states. The angular correlations between electrons and ions, deduced from the VIPCO data, imply that the nuclear dissociation is faster or at least comparable in the time scale with molecular rotation, and that the second electron emission occurs after the C⁺ and O* atoms are far apart. A possible generic identification of the intermediate states is suggested.