Dissociative double photoionization of N2 using synchrotron radiation: Appearance energy of the N2+ dication

Photoionization cross sections for the production of the doubly charged ion N++ from N2 have been measured by means of synchrotron radiation in the photon energy range from 50 to 110 eV. The appearance energy for N2+ has been determined as 55.2±0.2 eV, i.e., about 1.3 eV higher than the spectroscopic dissociation limit leading to the charge asymmetric dissociation channel N++(2P) +N(4S) at 53.9 eV. The onset of a second threshold at 59.9±0.2 eV is detected and the energy dependence of photoion intensities near the threshold regions is interpreted in terms of the Wannier theory. The production of the N++ dication is discussed in terms of direct and indirect mechanisms for dissociative charge asymmetric photoionization and by comparison with the potential energy curves of the intermediate N2++ dication. Experimental evidences for the opening of the Coulomb explosion channel N++ plus N+ at high photon energies are provided by measuring the kinetic energy release spectra of N++ fragments at selected photon energies.