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 N2+ from N-2 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 N2+(P-2)+N(S-4) 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 N2+ 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 N-2(2+) dication. Experimental evidences for the opening of the Coulomb explosion channel N2++N+ at high photon energies are provided by measuring the kinetic energy release spectra of N2+ fragments at selected photon energies. (c) 2007 American Institute of Physics.