Dissociative photoionization of N2 in the 24-32 eV photon energy range

Dissociative photoionization of N-2 is studied with synchrotron radiation in the 24-32 eV photon energy range. Branching ratios between the different dissociation limits are measured from coincidence time of flight ion spectra threshold photoelectron-photoion coincidence recorded for state-selected N-2(+) parent ions. In this energy range, N-2(+) molecular ions are observed to dissociate 2 only towards the three lowest dissociation limits. Dissociations towards the second and third ones, which correspond to the formation of NI(ID) and N (D-2) metastable states, respectively, occur right from their thermochernical onsets. From the second dissociation limit energy up to the third one, the N+ (I D) + N(S-4)/N+(P-3) + N (4S) branching ratio is almost constant and equal to 0.6:0.4, except at the energy of the C(2)Sigma(u)(+) (nu = 12) state, where this branching ratio is found to be equal to 0.5:0.5. From the third dissociation limit onset, N-2(+) ions fragment only towards this limit. Possible dissociation mechanisms 2 are discussed, involving a spin-orbit coupling between doublet and quartet states of N-2(+) to produce N+ (I D) + N (4 S) and a direct dissociation to produce 2 N+(P-3) + N (D-2). No dissociation towards the other limits has been observed, in particular to produce the N+ (S-1), N+ (S-5) and N(P-2) metastable states.