Photodissociation Dynamics in (N₂)_n⁺ Clusters

(N₂)_n⁺ cluster ions are produced and cooled in a pulsed-discharge supersonic expansion and studied with UV laser photodissociation and velocity-map imaging (VMI). All cluster sizes up to n = 15 absorb strongly near 355 nm, and those with n > 3 dissociate to produce both N₂⁺ and N₄⁺ photofragments. This suggests that the N₄⁺ ion is the chromophore in the larger clusters, consistent with the previous optical spectroscopy and bond energy determinations. Photofragment imaging of N₄⁺ produces an anisotropic distribution peaked along the laser polarization. Analysis of the maximum kinetic energy release produces a dissociation energy consistent with values determined in previous experiments. Dissociation of larger clusters produces N₂⁺ with significant kinetic energy values that do not change appreciably with cluster size. This suggests that the N₄⁺ core ion is not enclosed by the clustering of additional N₂ molecules. N₄⁺ fragments from larger clusters have somewhat lower kinetic energies than the N₂⁺ fragments, consistent with recombination or partial caging after dissociative recoil. However, the kinetic energy release of N₄⁺ is also considerable and it persists in the dissociation of larger clusters. This suggests that the N₄⁺ ion in these clusters resides near the surface and that the photodissociation and recombination are mediated by this surface rather than by a true caging effect.