Photodissociation Dynamics in (N2)n+ Clusters

Abstract

(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.

1

Mass spectrum of (N₂) _n ⁺ ions produced by the pulsed-discharge supersonic nozzle source.

2

Photodissociation mass spectra of different-sized nitrogen cluster cations measured in the imaging instrument configuration. The negative-going peak indicates the depletion of the selected parent ion and the positive peaks indicate the photofragments coming from it. All cluster sizes except N₄ ⁺ and N₆ ⁺ produce both N₂ ⁺ and N₄ ⁺ as fragment ions. Because of mass discrimination effects and unstable cluster source, the integrated areas of the parent depletion and fragments are not equal.

3

Photofragment image and fragment kinetic energy spectrum for the N₄ ⁺ ion dissociating to produce N₂ ⁺ (and N₂), measured at 355 nm.

4

Photofragment images and fragment kinetic energy spectra for the (N₂) _n ⁺ ions dissociating to produce N₂ ⁺ and (n – 1)­(N₂) neutrals, measured at 355 nm.

5

Photofragment images and fragment kinetic energy spectra for the (N₂) _n ⁺ ions dissociating to produce N₄ ⁺ and (n – 2)­(N₂) neutrals, measured at 355 nm.

6

Photofragment images and angular distributions for N₆ ⁺ and N₁₄ ⁺ measured at 355 nm. The upper frame in each set shows the angular distribution when ions of all kinetic energy values are included in the fits. The middle frames show the angular distribution of the low KER fraction of the signal, and the lower frames show the angular distribution of the high KER fraction of the signal. In both cases, the higher KER signal is much more anisotropic.