Fine-structure branching ratios N(²D_5/2)/N(²D_3/2) above threshold N(²D_5/2.3/2)+N(²D_5/2.3/2) for ¹⁴N₂ and ¹⁵N₂^†

Accurate measurements of the product spin-orbit finne-structure branching ratios are important for understanding the detailed photodissociation dynamics of small molecules. In this study, the atomic spin-orbit fine-structure branching rar tio N(²D_5/2)/N(²D_3/2) to the dissociation channel N(²D_5/2,3/2)+N(²D_5/2,3/2) is measured for the $c4′1Σu+(υ′=6)$ and $b′1Σu+(υ′=21)$ states of ¹⁴N₂, and the $b′1Σu+(υ′=20)$ and $b′1Σu+(υ′=21)$ states of ¹⁵N₂ by using the vacuum ultraviolet (VUV)pump VUV-probe time-sliced velocity-mapped ion imaging setup. The measurements show that the fine-structure branching ratio N(²D_5/2)/N(²D_3/2) is independent of the rotational level of the parent molecule (¹⁴N₂ or ¹⁵N₂) in each vibronic state, while it does show dependence on the vibronic characteristics. It is ~1.35 for the $c4′1Σu+(υ′=6)$ state of ¹⁴N₂ and $b′1Σu+(υ′=20)$ state of ¹⁵N₂, which are both close to the dissociation threshold N(²D_5/2,3/2)+N(²D_5/2,3/2); while it becomes ~1.00 for the $b′1Σu+(υ′=21)$ state of both ¹⁴N₂ and ¹⁵N₂, which are relatively far above the dissociation threshold. A possible change from a statistical process near the threshold to a diabatic process far above the threshold might have occurred to be responsible for the observed vibronic dependence of the branching ratio. Detailed informations on the potential energy curves and their mutual couplings near the dissociation threshold are highly desired for understanding the present experimental measurement.