Triplet state spectroscopy and photofragment dynamics of N2+2

Abstract

The photofragment spectrum of N²⁺₂→N⁺(³P_g)+N⁺(³P_g) has been studied using a coaxial laser–ion beam spectrometer. Transitions from the excited ³Σ⁺_u(v=0,1) state of N²⁺₂ to the predissociative ³Π_g(v=0) state result in two bands with dense rotational structure at 15 300 and 13 100 cm⁻¹. A complete analysis of the 27 rotational branches associated with the ³Π_g(v=0)←³Σ⁺_u(v=0) transition provides bond lengths and spectroscopic constants for both states. A perturbation is observed in the (0,0) band, caused by the interaction of ³Σ⁺_u(v=0) with ³Π_u(v≊10). From a deperturbation analysis, the coupling constants and the energy difference between the ³Π_u(v≊10) and ³Σ⁺_u(v=0) states are determined—ξ=1.5±0.1 cm⁻¹, η=0.220±0.003 cm⁻¹, and T_pert=365±25 cm⁻¹. A broad unstructured band at 16 400 cm⁻¹ arises from a transition out of v=0 in the ³Σ⁺_u state into a very short‐lived v=1 level of the ³Π_g state. Vibrational spacings between v=0 and v=1 in both electronic states are determined—³Σ⁺_uω₀=2210±20 cm⁻¹ and ³Π_g ω₀≊1100 cm⁻¹. The fragmentation of N²⁺₂ is explored by measuring the dependence of predissociation lifetimes on the rotational quantum number of the ³Π_g(v=0) state. Experimentally determined lifetimes of 50–70 ps for N’=2–17 are almost independent of N’ and indicate that the predissociation occurs by the interaction of ³Π_g(v=0) with the ³Σ⁻_g continuum rather than by tunneling through the ³Π_g barrier.