Non-Franck-Condon 2σ_u^-1 vibrational distributions in N₂⁺. An interchannel-coupled shape resonance observed by dispersed fluorescence

We report evidence for a molecular interchannel-coupled shape resonance in N₂ photoionization, observable in the vibrational branching ratios for the N₂⁺ (B ²Σ_u ⁺) state. The vibrational branching ratios for the N₂ ⁺ (B ²Σ_u⁺) state are determined from vibrationally resolved dispersed fluorescence measurements on the N₂⁺ (B ²Σ_u ⁺→X ²Σ_u⁺) band system. The results demonstrate that the partial cross section for the N ₂⁺ (B ²Σ_u⁺, υ′=1) level is selectively enhanced in the photon energy range 28 < hν_exc < 30 eV. A shape resonance occurs in this range for the 3σσ_grarr;εσ_u channel, and the observed enhancement is attributed to coupling between this channel and the 2σ_u→εσg channel. Vibrational branching ratio curves for the excited state ion also show a pronounced peak at hν_exc=22 eV, which shifts for the curves that sample the υ′=1 and υ′=2 levels of the ion, suggesting a few interpretations. The utility of coupling monochromatized fluorescence detection with synchrotron radiation excitation for extracting relative partial photoionization cross sections is demonstrated for the first time.