Non-Franck–Condon 2σ−1u vibrational distributions in N+2. 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 N+2(B 2Σ+u) state. The vibrational branching ratios for the N+2(B 2Σ+u) state are determined from vibrationally resolved dispersed fluorescence measurements on the N+2(B 2Σ+u→X 2Σ+g) band system. The results demonstrate that the partial cross section for the N+2(B 2Σ+u,v′=1) level is selectively enhanced in the photon energy range 28 <hνexc<30 eV. A shape resonance occurs in this range for the 3σg→εσu channel, and the observed enhancement is attributed to coupling between this channel and the 2σu→εσg channel. Vibrational branching ratio curves for the excited state ion also show a pronounced peak at hνexc=22 eV, which shifts for the curves that sample the v′=1 and v′=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.