Coupling between the vibrational motion of core-excited and valence-ionized states of N₂

Fluorescence emission of the N₂ ⁺ (A ²Π_u) and N₂ ⁺ (B²Σ_u ⁺) states has been investigated after resonant excitation of the N₂ (1s^-1-π*) resonance with monochromatized synchrotron radiation. The vibrational structure of the core-excited state as well as of the final ionic state has been resolved in the photoexcitation and in the dispersed fluorescence spectra, respectively. A detailed analysis of the experimental spectra and a comparison with theoretical results show a strong coupling between the vibrational levels in the core-excited and the ionic state produced via autoionization. The N₂ ⁺ (A ²Π_u) potential curve is almost parallel to the core-excited state which causes mainly (Δv = 0) transitions in the autoionization process. For the N₂ ⁺ (B ²Σ_u ⁺) state, the production via the π* resonance leads to the population of higher vibrational states which are only very weakly populated by direct ionization. The calculations reproduce the observed structures and underline the complementary character of fluorescence and photoelectron spectroscopy.