Vibrationally resolved photoelectron studies of the 7σ−1 channel in N2O

We present vibrationally resolved photoelectron studies of the 000, 100, 200, and 001 modes of the à state (7σ−1) of N2 O+ in the 17.4–26 eV photon-energy range. The vibrational branching ratios σ(100)/σ(000) and σ(001)/σ(000) agree very well with fluorescence measurements by Kelly et al. and qualitatively with recent theoretical predictions of Braunstein and McKoy. The large non-Franck–Condon variations in the σ(100)/σ(000) and σ(200)/σ(000) branching ratios are associated with a predicted 7σ→εσ shape resonance near 20 eV. Overall, the vibrational branching ratios imply lower resonant energies for the stretching modes (100 and 200) and a similar resonant energy for the asymmetric stretch (001), compared with the 000 mode. The vibrational asymmetry parameters (β) display a strong variation with energy which is qualitatively reproduced by theory; however, the experimental values for β(100) and β(001) exhibit additional structure around 20 eV. When combined with theory and recent fluorescence data, these results help to demonstrate a correlation of shape resonance energy with overall molecular length (RN–N+RN–O); this important result implies a resonant state which is localized on the entire triatomic molecular frame rather than on the N–N or N–O components.