Bend-stretch Fermi resonance in NO2+ observed by delayed pulsed-field ionization zero-electron kinetic energy photoelectron spectroscopy

By means of zero-electron kinetic energy (ZEKE) threshold photoelectron spectroscopy, we examine bend-stretch Fermi resonance in NO2+. Accurate fundamental frequencies are obtained by using the (000) 3pσ Rydberg state as a reference state for all ionic vibrational ZEKE spectra. The degree of (100)–(02°0) Fermi resonance observed in NO2+ is comparable to that found in the NO2 3pσ Rydberg state, and results for both systems agree well with recent CCSD(T) and MRD-CI ab initio calculations. Comparison of the vibrational frequencies of the 3pσ Rydberg state and the cation suggests that the presence of the Rydberg electron slightly perturbs the harmonic potential and its second-order anharmonicities, but has little effect on the cubic coupling term associated with core Fermi resonance.