Resonant multiphoton ionization of NO via the a ²Σ ⁺ state: Photoelectron spectra and angular distributions

Photoelectron spectra resulting from the two-photon resonant, four-photon ionization of NO via the A ²Σ⁺ state are reported for several rotational levels in the A, υ=0 manifold. The observed final ionic state vibrational distributions are found to be consistent with and confirm spectroscopic studies which have assigned intense features in the four-photon A state MPI spectrum with accidental resonances in the energy range of the third photon. The upper states in the strong accidental double resonance transitions are assigned to Rydberg vibronic levels which are strongly perturbed by nearby valence states. Photoelectron angular distributions were also measured for two vibronic transitions and these are interpreted on the basis of a simple, one-electron description of the ionization step.