The role of dipole-forbidden autoionizing resonances in non-resonant one-color two-photon single ionization of N₂

We present an experimental and theoretical energy- and angle-resolved study on the photoionization dynamics of non-resonant one-color two-photon single valence ionization of neutral N₂ molecules. Using 9.3 eV photons produced via high harmonic generation and a 3-D momentum imaging spectrometer, we detect the photoelectrons and ions produced from one-color two-photon ionization in coincidence. Photoionization of N₂ populates the X ²Σ^+_g, A ²Π_u, and B ²Σ^+_u ionic states of N₂^+, where the photoelectron angular distributions associated with the X ²Σ^+_g and A ²Π_u states both vary with changes in photoelectron kinetic energy of only a few hundred meV. We attribute the rapid evolution in the photoelectron angular distributions to the excitation and decay of dipole-forbidden autoionizing resonances that belong to series of different symmetries, all of which are members of the Hopfield series, and compete with the direct two-photon single ionization.