N 1s photoelectron angular distributions from fixed-in-space NO₂ molecules: Stereodynamics and symmetry considerations

Angular distributions of N 1s photoelectrons from fixed-in-space NO ₂ molecules have been measured over the energy region of shape resonance and above. A multiple-coincidence velocity-map imaging technique for observation of molecular frame photoelectron angular distributions (MF-PADs) has been extended to nonlinear molecular targets. Density functional theory calculations have also been conducted to elucidate the photoionization dynamics and shape resonance in the N 1s photoionization of NO₂. Results show that the N 1s MF-PADs exhibit strong shape variation as a function of both photoelectron kinetic energy and symmetries of final states, whereas asymmetry parameters of laboratory frame PADs show a local minimum around the shape resonance region and increase monotonically as the photon energy increases. Over the shape resonance, the spatial shape of the photoelectron wave function with b₂ -symmetry closely resembles that of 5 b2 unoccupied molecular orbital of NO₂, although the MF-PAD pattern for b₂ -symmetry does not correspond directly to the 5 b2 orbital shape. At higher kinetic energy of 90 eV, MF-PADs become less structured, but still show a significant dependence on the symmetry of final states.