Velocity-map imaging of near-threshold photoelectrons in Ne and Ar

The photoionization of Ne and Ar has been studied in the region between the (2)P(3/2) and (2)P(1/2) thresholds using a velocity-map imaging (VMI) spectrometer. The VMI technique provides a two-dimensional overview of the ionization cross section versus photon energy and emission angle. In these regions the neutral Rydberg states converging to the (2)P(1/2) ion state affect both the ionization cross section and the asymmetry parameter of the photoelectron angular distribution, which both display Fano line shapes. The results are compared with relativistic multichannel quantum-defect calculations.