Fig. 1
From: Symmetry breakdown of electron emission in extreme ultraviolet photoionization of argon
Exemplary electron kinetic energy spectrum and PADs in the Cooper minimum of atomic argon Spectral cut-outs for the first step of ionization into Ar+ (3p5) (left) and two-photon sequential ionization into Ar2+ (3p4) 1S (green), 1D (red), and 3P (purple) ionic states (right) at a photon energy of 51 eV. The intensity of the second ionization step is scaled up by a factor of 5. The orange and gray arrows below the 1S and 1D indicate overlapping contributions from 3s ionization (see text). The depicted spectrum corresponds to only one of the 16 spectrometers at an angle of 45° with respect to the beam propagation for clearer illustration. For the final states Ar+ (3p5) (blue, left) and Ar2+ (3p4) 3P (purple, right), the three-dimensional photoelectron angular distribution (PAD) patterns are shown in the upper insets based on the experimental data of all 16 spectrometers (alignment plane shown in the insets, further details in the Methods section) to illustrate the asymmetry of the PADs in the Cooper minimum. The underlying coordinate system is depicted for better orientation (upper left, see also Eq. (1))
