Direct Probe of the Shape Resonance Mechanism in2σg-Shell Photoionization of theN2Molecule

Angular distributions of photoelectrons from a $2{\sigma }_g$ shell of fixed-in-space ${\mathrm{N}}_2$ molecules have been measured for left- and right-elliptically polarized and for linearly polarized light at several photon energies in the region of ${\sigma }^{*}$ shape resonance. That allowed the determination of a set of dipole matrix elements and phase shift differences characterizing the process. These data clearly show the enhancement of the $f{\sigma }_u$ partial cross section in the resonance simultaneously with an abrupt increase of the corresponding phase shift by $\pi$, which is the first experimental demonstration of the nature of the ${\sigma }^{*}$ shape resonance in homonuclear diatomic molecules.