Single-photon double ionization of H ₂ away from equilibrium: A showcase of two-center electron interference

We demonstrate the effect of two-center interference on single-photon double ionization [double photoionization (DPI)] of the aligned H ₂ molecule when it shrinks or expands from the equilibrium internuclear distance. This interference affects the first stage of the DPI process in which the primary photoelectron is ejected predominantly along the polarization axis of light and its geometrical interference factor is most sensitive to the internuclear distance in the parallel (Σ) orientation of the internuclear and polarization axes. This effect is responsible for strong modification of the DPI amplitude in the parallel orientation while the corresponding amplitude for the perpendicular (Π) orientation is rather insensitive to the internuclear distance. The combination of these two factors explains the profound kinetic energy release effect on the fully differential cross sections of DPI of H ₂.