Orbital effects on tunnel-electron momentum distributions: Ar and H2 studied by electron–ion coincidence momentum imaging

Three-dimensional momentum distributions of tunnel-electrons produced from Ar and H2 in circularly polarized intense laser fields (1035 nm, 35 fs, 2×1014 W/cm2) have been measured by electron–ion coincidence momentum imaging. The photoelectrons recorded with a gas mixture of Ar and H2 exhibit torus-like distributions in the momentum space. Although ionization potentials are similar (15.8 eV for Ar and 15.4 eV for H2), significant differences are identified in the radius and the width of the photoelectron torus recorded in coincidence with the counterpart ions (Ar+ and H2+). The differences are confirmed by theoretical calculations based on the strong-field approximation, showing that the orbital character of ionizing electrons is encoded in the momentum distribution of tunnel electrons.