The role of the atomic potential in the regime of strong-field tunnelling ionization: imprints on longitudinal and 2D momentum distributions

We present a theoretical study of strong-field ionization of selected rare-gas atoms and hydrogen in the tunnelling regime, using two- and ten-cycle laser pulses. For two- cycle pulses, ionization occurs via the tunnelling process, and the Coulombic long-range part of the atomic potentials leads to bimodal longitudinal momentum distributions. The momentum distributions are not sensitive to the electronic structure of the probed atom. For ten-cycle pulses, a multiphoton picture emerges even in the tunnelling regime, and the momentum distributions are influenced by the internal structure of the probed atom. The effect of the internal structure is clearly demonstrated in the case of Ar, where the longitudinal momentum distribution obtained using a ten-cycle pulse peaks at zero momentum.