Coulomb singularity in the transverse momentum distribution for strong-field single ionization

We present high-resolution momentum distributions of ions and electrons created in single ionization of He, Ne and Ar targets by intense (0.15-2 PW cm(-2)) short-pulsed (25 fs) linearly polarized laser fields in the direction perpendicular to the polarization. Instead of a Gaussian shape predicted by standard tunnelling theory, the experimental data exhibit a sharp cusp-like peak at zero transverse momentum. The comparison of experimental data with (i) calculations performed within the strong-field approximation employing a Coulomb-Volkov wavefunction to model the final electron state and (ii) results of recent semiclassical calculations, shows that the `cusp' appears due to the long-range Coulomb interaction between the emitted electron and the remaining ion. A similar structure was previously observed for ion-atom collisions.