Time evolution of the lateral-velocity distribution for a strong-field-ionization process

We study time development of a cusp in the lateral-velocity distribution for the process of strong-field ionization. The lateral-velocity distribution is computed using an ab initio quantum mechanical procedure for the moments of time inside and after the end of the laser pulse. We show that at the moment of time corresponding to the midpoint of the laser pulse the lateral-velocity distribution is a smooth Gaussian curve, its parameters agreeing very well with the predictions of the tunelling theories. At the moment of time corresponding to the end of the pulse the lateral-velocity distribution narrows considerably, showing the initial stage of the cusp-formation process due to the Coulomb focusing effect. Following evolution of the ionized wave packet yet further in time we consider the cusp formation in detail.