On the origin of the cusp in the transverse momentum distribution for the process of strong field ionization

We study the origin of the cusp-structure in the transverse or lateral electron momentum distribution (TEMD) for the process of tunelling ionization driven by a linearly polarized laser pulse. We show that appearance of the cusp in the TEMD can be explained as follows. Projection on the set of the Coulomb scattering states leads to appearance of "elementary" cusps which have simple structure as functions of the lateral momentum. This structure is independent of the detailed dynamics of the ionization process and can be described analytically. These "elementary" cusps can be used to describe the cusp-structure in TEMD.