What can the measurement of the transverse velocity of atoms tell us about strong-field interactions with atoms?

In this paper we present a theoretical and experimental investigation of the ionization of atoms though the interaction of light in the strong-field regime. We show that the measurement of the transverse electron momentum distribution of ionised atoms as a function of the ellipticity of the ionizing light in the over-the-barrier and the tunnelling regime evolve in qualitatively different ways and in the case of the use of linearly polarized light demonstrates that the strong-field approximation is not valid.