choice would have been to use as continuum state the exact atom scattering states. The time evolution of simple plane wave states with the Volkov Hamiltonian is given by: here for consistency with Eq. the evolution has already been properly converted into the length gauge. As a consequence, the final momentum distribution of a single electron in a single-level atom, with ionization potential , is expressed as: where, is the dipole expectation value (or transition dipole moment), and is the semiclassical action. The result of Eq. is the basic tool to understand phenomena like: The high-harmonics generation process, which is typically the result of strong field interaction of noble gases with an intense low-frequency pulse, Attosecond pump-probe experiments with simple atoms. The debate on tunneling time. Weak Attosecond pulse-strong-IR-fields-atoms interactions Attosecond pump-probe experiments with simple atoms is a fundamental tool to measure the time duration of an attosecond pulse and to explore several quantum proprieties of matter. This kind of experiments can be easily described within strong field approximation by exploiting the results of Eq. , as discussed below. As a simple model, consider the interaction between a single active electron in a single-level atom and two fields: an intense femtosecond infrared (IR) pulse (, and a weak attosecond pulse (centered in the extreme ultraviolet (XUV) region) . Then, by substituting these fields to it results with . At this point, we can divide Eq. in two contributions: direct ionization and strong field ionization (multiphoton regime), respectively. Typically, these two terms are relevant in different energetic regions of the continuum. Consequently, for typical experimental condition, the latter process is disregarded, and only direct ionization from the attosecond pulse is considered. Then, since the attosecond pulse is weaker than the infrared one, it holds . Thus, is typically neglected in Eq. . In addition to that, we can re-write the attosecond pulse as a delayed function with respect to the IR field, . Therefore, the probability distribution, , of finding an electron ionized in the continuum with momentum , after the interaction has occurred (at ), in a pump-probe experiments, with an intense IR pulse and a delayed-attosecond XUV pulse, is given by: with Equation describes the photo-ionization phenomenon of two-color interaction (XUV-IR) with a single level atom and single active electron. This peculiar result can be regarded as a quantum interference process between all the possible ionization paths, started by