text: 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 photoionization 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 a delayed XUV attosecond pulse, with a following motion in the continuum states driven by a strong IR field.
  The resulting 2D photo-electron (momentum, or equivalently energy, vs delay) distribution is called streaking trace.