Interpretation of the ultrafast optical measurements of time delay in the ionization of Coulomb systems

We consider the time delay of electron detachment from a Coulomb center in the process of ionization. It is shown that the attosecond streaking, an ultrafast optical method most commonly used for ionization time delay measurement, can be formally described by placing a virtual detector of the arrival time delay at a certain distance from the center of the system. This approach allows derivation of a simple formula for Coulomb-laser coupling that perfectly agrees with the results of numerical solution of the appropriate time-dependent Schrödinger equation.