Extraction of the attosecond time delay using the soft photon approximation

The concept of time delay was introduced in particle scattering theory by Wigner [1] and his contemporaries (see Ref. [2] for a comprehensive review). It is a quantity related to the phase of the complex scattering amplitude which provides an insight into development of the scattering process in time. In recent years, this idea has made a dramatic comeback when it was realized that the time delay can be measured experimentally in photoionization processes. This has led to many interesting and not yet fully understood results such as observation of a considerable time delay between photoelectrons emitted from the 2s and 2p sub-shells in neon [3], or an experimental determination of the tunneling time in a ionization event [4].