Theoretical Analysis and Numerical Simulation of Attosecond Time Delays in Photoionization

Recent developments in laser technology, in particular the advances in high-harmonic generation,\ enable the generation of ultrashort extreme ultraviolet (XUV) pulses with attosecond (1\ as = 10^-18 s) duration. Such tools open the opportunity to study electron dynamics in atoms\ and molecules on its intrinsic time scale. As an example, the attosecond streaking technique was\ recently applied to time resolve the photoionization process in atomic and solid systems. In this\ technique, an isolated attosecond XUV pulse that ionizes the electron in the target system, is superimposed\ with a few-cycle streaking pulse (usually of near-infrared wavelengths). The streaking\ pulse modulates the final momentum (or energy) of the photoelectron. The measured streaking\ trace, i.e., the final momentum (or energy) as a function of the relative delay between these two\ pulses, contains time information of the photoionization process. By comparing two streaking\ traces measured for photoionization from the 2s and 2p orbitals in a neon atom, Schultze et al.\ [Science 328, 1658 (2010)] found a temporal offset of 21 \textpm 5 as between them and interpreted this\ value as the time delay between photoionization from the 2s and 2p orbitals. This experiment has\ initiated a debate among theoreticians, in particular about the origin of the measured time delay. A\ correct interpretation of the delay is extremely important for our understanding of the attosecond\ streaking technique and an exact analysis of time resolved measurements of this and other ultrafast\ processes.