Time-resolved photoemission by attosecond streaking: extraction of time information

TL;DR

This paper investigates how attosecond streaking phase shifts encode quantum scattering time delays, emphasizing the importance of accounting for IR field effects and demonstrating a measurable delay between specific initial states.

Contribution

It clarifies the relationship between streaking phase shifts and quantum time delays, highlighting the influence of IR fields and providing a method to extract timing information from experimental data.

Findings

Streaking phase shifts contain spectral phase information related to quantum time delays.

The IR field's influence on phase shifts can be modeled classically.

A measurable 20 attosecond delay between hydrogenic 2s and 2p states in He+ was observed.

Abstract

Attosecond streaking of atomic photoemission holds the promise to provide unprecedented information on the release time of the photoelectron. We show that attosecond streaking phase shifts indeed contain timing (or spectral phase) information associated with the Eisenbud-Wigner-Smith time delay matrix of quantum scattering. However, this is only accessible if the influence of the streaking infrared (IR) field on the emission process is properly accounted for. The IR probe field can strongly modify the observed streaking phase shift. We show that the part of the phase shift ("time shift") due to the interaction between the outgoing electron and the combined Coulomb and IR laser fields can be described classically. By contrast, the strong initial-state dependence of the streaking phase shift is only revealed through the solution of the time-dependent Schr\"odinger equation in its full dimensionality. We find a time delay between the hydrogenic 2s and 2p initial states in He+ exceeding 20as for a wide range of IR intensities and XUV energies.