Asymmetry in attosecond streaking from a degenerate state

By the numerical solution to the time-dependent Schrodinger equation, we observe a remarkable asymmetry in the photoelectron streaking spectra when the He+ ion is ionized by an attosecond pulse from the 2s state in the presence of a weak infrared pulse. An analytical theory based on the modified perturbation theory is developed to quantitatively account for the underlying mechanism. We find that a transition occurs between the degenerate 2s and 2p states under the influence of the infrared electric field before the arrival of the attosecond pulse. The observed asymmetry in the attosecond streaking is formed by the interference of photoelectrons ionized from the 2s and 2p states. Using the sensitivity of the asymmetry to the infrared (IR) electric field at the moment of attosecond photoionization, we show that the present finding provides a highly reliable way to fully characterize the IR waveform in the streaking experiment.National Program on Key Basic Research Project of China (973 Program) [2013CB922402]; National Natural Science Foundation of China (NSFC) [11574010]SCI(E)ARTICLE29