Photoelectron -- residual-ion correlation in angle-resolved streaked shake-up ionization of helium  

Streaked photoelectron emission spectra access the correlated dynamics of photoelectrons and residual target electrons with attosecond temporal resolution. We calculated ab initio and measured single-ionization spectra for photoemission from helium atoms by co-linearly polarized ultrashort XUV and assisting few-femtosecond IR pulses [1]. Distinguishing direct and shake-up ionization resulting in ground-state and excited (n=2,3) He+ residual ions, respectively, we examined the effects of the correlated photoemission dynamics on the photoelectron phase-accumulation as a function of the observable photoelectron detection direction and kinetic energy, and XUV -- IR pulse delay. We tracked the dynamical evolution of the residual ion in relative streaked photoemission delays and found dominant contributions for shake-up emission from the residual-ion - photoelectron interaction. These are in very good and fair agreement, respectively, for n=2 and n=3 shake-up photoemission along the pulse-polarization directions (Ɵ=0), with previous experimental and theoretical investigations [2]. Our calculations predict a strong 1/cos Ɵ dependence of the photoemission delay for shake-up ionization due to the coupling between the photoelectron and evolving residual-ion charge distribution in the IR-laser field that we started to scrutinize against our angle-resolved photoemission measurements [3].