Convergent close-coupling calculations of two-photon double ionization of helium

We apply the convergent close-coupling (CCC) formalism to the problem of two-photon double ionization of helium. The electron-photon interaction is treated perturbatively whereas the electron-electron interaction is included in full. The integrated two-photon double ionization cross-section is substantially below non-perturbative literature results. However, the pattern of the angular correlation in the two-electron continuum is remarkably close to the non-perturbative time-dependent close-coupling calculation of Hu {\em et al} [J. Phys. {\bf B38}, L35 (2005)]Comment: 15 pages, 6 figure

Delay in atomic photoionization

We analyze the time delay between emission of photoelectrons from the outer valence $ns$ and $np$ sub-shells in noble gas atoms following absorption of an attosecond XUV pulse. By solving the time dependent Schr\"odinger equation and carefully examining the time evolution of the photoelectron wave packet, we establish the apparent "time zero" when the photoelectron leaves the atom. Various processes such as elastic scattering of the photoelectron on the parent ion and many-electron correlation affect the quantum phase of the dipole transition matrix element, the energy dependence of which defines the emission timing. This qualitatively explains the time delay between photoemission from the $2s$ and $2p$ sub-shells of Ne as determined experimentally by attosecond streaking [{\em Science} {\bf 328}, 1658 (2010)]. However, with our extensive numerical modeling, we were only able to account for less than a half of the measured time delay of $21\pm5$ as. We argue that the XUV pulse alone cannot produce such a large time delay and it is the streaking IR field that is most likely responsible for this effect.Comment: 5 pages, 2 figure

On the use of the Kramers-Henneberger Hamiltonian in multi-photon ionization calculations

We employ the Kramers-Henneberger Hamiltonian for time-independent calculations of multi-photon ionization of atoms with one and two electrons. As compared to the electromagnetic interaction in the length and velocity gauges, the presently employed Kramers-Henneberger gauge has an advantage of the dipole matrix elements for the free-free electron transitions being finite and well-defined quantities. This circumstance simplifies considerably the computations and allows to obtain accurate results for the two-photon ionization of realistic atomic systems.Comment: 16 pages, 1 figur