J. M. Ngoko Djiokap

Enhanced asymmetry in few-cycle attosecond pulse ionization of He in the vicinity of autoionizing resonances

By solving the two-active-electron, time-dependent Schr¨ odinger equation in its full dimensionality, we investigate the carrier-envelope phase (CEP) dependence of single ionization of He to the He + (1s) state triggered by an intense few-cycle attosecond pulse with carrier frequency ! corresponding to the energy ¯! = 36eV. Effects of electron correlations are probed by comparing projections of the final state of the two-electron wave packet onto field-free highly correlated Jacobi matrix wave functions with projections onto uncorrelated Coulomb wave functions. Significant differences are found in the vicinity of autoionizing resonances. Owing to the broad bandwidths of our 115 and 230 as pulses and their high intensities (1-2PWcm 2 ), asymmetries are found in the differential probability for ionization of electrons parallel and antiparallel to the linear polarization axis of the laser pulse. These asymmetries stem from interference of the one- and two-photon ionization amplitudes for producing electrons with the same momentum along the linear polarization axis. Whereas these asymmetries generally decrease with increasing ionized electron

Evidence of the 2s2p({sup 1}P) doubly excited state in the harmonic generation spectrum of helium

By solving the two-active-electron time-dependent Schroedinger equation in an intense, ultrashort laser field, we investigate evidence of electron correlations in the high-order harmonic generation spectrum of helium. As the frequency of the driving laser pulse varies from 4.6 to 6.6 eV, the 13th, 11th, and 9th harmonics sequentially become resonant with the transition between the ground state and the isolated 2s2p({sup 1}P) autoionizing state of helium, which dramatically enhances these harmonics and changes their profiles. When each of the 9th and 13th harmonics are in resonance with this autoionizing state, there is also a low-order multiphoton resonance with a Rydberg state, resulting in a particularly large enhancement of these harmonics relative to neighboring harmonics. When the 11th harmonic is in resonance with the 2s2p({sup 1}P) autoionizing state, the 13th harmonic is simultaneously in resonance with numerous higher-energy autoionizing states, resulting in a competition between these two harmonics for intensity. These results demonstrate that even electron correlations occurring over a narrow energy interval can have a significant effect on strong-field processes such as harmonic generation.

Electron Matter-Wave Vortices in Double Photoionization of Helium by Attosecond Pulses

Attosecond Pulses J.M. Ngoko Djiokap∗ 1, A.V. Meremianin†, N.L. Manakov†, S.X. Hu‡, L.B. Madsen§, and Anthony F. Starace∗ 2 ∗ Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588-0299, USA † Department of Physics, Voronezh State University, Voronezh 394006, Russia ‡ Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14623-1299, USA § Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark