Session P7: Theory of Attosecond and Strong Field Physics

Bichromatic control of photoelectron angular distribution for ionization via intermediate autoionizing states by ultrafast XUV pulses

Control of the angular dependence of photoelectrons from ionization of the Li atom by two ultrafast extreme-ultraviolet (XUV) pulses is investigated using an extension of a recently developed implementation of the muticonfiguration time-dependent Hartree Fock (MCTDHF) method. One-photon direct ionization and two-photon ionization via intermediate autoionizing states well above the ionization threshold combine to determine the angular distribution of the ejected photoelectron in these calculations. Two coincident attosecond pulses with different colors provide an opportunity for manipulating the ionization process not available with much longer pulses. It is shown that the resulting asymmetric angular distribution of photoelectrons is sensitive to and can be controlled by the difference between the carrier envelope phases of the two phase-locked XUV pulses of different frequencies.   

Two-photon double ionization of atomic beryllium by ultrashort laser pulses

A time-dependent formalism for evaluating ionization amplitudes and generalized cross sections for two-electron atoms previously used to study the correlated electron dynamics of helium under ultrashort laser pulses~[1] is adapted to study similar processes involving the $2s^2$ valence shell of atomic beryllium in the presence of a fully-occupied $1s^2$ core shell. The similar symmetry of the overall process in two-photon double ionization permits a direct comparison between Be and He atoms, revealing details about the nature of electron correlation within these two atoms whose impact is manifest in the continuum electron dynamics. In particular, consequences of the different shell structures of the initial states for He and Be are prominent when considering sequential double ionization processes.\\[4pt] [1] A. Palacios, T.N. Rescigno and C.W. McCurdy.{\em Phys. Rev. A} {{\bf 79} 033402, (2009)}   

Carrier-Envelope-Phase-Induced Asymmetries in Double Ionization of Helium by an Intense Few-Cycle XUV Pulse

A complete formulation of the carrier-envelope-phase (CEP) dependence of electron angular distributions in double ionization of He by an arbitrarily-polarized, few-cycle, intense XUV pulse is carried out using perturbation theory (PT) in the pulse amplitude.\footnote{J.M. Ngoko Djiokap, N.L. Manakov, A.V. Meremianin and A.F. Starace, Phys.~Rev.~A~\textbf{88}, 053411 (2013).} The broad pulse bandwidth induces interference of first- and second-order PT amplitudes producing thus asymmetric angular distributions which can be controlled by the CEP of the pulse. For linear polarization of the pulse, our PT parametrization is in excellent agreement with results of solutions of the full-dimensional, two-electron time-dependent Schr\"{o}dinger equation, validating thus the PT approach.   

Adiabatic Hyperspherical Study of One-dimensional Hydrogen Molecule

We present a calculation of the adiabatic hyperspherical potentials for one-dimensional H$_2$. Although the adiabatic hyperspherical representation has proven very useful in understanding atomic systems, especially highly correlated states like doubly excited states, it has not yet been applied to the electronic and nuclear degrees of freedom for a molecule more complicated than H$_2^+$. We thus present the first such calculation, albeit for a one-dimensional model of H$_2$. Our model, however, is chosen to exactly reproduce the three-dimensional H$_2$ and H$_2^+$ ground Born-Oppenheimer potentials. One of our goals is to identify and understand the role of doubly excited states --- which can be readily identified in the adiabatic hyperspherical representation, unlike standard quantum chemistry. We illustrate the method with an application to attosecond physics. We also want to take advantage of the fact that the adiabatic hyperspherical representation produces well defined and discrete effective potentials for all ionization channels to help understand processes like strong-field dissociative ionization. These topics, and others, will be discussed.   

Time- and Frequency-Dependent Imaging of Nuclear Dynamics in Laser-Excited Nobel-Gas Dimers

We study the nuclear dynamics of noble-gas dimer ions resolved in time using intense ultrashort pump in combination with delayed probe laser pulses. We compare our time-dependent numerical results with those from a complementary description of the same basic dynamics in the frequency domain. This alternative analysis is based on the Fourier transformation of the time- and internuclear-separation-dependent wavefunction probability density or, equivalently, the Fourier transformation of the delay-dependent kinetic-energy-release spectra [1]. Specifically, for pump-laser excited diatomic molecules, it allows for the characterization of their nuclear motion in terms of coherently superimposed stationary vibrational states and the mapping of the laser-dressed nuclear potential curves, thereby supplementing the time-domain formulation [2], as we will demonstrate for the sequence He$_{2}^{+}$ to Xe$_{2}^{+}$ of dimer cations.\\[4pt] [1] M. Magrakvelidze et al., Phys. Rev. A 79, 033410 (2009); Phys. Rev. A 86, 023402 (2012); Phys. Rev. A 88, 013413 (2013);\\[0pt] [2] U. Thumm et al., Phys. Rev. A 77, 063401 (2008). This work was supported by the NSF and DOE.   

Model for atomic dielectric response in strong, time-dependent laser fields

A nonlocal quantum mechanical model is presented for calculating the atomic dielectric response to a strong laser electric field. By replacing the Coulomb potential with a nonlocal potential in the Schrodinger equation, a 3+1D calculation of the time-dependent electric dipole moment can be reformulated as a 0+1D integral equation that retains the 3D dynamics, while offering significant computational savings. The model is benchmarked against an established ionization model and \textit{ab initio} simulation of the time-dependent Schrodinger equation. The reduced computational overhead makes the model a promising candidate to incorporate full quantum mechanical time dynamics in laser pulse propagation simulations.   

Confinement and cavity effects in time-domain photoionization and recombination of Kr@C60

Photoionization (PI) and radiative recombination (RR) of the endofullerene molecule, Kr@C60, are studied in a framework of time-dependent local density approximation (TDLDA) augmented by the Leeuwen and Baerends exchange-correlation functional [1]. Phases of the dipole matrix elements corresponding to transitions to (PI) and from (RR) the continuum, involving atomic-type, fullerene-type and atom-fullerene hybrid levels of the molecule, are considered. Associated Wigner-Smith time-delays are extracted. Effects of the confinement on the phase and time-delay at Kr Cooper minima, and that of the C60 cavity at C60 cross section minima have been examined. Further, intercoulombic decay (ICD) and hybrid Auger-ICD [2] resonances, direct results of the confinement, are found to considerably enrich the time-domain response of the molecule. \\[4pt] [1] G. Dixit, H.S. Chakraborty, and M.E. Madjet, Phys. Rev. Lett.111, 203003(2013).\\[0pt] [2] M.H. Javani, J.B. Wise, R. De, M.E. Madjet, S.T. Manson, and H.S. Chakraborty, arXiv:1312.2144 [physics.atm-clus].