arXiv subjects

A. S. Kheifets

Publications and source records attributed to A. S. Kheifets.

34 records · Page 2Linked to original sources

arXiv2014

Attosecond time delay in the photoionization of endohedral atoms A@C$_{60}$: A new probe of confinement resonances

The effects of confinement resonances on photoelectron group delay (Wigner time delay) following ionization of an atom encapsulated inside a C$_{60}$ cage have been studied theoretically using both relativistic and non-relativistic random phase approximations. The results indicate clearly the resonant character of the confinement oscillations in time delay of the $4d$ shell of Xe@C$_{60}$ and present a most direct manifestation of Wigner time delay. These oscillations were missed in a previous theoretical investigation of Ar@C$_{60}$ [PRL 111, 203003 (2013)]

physics.atom-ph↗

arXiv2013

Strong-field ionization of He by elliptically polarized light in attoclock configuration

We perform time-dependent calculations of strong-field ionization of He by elliptically polarized light in configuration of recent attoclock measurements of Boge {\em et al} [PRL {\bf 111}, 103003 (2013)]. By solving a 3D time-dependent Schrödinger equation, we obtain the angular offset $θ_m$ of the maximum in the photoelectron momentum distribution in the polarization plane relative to the position predicted by the strong field approximation. This offset is used in attoclock measurements to extract the tunneling time. Our calculations clearly support the set of experimental angular offset values obtained with the use of non-adiabatic calibration of the {\em in situ} field intensity, and disagree with an alternative set calibrated adiabatically. These findings are in contrast with the conclusions of Boge {\em et al} who found a qualitative agreement of their semiclassical calculations with the adiabatic set of experimental data. This controversy may complicate interpretation of the recent atto-clock measurements.

physics.atom-ph↗

arXiv2013

Extraction of attosecond time delay using the soft photon approximation

We use the soft photon approximation to extract the Wigner time delay from atomic two-color photoionization experiments. Unlike the strong field approximation, the present method does not require introduction of the Coulomb-laser coupling corrections and enables one to extract the Wigner time delay directly from attosecond time delay measurements.

physics.atom-ph↗

arXiv2013

Time delay in valence shell photoionization of noble gas atoms

We use the non-relativistic random phase approximation with exchange to perform calculations of valence shell photoionization of Ne, Ar, Kr and Xe from their respective thresholds to photon energy of 200 eV. The energy derivative of the complex phase of the photoionization matrix elements is converted to the photoelectron group delay that can be measured in attosecond streaking or two-photon transitions interference experiments. Comparison with reported time delay measurements in Ne and Ar at a few selected photon energies is made. Systematic mapping of time delay across a wide range of photon energies in several atomic targets allows to highlight important aspects of fundamental atomic physics that can be probed by attosecond time delay measurements.

physics.atom-ph↗

arXiv2012

Photoemission time-delay measurements and calculations close to the 3s ionization minimum in Ar

We present experimental measurements and theoretical calculations of photoionization time delays from the $3s$ and $3p$ shells in Ar in the photon energy range of 32-42 eV. The experimental measurements are performed by interferometry using attosecond pulse trains and the infrared laser used for their generation. The theoretical approach includes intershell correlation effects between the 3s and 3p shells within the framework of the random phase approximation with exchange (RPAE). The connection between single-photon ionization and the two-color two-photon ionization process used in the measurement is established using the recently developed asymptotic approximation for the complex transition amplitudes of laser-assisted photoionization. We compare and discuss the theoretical and experimental results especially in the region where strong intershell correlations in the 3s to kp channel lead to an induced "Cooper" minimum in the 3s ionization cross-section.

physics.atom-ph↗

arXiv2010

Timing analysis of two-electron photoemission

We predict a significant delay of two-electron photoemission from the helium atom after absorption of an attosecond XUV pulse. We establish this delay by solving the time dependent Schrödinger equation and by subsequent tracing the field-free evolution of the two-electron wave packet. This delay can also be related to the energy derivative of the phase of the complex double photoionization (DPI) amplitude which we evaluate by the convergent close-coupling method. Our observations prompt future attosecond streaking experiments on DPI of He which can elucidate various mechanisms of this strongly correlated ionization process.

physics.atom-ph↗

arXiv2010

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 Schrödinger 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.

physics.atom-ph↗

arXiv2009

Tailoring the waveforms to extend the high-order harmonic generation cut-off

Increase of the cut-off value in the high order harmonics generation process is demonstrated for a special case of the driving field composed of several harmonics of a given frequency. It is shown that a moderate, of the order of 20%, increase in the cut-off value can be achieved. This result possibly constitutes an upper limit for the increase in the cut-off value, attainable for a class of the waveforms considered in the paper.

physics.atom-ph↗

arXiv2006

Two-photon double ionization of helium in the region of photon energies 42-50 eV

We report the total integrated cross-section (TICS) of two-photon double ionization of helium in the photon energy range from 42 to 50 eV. Our computational procedure relies on a numerical solution of the time-dependent Schrödinger equation on a square-integrable basis and subsequent projection of this solution on a set of final states describing two electrons in continuum. Close to the threshold, we reproduce results previously known from the literature. The region 47-50 eV seems to have been previously unexplored. Our results suggest that TICS, as a function of the photon energy, grows monotonously in the region 42-50 eV. We also present fully resolved triple differential cross sections for selected photon energies.

physics.atm-clus↗

arXiv2006

Different escape modes in two-photon double ionization of helium

The quadrupole channel of two-photon double ionization of He exhibits two distinctly different modes of correlated motion of the photoelectron pair. The mode associated with the center-of-mass motion favors a large total momentum which is maximazed at a parallel emission. However, the mode associated with the relative motion favors an antiparallel emission. This difference is manifested in a profoundly different width of the angular correlation functions corresponding to the center-of-mass and relative motion modes.

physics.atom-ph↗

arXiv2006

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)]

physics.atom-ph↗

arXiv2005

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.

physics.atom-ph↗

arXiv2005

Calculations of total photoionization cross-section for^M two-electron atomic systems

We outline a non-perturbative procedure for calculating the total photoionization cross-section of two-electron atomic systems. The procedure is based on the Floquet-Fourie representation of the solution of the time-dependent Schrodinger equation. The Floquet-Fourie ansatz produces a set of equations which is recast into a generalized eigenvalue problem by means of the complex rotation procedure. With the use of the Hylleraas-type basis functions, the total photoionization cross-sections are obtained within the accuracy of a fraction of a percent. The total photoionization cross-sections for neutral helium are in good agreement with the convergent close-coupling calculations of Kheifets and Bray [Phys. Rev. A {\bf 58}, 4501 (1999)] but deviate notably from the experimental data of Samson {\em et al.} [J. Phys. B {\bf 27} 887 (1994)].

physics.atom-ph↗

arXiv2005

Lippmann-Schwinger description of multiphoton ionization

We outline a formalism and develop a computational procedure to treat the process of multiphoton ionization (MPI) of atomic targets in strong laser fields. We treat the MPI process nonperturbatively as a decay phenomenon by solving a coupled set of the integral Lippmann-Schwinger equations. As basic building blocks of the theory we use a complete set of field-free atomic states, discrete and continuous. This approach should enable us to provide both the total and differential cross-sections of MPI of atoms with one or two electrons. As an illustration, we apply the proposed procedure to a simple model of MPI from a square well potential and to the hydrogen atom.

physics.atom-ph↗

arXiv2004

EMS Measurement of the Valence Spectral Function of Silicon - a test of Many-body Theory

The spectral function A(q,omega) of silicon has been measured along a number of symmetry directions using high-energy high-resolution electron momentum spectroscopy. It is compared with first-principles calculations based on the interacting one-electron Green's function which is evaluated in the GW and the cumulant expansion approximations. Positions of the quasiparticle peaks (dispersion), their widths (lifetimes), and the extensive satellite structures are measured over a broad range of energies and momenta. The band dispersions are well described by both calculations, but the satellite predicted by the GW calculation is not observed. Unlike the GW calculation, the cumulant expansion calculation gives a significantly better description of the shape and momentum dependence of the satellite structure, presenting a promising approach for studying high-energy excitations.The spectral function A(q,omega) of silicon has been measured along a number of symmetry directions using high-energy high-resolution electron momentum spectroscopy. It is compared with first-principles calculations based on the interacting one-electron Green's function which is evaluated in the GW and the cumulant expansion approximations. Positions of the quasiparticle peaks (dispersion), their widths (lifetimes), and the extensive satellite structures are measured over a broad range of energies and momentum dependence of the satellite structure, presenting a promising approach for studying high-energy excitations.

cond-mat↗

arXiv2003

Convergent calculations of double ionization of helium: from ($γ$,2e) to (e,3e) processes

The first absolute (e,3e) measurements, by Lahmam-Bennani et al [Phys. Rev. A {\bf 59}, 3548 (1999)], have been recently approximately reproduced by Berakdar [Phys. Rev. Lett. {\bf 85}, 4036 (2000)] and supported by Jones and Madison [Phys. Rev. Lett. {\bf 91}, 07321 (2003)], but with widely differing conclusions. The former indirectly implied that the Born-CCC-based calculations of Kheifets et al [J. Phys. B {\bf 32}, 5047 (1999) were invalid due to the reliance on the 1st Born approximation. The latter argued that the 1st Born approximation was valid, but the wrong initial state was used. We investigate these claims and find that the original calculations of Kheifets et al are reproduced whether the 2nd Born approximation is incorporated or if we use a ground state similar to that of Jones and Madison, but appropriately corrected as done by Le Sech and co-workers [J. Phys. B {\bf 23}, L739 (1990)].

physics.atom-ph↗