Single Active Electron Approximation Research Articles

Non-perturbative solution of the time-dependent Schrödinger equation describing H2 in intense short laser pulses

A method for solving the time-dependent Schrödinger equation describing the electronic motion of molecular hydrogen exposed to very short intense laser pulses has been developed. The fully correlated three-dimensional time-dependent electronic wavefunction is expressed in terms of field-free wavefunctions. These are obtained from a configuration-interaction calculation where the one-electron basis functions are built from B splines. The reliability of the method is tested by comparing results in the low-intensity regime to the prediction of lowest order perturbation theory. The onset of non-perturbative effects is shown for higher intensities and the validity of the single-active electron approximation is briefly discussed. Finally, the ability of the method to calculate photoelectron spectra including above-threshold-ionization peaks is demonstrated.

Above-threshold ionization beyond the dipole approximation

A generalization of the analytical theory of above-threshold ionization in the single active electron approximation is developed while taking into account leading non-dipole and relativistic corrections in the starting Hamiltonian. Special interest is placed on the high energy part of the photoelectron spectrum which consists of a plateau and a characteristic cutoff. It is shown that the correction due to the magnetic component of the laser field gives rise to a decrease of the plateau height, an increase of the maximal cutoff energy, and a drift of the emitted electrons in propagation direction of the laser field. Furthermore, the influence of the relativistic mass shift may become non-neglible by reducing the cutoff energy significantly. Spin effects or the Zitterbewegung play a comparably minor role in the investigated parameter regime of suboptical frequencies and high but not ultra-high laser intensities.

Absolute Ionization Rates of Multielectron Transition Metal Atoms in Strong Infrared Laser Fields

We report on nonresonant strong field ionization of the multielectron transition metal atoms V, Nb, Ta, Ni, and Pd. Operating in the adiabatic regime (lambda = 1.5 microm), we quantitatively determined both (i) the first charge state saturation intensities and (ii) the absolute ionization rates for intensities ranging from threshold up to 3 x 10(14) W/cm2. We observed a dramatic suppression of ionization relative to single active electron approximation expectations. We suggest that this derives from dynamic polarization or screening effects within the multielectron atom, stressing a need for many-body theories of strong field ionization.

Dynamics of strong-field above-threshold ionization of argon: Comparison between experiment and theory

We record angle-resolved electron-momentum distributions from 800-nm short-pulse laser ionization of argon and compare our data with numerical solutions of the time-dependent Schrodinger equation. A model potential of argon and the single active electron approximation are used. The calculation shows quantitative agreement in all dominant features of the experimental results. The energy and angular distributions of the photoelectrons, together with numerical simulations, allow us to identify the multiple processes involved during the interaction, such as channel switching, multiphoton resonant and nonresonant ionization, and ac Stark splitting.

Strong-field photoionization of argon: A comparison between experiment and the SAE approximation

We monitor the intensity dependence of photoionization of argon at 800 nm at pulse durations of 100fs employing momentum-resolved photoelectron imaging spectroscopy. We compare our data with numerical solutions of the time-dependent Schrödinger equation (TDSE) using a model potential for argon and the single active electron approximation (SAE). We find quantitative agreement in all dominant features in energy and angle. The energy and angular distributions of the photoelectrons allow us to identify selective dominant ionization channels.

Quantum path distributions for high-order harmonics in rare gas atoms

We present quantum path distributions of high-order harmonics produced by rare gas atoms interacting with an intense 810 nm laser field, obtained by numerical integration of the time-dependent Schrodinger equation within the single active electron approximation. We find that the distributions are sensitive to the atomic potentials, and differ from the distributions predicted by the strong field approximation. We demonstrate that these differences can lead to significant differences in the time-frequency behavior of the harmonics produced in a macroscopic nonlinear medium.

High-order harmonic generation in atomic clusters with a two-dimensional model

High-order harmonic generation in atom clusters interacting with an intense linearly polarized laser field is studied by means of numerical simulations of the time-dependent Schrödinger equation for a two-dimensional model cluster in the single-active electron approximation. The well-known first and second cutoffs (in the atomic and molecular cases) are not clear in cluster calculations. In fact, the structure of the spectrum consists of a long plateau that extends far beyond the IP+3.17 UP limit and a collection of high-order harmonics with decreasing but significant intensities up to extremely high frequencies.

High harmonic generation beyond the electric dipole approximation

A generalization of the analytical theory of high harmonic generation in the long wavelength limit and in the single active electron approximation is developed taking into account the magnetic dipole and electric quadrupole interaction. Quantum mechanical and classical theories are found to be in excellent agreement, which allows one to explain the influence of multipole effects in terms of an intuitive picture. For Ti:S lasers ( 0.8 &mgr;m) multipole contributions are found to be small below an intensity of about 10(17) W/cm(2), at which harmonic radiation with photon energies of several keV is generated. This promises the extension of high harmonic generation well into the sub-nm wavelength regime.

Ionization Above the Coulomb Barrier

The interaction of noble gases with laser pulses in the near-infrared and visible wavelength regime is dominated by above barrier ionization while tunnel ionization plays a subordinate role. We develop a theory of ionization in this parameter range. A condition for the applicability of the quasistatic approximation in the above barrier ionization regime is obtained. Ionization of arbitrary atoms by a laser pulse may then be calculated from static ionization rates. The static ionization rate of He is obtained by solving the full two-electron Schr\odinger equation. Our analysis yields a verification of the single active electron approximation and indicates the possibility to create keV radiation by high harmonic generation in a He gas.

Nonsequential Double Ionization of Helium

Electron correlation effects in strong laser fields are investigated by using a simplified two electron model to calculate the double ionization rate in helium. In our model we make a correction to the single active electron approximation by including the effect of the outer electron on the inner one through a time-dependent potential. Using this approach we are able to investigate the nonsequential double ionization observed in recent experiments. {copyright} {ital 1997} {ital The American Physical Society}

Harmonic generation beyond the saturation intensity in helium.

We present results from numerical simulations of the interaction of an intense ultrashort (100 fs) laser pulse with a neutral helium (He) atom and its ion ${\mathrm{He}}^{+}$. The simulations are done for a number of laser intensities in the range of ${10}^{14}$--${10}^{16}$ W/${\mathrm{cm}}^{2}$ at the KrF-laser wavelength. These intensities range from close to well above the saturation intensity of the neutral atoms and it is therefore necessary to study the contribution to harmonics from neutral atoms as well as ions. Our approach is based on a two-step procedure where we first calculated the neutral-atom response in the single active electron approximation. We observe that harmonics are still produced by the neutral atoms above the saturation intensity, but the simple classical law for the cutoff is not obeyed. In the second step we calculated the response of ions. We show that the contribution from ions extends through and beyond the region where the contribution from the neutral atoms disappears. We find good agreement between our numerical results and the experimental ones. We also present some ideas about the importance of the laser defocusing and phase matching at such high intensities.

Atoms in a strong laser field

The physical effects induced on the atomic electrons by an intense linearly polarized laser field are reviewed. Although most of the ideas presented here apply to many different atoms in what is called the single active electron approximation, only hydrogen-like atoms will be considered in the review. The application of the "photoelectric effect ideas" to terawatt laser pulses gives a lot of surprising results, most of them related to the suppression of ionization. The emphasis is put on the unification of different existing theories and the identification of the basic ionization mechanisms.

Above threshold ionization beyond the high harmonic cutoff.

We present high sensitivity electron energy spectra for xenon in a strong 50 ps, 1.053 \ensuremath{\mu}m laser field. The above threshold ionization distribution is smoothly decreasing over the entire kinetic energy range (0--30 eV), with no abrupt changes in the slope. This is in direct contrast to the sharp cutoff observed in xenon optical harmonic generation spectra. Calculations using the single active electron approximation show excellent agreement with the observed electron distributions. These results directly address the unresolved relationship between the electron and photon emission from an atom in an intense field.

Eikonal exchange amplitudes

The exchange scattering of electrons by atomic hydrogen is considered in the eikonal approximation. The six-dimensional integrals for the ''post'' and ''prior'' forms of the eikonal exchange amplitudes are reduced to two-dimensional integral expressions. A comparison of the eikonal-exchange cross section, nonexchange Glauber, and experimental results for 50-, 100-, and 200-eV electron-hydrogen elastic scattering is given. Numerical convergence problems were not encountered in the evaluation of the scattering amplitudes and the procedure demonstrates that eikonal exchange calculations are almost as easily done as Glauber calculations in the single active electron approximation. (AIP)