Dieter Bauer

EXACT FIELD IONIZATION RATES IN THE BARRIER-SUPPRESSION REGIME FROM NUMERICAL TIME-DEPENDENT SCHRODINGER-EQUATION CALCULATIONS

Numerically determined ionization rates for the field ionization of atomic hydrogen in strong and short laser pulses are presented. The laser pulse intensity reaches the so-called ``barrier-suppression ionization regime where field ionization occurs within a few half laser cycles. Comparison of our numerical results with analytical theories frequently used shows poor agreement. An empirical formula for the ``barrier-suppression ionization rate is presented. This rate reproduces very well the course of the numerically determined ground-state populations for laser pulses with different length, shape, amplitude, and frequency.

C

SUMMARY We study the interaction of strong femtosecond laser pulses with the C

_{60}

_{60}

in intense femtosecond laser pulses: nonlinear dipole response and ionization

nmolecule employing time-dependent density functional theory with the ionic nbackground treated in a jellium approximation. The laser intensities considered nare below the threshold of strong fragmentation but too high for perturbative ntreatments such as linear response. The nonlinear response of the model to nexcitations by short pulses of frequencies up to 45eV is presented and analyzed nwith the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond nlaser pulses of 800nm wavelength ionization is found to occur multiphoton-like nrather than via excitation of a ``giant resonance.

Dynamical symmetries and harmonic generation

We discuss harmonic generation in the case of laser field-dressed Hamiltonians that are invariant under so-called dynamical symmetry operations. Examples for such systems are molecules which exhibit a discrete rotational symmetry of order N (e.g. benzene with N = 6) interacting with a circularly polarized laser field and single atoms in a bichromatic field, with the two lasers having circular polarizations. Within a general group theory approach we study the harmonics one obtains from the interaction of a laser pulse and a circular molecule. When the system is in a pure field-dressed state the known selection rule kN ± 1, k = 1, 2, 3 ,... results. However, other lines are observed when recombinations with states of a symmetry different from the initial one become important. This is the case for realistic laser pulses (i.e. with a finite duration), in particular when the fundamental laser frequency (or one of its multiples) is resonant with a transition between field-dressed states. Numerical ab initio simulations, confirming our analytical calculations and illustrating the power of the group theory approach, are presented.

Time-dependent density functional theory applied to nonsequential multiple ionization of Ne at 800 nm

Time-dependent density functional theory (TDDFT) is employed to study the interaction of a Ne atom with short and strong 800,nm laser pulses. In the intensity regime covered (10;14-10;16 W/cm;2) up to triply ionized Ne is observed. Good quantitative agreement with the experimental Ne;+ ion-yield (and the Ne;2+ -yield near saturation) is obtained. Nonsequential ionization (NSI) leads to a strong increase of the probability for double and triple ionization when compared to a single active electron (SAE)-approach. A NSI-knee is observed but the agreement with its experimental counterpart is not satisfactory.

Modeling field ionization in an energy conserving form and resulting nonstandard fluid dynamics

A fluid model that takes the field ionization energy correctly into account is presented for the first time by introducing an energy conserving ionization current as a source term in the wave equation. Nonstandard type fluid equations result from the finite ejection energy of the electrons in the field ionization process. The energy and momentum distributions of the ejected electrons are obtained from the time-dependent Schrodinger equation and classical Monte Carlo calculations. Characteristic results of how field ionization influences the pulse propagation, and some extremely nonlinear features caused by the ionization current are given.

Harmonic generation in ring-shaped molecules

We study numerically the interaction between an intense circularly polarized laser field and an electron moving in a potential that has a discrete cylindrical symmetry with respect to the laser-pulse propagation direction. This setup serves as a simple model, e.g., for benzene and other aromatic compounds. From general symmetry considerations, within a Floquet approach, selection rules for the harmonic generation [O. Alon et al., Phys. Rev. Lett. 80, 3743 (1998)] have been derived recently. Instead, the results we present in this paper have been obtained solving the time-dependent Schrodinger equation ab initio for realistic pulse shapes. We find a rich structure that is not always dominated by the laser harmonics.

Plasma formation through field ionization in intense laser–matter interaction

Optical field ionization is the earliest and fastest plasma-generating process during the interaction of intense laser light with matter. By using short and rapidly rising laser pulses, the free electron density may turn from being transparent for an incoming laser pulse to reflective in less than half a laser cycle, that is, on a subfemtosecond timescale. Extremely nonlinear optical effects arise as a consequence of this. In this article, the basics of optical field ionization that are relevant in analytical or numerical studies of intense laser–matter interactions are reviewed. Several macroscopic effects of field ionization in the interaction of intense laser pulses with solid targets are briefly surveyed.

Harmonic generation by atoms in circularly polarized two-color laser fields with coplanar polarizations and commensurate frequencies

The generation of harmonics by atoms or ions in a two-color, coplanar field configuration with commensurate frequencies is investigated through both an analytical calculation based on the Lewenstein model and the numerical ab initio solution of the time-dependent Schrodinger equation of a two-dimensional model ion. Through the analytical model, selection rules for the harmonic orders in this field configuration, a generalized cutoff for the harmonic spectra, and an integral expression for the harmonic dipole strength are provided. The numerical results are employed to test the predictions of the analytical model. The scaling of the cutoff as a function of both one of the laser intensities and frequency ratio