D. Bauer

Publications (10)0 Total impact

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  Available from: ArXiv

  Article: Harmonic generation by atoms in circularly polarized laser fields: far-off and near resonances regimes

  F. Ceccherini, N. Davini, D. Bauer, F. Cornolti
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  ABSTRACT: The generation of harmonics by atoms interacting with two circularly polarized and frequency related laser fields is addressed through ab initio numerical simulations. A detailed charaterization of a few specific harmonics is given. In particular, the two different cases where the total energy absorbed through photons is far-off or close to the energy gap between different atomic states are investigated. It is found that the conversion efficiency in the harmonic generation is strongly dependent on the inner atomic structure and in certain specific cases it can be significantly enhanced within a small frequency range. Comment: Submitted to Appl. Phys. B, 4 pages
  11/2003;
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  Available from: ArXiv

  Article: Dynamical ionization ignition of clusters in intense and short laser pulses

  D. Bauer, A. Macchi
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  ABSTRACT: The electron dynamics of rare gas clusters in laser fields is investigated quantum mechanically by means of time-dependent density functional theory. The mechanism of early inner and outer ionization is revealed. The formation of an electron wave packet inside the cluster shortly after the first removal of a small amount of electron density is observed. By collisions with the cluster boundary the wave packet oscillation is driven into resonance with the laser field, hence leading to higher absorption of laser energy. Inner ionization is increased because the electric field of the bouncing electron wave packet adds up constructively to the laser field. The fastest electrons in the wave packet escape from the cluster as a whole so that outer ionization is increased as well. Comment: 8 pages, revtex4, PDF-file with high resolution figures is available from http://mitarbeiter.mbi-berlin.de/bauer/publist.html, publication no. 24. Accepted for publication in Phys. Rev. A
  04/2003;
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  Available from: arxiv.org

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

  F. Ceccherini, D. Bauer, F. Cornolti
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  ABSTRACT: 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 Schroedinger equation of a two-dimensional model ion. Through the analytical model, selection rules for the harmonic orders in this field configuration, a generalized cut-off for the harmonic spectra, and an integral expression for the harmonic dipole strength is provided. The numerical results are employed to test the predictions of the analytical model. The scaling of the cut-off as a function of both, one of the laser intensities and frequency ratio $\eta$, as well as entire spectra for different $\eta$ and laser intensities are presented and analyzed. The theoretical cut-off is found to be an upper limit for the numerical results. Other discrepancies between analytical model and numerical results are clarified by taking into account the probabilities of the absorption processes involved. Comment: 8 figures
  11/2002;
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  Available from: arxiv.org

  Article: Two-color stabilization of atomic hydrogen in circularly polarized laser fields

  D. Bauer, F. Ceccherini
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  ABSTRACT: Dynamic stabilization of atomic hydrogen against ionization in high-frequency single- and two-color, circularly polarized laser pulses is observed by numerically solving the three-dimensional, time-dependent Schr\"odinger equation. The single-color case is revisited and numerically determined ionization rates are compared with both, exact and approximate high-frequency Floquet rates. The position of the peaks in the photoelectron spectra can be explained with the help of dressed initial states. In two-color laser fields of opposite circular polarization the stabilized probability density may be shaped in various ways. For laser frequencies $\omega_1$ and $\omega_2=n\omega_1$, $n=2,3,...$ and sufficiently large excursion amplitudes $n+1$ distinct probability density peaks are observed. This may be viewed as the generalization of the well-known ``dichotomy'' in linearly polarized laser fields, i.e, as ``trichotomy,'' ``quatrochotomy,'' ``pentachotomy'' etc. All those observed structures and their ``hula-hoop''-like dynamics can be understood with the help of high-frequency Floquet theory and the two-color Kramers-Henneberger transformation. The shaping of the probability density in the stabilization regime can be realized without additional loss in the survival probability, as compared to the corresponding single-color results. Comment: 10 pages, REVTeX4, 11 eps-figures, see also http://www.physik.tu-darmstadt.de/tqe/dieter/publist.html for a manuscript with higher-quality figures
  08/2002;
• Article: Fullerenes and molecules in strong laser pulses: Collective dynamics, field ionization and harmonic generation

  D. Bauer, F. Ceccherini, A. Macchi, F. Cornolti
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  ABSTRACT: Computational studies of many-electron systems in strong laser fields are presented. The ionization and nonlinear dipole response of the ball-shaped C60 fullerene molecule are investigated with a time-dependent density functional approach and a jellium approximation for the ionic background. We find that C60 ionization at 800 nm wavelength occurs multiphoton-like rather than via excitation of a “giant” resonance. Harmonic generation from the interaction of a circularly polarized laser field and a molecule with a discrete rotational symmetry is also studied, and spectra and selection rules are interpreted via a general group theory approach. © 2002 American Institute of Physics.
  AIP Conference Proceedings. 04/2002; 611(1):276-281.
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  Available from: arxiv.org

  Article: Dynamical symmetries and harmonic generation

  F. Ceccherini, D. Bauer, F. Cornolti
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  ABSTRACT: 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 \pm 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. Comment: Submitted to Journal of Physics B
  07/2001;
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  Available from: arxiv.org

  Article: C$_{60}$ in intense femtosecond laser pulses: nonlinear dipole response and ionization

  D. Bauer, F. Ceccherini, A. Macchi, F. Cornolti
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  ABSTRACT: We study the interaction of strong femtosecond laser pulses with the C$_{60}$ molecule employing time-dependent density functional theory with the ionic background treated in a jellium approximation. The laser intensities considered are below the threshold of strong fragmentation but too high for perturbative treatments such as linear response. The nonlinear response of the model to excitations by short pulses of frequencies up to 45eV is presented and analyzed with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond laser pulses of 800nm wavelength ionization is found to occur multiphoton-like rather than via excitation of a ``giant'' resonance. Comment: 14 pages, including 1 table, 5 figures
  06/2001;
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  Available from: arxiv.org

  Article: A numerical ab initio study of harmonic generation from a ring-shaped model molecule in laser fields

  D. Bauer, F. Ceccherini
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  ABSTRACT: When a laser pulse impinges on a molecule which is invariant under certain symmetry operations selection rules for harmonic generation (HG) arise. In other words: symmetry controls which channels are open for the deposition and emission of laser energy---with the possible application of filtering or amplification. We review the derivation of HG selection rules and study numerically the interaction of laser pulses with an effectively one-dimensional ring-shaped model molecule. The harmonic yields obtained from that model and their dependence on laser frequency and intensity are discussed. In a real experiment obvious candidates for such molecules are benzene, other aromatic compounds, or even nanotubes. Comment: 5 pages, 3 figures
  01/2001;
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  Available from: ArXiv

  Article: Electron correlation vs. stabilization: A two-electron model atom in an intense laser pulse

  D. Bauer, F. Ceccherini
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  ABSTRACT: We study numerically stabilization against ionization of a fully correlated two-electron model atom in an intense laser pulse. We concentrate on two frequency regimes: very high frequency, where the photon energy exceeds both, the ionization potential of the outer {\em and} the inner electron, and an intermediate frequency where, from a ``single active electron''-point of view the outer electron is expected to stabilize but the inner one is not. Our results reveal that correlation reduces stabilization when compared to results from single active electron-calculations. However, despite this destabilizing effect of electron correlation we still observe a decreasing ionization probability within a certain intensity domain in the high-frequency case. We compare our results from the fully correlated simulations with those from simpler, approximate models. This is useful for future work on ``real'' more-than-one electron atoms, not yet accessible to numerical {\em ab initio} methods. Comment: 8 pages, 8 figures in an extra ps-file, submitted to Phys. Rev. A, updated references and shortened introduction
  03/1999;
• Article: Dynamical ionization ignition of clusters in intense short laser pulses

  D. Bauer
  [show abstract] [hide abstract]
  ABSTRACT: The electron dynamics of rare-gas clusters in laser fields is investigated quantum mechanically by means of time-dependent density-functional theory. The mechanism of early inner and outer ionization is revealed. The formation of an electron wave packet inside the cluster shortly after the first removal of a small amount of electron density is observed. By collisions with the cluster boundary the wave-packet oscillation is driven into resonance with the laser field, hence leading to higher absorption of laser energy. Inner ionization is increased because the electric field of the bouncing electron wave packet adds up constructively to the laser field. The fastest electrons in the wave packet escape from the cluster as a whole so that outer ionization is increased as well.
  Phys. Rev. A. 68(3).