We present accurate time-dependent ab initio calculations on fully differential and total integrated (generalized) cross sections for the nonsequential two-photon double ionization of helium at photon energies from 40 to 54 eV. Our computational method is based on the solution of the time-dependent Schroedinger equation and subsequent projection of the wave function onto Coulomb waves. We compare our results with other recent calculations and discuss the emerging similarities and differences. We investigate the role of electronic correlation in the representation of the two-electron continuum states, which are used to extract the ionization yields from the fully correlated final wave function. In addition, we study the influence of the pulse length and shape on the cross sections in time-dependent calculations and address convergence issues.Comment: 14 pages, 10 figures; final version (acknowledgements and reference added, typos fixed

The coupling of a quantum mechanical system to open decay channels has been theoretically studied in numerous works, mainly in the context of nuclear physics but also in atomic, molecular and mesoscopic physics. Theory predicts that with increasing coupling strength to the channels the resonance widths of all states should first increase but finally decrease again for most of the states. In this letter, the first direct experimental verification of this effect, known as resonance trapping, is presented. In the experiment a microwave Sinai cavity with an attached waveguide with variable slit width was used.PACS numbers: 03.65.Nk, 84.40.Az, 85.30.Vw Since more than ten years, interference phenomena in open quantum systems have been studied theoretically in the framework of different models. Common to all these studies is the appearance of different time scales as soon as the resonance states start to overlap see [1] and the recent papers [2] with references therein). Some of the states align with the decay channels and become short-lived while the remaining ones decouple to a great deal from the continuum and become long-lived (trapped). Due to this phenomenon, the number of relevant states will, in the short-time scale, be reduced while the system as a whole becomes dynamically stabilized. The phenomenologically introduced doorway states in nuclear physics provide an example for the alignment of the short-lived states with the channels [3]. Calculations for microwave resonators showed that the trapped resonance states can be identified in the time-delay function and that short-lived collective modes are formed at large openings of the resonator [4]. Resonance narrowing is inherent also in the Fano formalism [5]. Similar effects have been found in the linewidths in a semiconductor microcavity with variable strength of normal-mode coupling [6]. In spite of the many theoretical studies, the effect of resonance trapping has not yet been verified unambigously in an experiment. A theoretical study of neutron resonances in nuclei as a function of the interaction of a doorway state with narrow resonances [7] allowed only to draw the conclusion that resonance trapping is not in contradiction with experimental data. For a clear experimental demonstration of the trapping effect, the coupling strength to the decay channels should be tunable, which was not possible in all above mentioned experiments.The mechanism of resonance trapping can be illustrated best on the basis of a schematical model. In an open quantum system the resonance states are allowed to decay, i. e. their energies are complex,The Hamilton operator is non-hermitian,Here H 0 describes the N discrete states of the closed quantum system coupled to K decay channels by the N × K matrix V . H 0 and V V † are hermitian and α is a real parameter for the total coupling strength between the closed system and the channels. The complex eigenvalues of H give the energy positions E R and widths Γ R of the resonance states. Studies on the basis of this model were perfo...

Within a semiclassical description of above-threshold ionization (ATI) we identify the interplay between intracycle and intercycle interferences. The former is imprinted as a modulation envelope on the discrete multiphoton peaks formed by the latter. This allows one to unravel the complex interference pattern observed for the full solution of the time-dependent Schrödinger equation (TDSE) in terms of diffraction at a grating in the time domain. These modulations can be clearly seen in the dependence of the ATI spectra on the laser wavelength. Shifts in energy modulation result from the effect of the long Coulomb tail of the atomic potential.Tunneling ionization is a highly nonlinear quantummechanical phenomenon induced by intense laser pulses (> ∼ 10 14 W/cm 2 ). Electrons are emitted by tunneling through the potential barrier formed by the combination of the atomic potential and the external strong field. Tunneling has recently attracted increasing interest as a probe of the atomic and molecular structure [1-3]. Tunneling occurs within each optical cycle predominantly around the maxima of the absolute value of the electric field. The interference of the successive bursts of ejected electrons reaching the same final momentum gives rise to features in photoelectron energy and momentum distribution which are markedly different from typical above-threshold ionization (ATI) spectra by multicycle lasers. This temporal double-slit interference has recently been studied both experimentally [1,4] and theoretically [5]. On the other hand, the ATI peaks separated by a photon energy can be themselves viewed as an interference pattern formed by electron bursts repeated each optical cycle. Details of the interplay between these intra-and intercycle interferences have not yet been clearly identified and analyzed, to the best of our knowledge.In this Rapid Communication, we study the influence of different interference processes on ATI spectra generated by multicycle laser pulses. We clarify the underlying mechanism within a simple one-dimensional (1D) model employing classical trajectories. Within the framework of the strongfield approximation (SFA) [6] the qualitative features, the modulation of the ATI peaks akin to the modulation of Bragg peaks by the structure factor in crystal diffraction, can be unambiguously identified in the ATI spectrum determined from the full solution of the three-dimensional time-dependent Schrödinger equation (TDSE). The multicycle laser pulse thus acts as a diffraction grating in the time domain. Quantitative deviations between the SFA predictions and the full TDSE can be traced to the Coulomb tail of the atomic potential affecting this modulation. The latter opens up the opportunity to observe effects of the atomic potential in easy-to-obtain photoelectron spectra after ionization by multicycle laser pulses.Our simple semiclassical model of photoelectron spectra is based on the 1D "simple man's model (SMM)" [6][7][8]. Let us consider an atom interacting with a linearly polarized laser pulse. The...

We investigate the dependence of the intensity of radiation due to high-harmonic generation as a function of the wavelength lambda of the fundamental driver field. Superimposed on a smooth power-law dependence observed previously, we find surprisingly strong and rapid fluctuations on a fine lambda scale. We identify the origin of these fluctuations in terms of quantum path interferences with up to five returning orbits significantly contributing.