Lithium Ionization by a Strong Laser Field

We study ab initio computations of the interaction of Lithium with a strong laser field. Numerical solutions of the time-dependent fully-correlated three-particle Schroedinger equation restricted to the one-dimensional soft-core approximation are presented. Our results show a clear transition from non-sequential to sequential double ionization for increasing intensities. Non sequential double ionization is found to be sensitive to the spin configuration of the ionized pair. This asymmetry, also found in experiments of photoionization of Li with synchrotron radiation, shows the evidence of the influence of the exclusion principle in the underlying rescattering mechanism

Momentum distributions in time-dependent density functional theory: Product phase approximation for non-sequential double ionization in strong laser fields

We investigate the possibility to deduce momentum space properties from time-dependent density functional calculations. Electron and ion momentum distributions after double ionization of a model Helium atom in a strong few-cycle laser pulse are studied. We show that, in this case, the choice of suitable functionals for the observables is considerably more important than the choice of the correlation potential in the time-dependent Kohn-Sham equations. By comparison with the solution of the time-dependent Schroedinger equation, the insufficiency of functionals neglecting electron correlation is demonstrated. We construct a functional of the Kohn-Sham orbitals, which in principle yields the exact momentum distributions of the electrons and the ion. The product-phase approximation is introduced, which reduces the problem of approximating this functional significantly.Comment: 8 pages, 5 figures, RevTeX

Second-harmonic generation in silicon waveguides strained by silicon nitride

Silicon photonics meets the electronics requirement of increased speed and bandwidth with on-chip optical networks. All-optical data management requires nonlinear silicon photonics. In silicon only third-order optical nonlinearities are present owing to its crystalline inversion symmetry. Introducing a second-order nonlinearity into silicon photonics by proper material engineering would be highly desirable. It would enable devices for wideband wavelength conversion operating at relatively low optical powers. Here we show that a sizeable second-order nonlinearity at optical wavelengths is induced in a silicon waveguide by using a stressing silicon nitride overlayer. We carried out second-harmonic-generation experiments and first-principle calculations, which both yield large values of strain-induced bulk second-order nonlinear susceptibility, up to 40pm/V at 2.300 nm. We envisage that nonlinear strained silicon could provide a competing platform for a new class of integrated light sources spanning the near- to mid-infrared spectrum from 1.2 to 10 micron

Adiabatic Theory of Electron Detachment from Negative Ions in Two-Color Laser Field

Negative ion detachment in bichromatic laser field is considered within the adiabatic theory. The latter represents a recent modification of the famous Keldysh model for multiphoton ionization which makes it quantitatively reliable. We calculate angular differential detachment rates, partial rates for particular ATD (Above Threshold Detachment) channels and total detachment rates for the Hydrogen ion in a bichromatic field with 1:3 frequency ratio and various phase differences. Reliability of the present, extremely simple approach is testified by comparison with much more elaborate earlier calculations.Comment: 22 pages, 6 Postscript figure

Mathematical Properties of a New Levin-Type Sequence Transformation Introduced by \v{C}\'{\i}\v{z}ek, Zamastil, and Sk\'{a}la. I. Algebraic Theory

\v{C}\'{\i}\v{z}ek, Zamastil, and Sk\'{a}la [J. Math. Phys. \textbf{44}, 962 - 968 (2003)] introduced in connection with the summation of the divergent perturbation expansion of the hydrogen atom in an external magnetic field a new sequence transformation which uses as input data not only the elements of a sequence $\{s_n \}_{n=0}^{\infty}$ of partial sums, but also explicit estimates $\{\omega_n \}_{n=0}^{\infty}$ for the truncation errors. The explicit incorporation of the information contained in the truncation error estimates makes this and related transformations potentially much more powerful than for instance Pad\'{e} approximants. Special cases of the new transformation are sequence transformations introduced by Levin [Int. J. Comput. Math. B \textbf{3}, 371 - 388 (1973)] and Weniger [Comput. Phys. Rep. \textbf{10}, 189 - 371 (1989), Sections 7 -9; Numer. Algor. \textbf{3}, 477 - 486 (1992)] and also a variant of Richardson extrapolation [Phil. Trans. Roy. Soc. London A \textbf{226}, 299 - 349 (1927)]. The algebraic theory of these transformations - explicit expressions, recurrence formulas, explicit expressions in the case of special remainder estimates, and asymptotic order estimates satisfied by rational approximants to power series - is formulated in terms of hitherto unknown mathematical properties of the new transformation introduced by \v{C}\'{\i}\v{z}ek, Zamastil, and Sk\'{a}la. This leads to a considerable formal simplification and unification.Comment: 41 + ii pages, LaTeX2e, 0 figures. Submitted to Journal of Mathematical Physic

High Order Harmonic Generation and Atomic Stabilization in Ultra-Intense Laser Pulses

We present results of recent numerical simulations on the non-linear response of a single-electron atom submitted to an intense laser pulse. We address both the questions of the light scattered by the atom and the possible stabilization against ionization. One of the results of our simulations is the the presence of hyper-Raman lines in the high-order harmonic spectra. We suggest that these lines could be a signature of atomic stabilization

SOME NEW PERSPECTIVES IN BREMSSTRAHLUNG RESEARCH

Nous présentons les résultats de calculs théoriques nouveaux des amplitudes de transition et sections efficaces relatives à deux types distincts de processus associés au bremsstrahlung : a) Transitions stimulées à un photon (transitions "libre-libre") ayant lieu au cours de collisions électron-atome d' hydrogène, en présence d' un champ extérieur intense. b) Bremsstrahlung à deux photons, correspondant à l'émission spontanée de deux photons distincts lors d'une seule collision électron-atome. Les caractéristiques essentielles de ces processus sont mises en évidence et discutées.We report the results of new theoretical calculations of the transition amplitudes and cross-sections for two distinct bremsstrahlung-related processes : i) Stimulated one-photon transitions (Free-Free Transitions) occuning in the course of electron-H atom collisions, in the presence of a strong extemal field. ii) Two-photon Bremsstrahlung, corresponding to the spontaneous emission of two distinct photons in a single electron-atom collision event. The main features of these processes will be outlined and discussed

Two-photon transitions in hydrogenic atoms via a Sturmian basis for the Dirac Coulomb Green's function

Using a Sturmian-like expansion of the (first-order) Dirac Coulomb Green's function, the two-photon probability amplitude for the $\vert 1^2S_{1/2} \rangle \rightarrow \vert 2^2S_{1/2} \rangle$ transition in hydrogenic systems is calculated as a function of nuclear charge Z. A comparison between relativistic and non-relativistic treatments is made, taking care of the influence of retardation in both cases