Nondispersive two-electron wave packets in driven helium

We provide a detailed quantum treatment of the spectral characteristics and of the dynamics of nondispersive two-electron wave packets along the periodically driven, collinear frozen planet configuration of helium. These highly correlated, long-lived wave packets arise as a quantum manifestation of regular islands in a mixed classical phase space, which are induced by nonlinear resonances between the external driving and the unperturbed dynamics of the frozen-planet configuration. Particular emphasis is given to the dependence of the ionization rates of the wave packet states on the driving field parameters and on the quantum mechanical phase space resolution, preceded by a comparison of 1D and 3D life times of the unperturbed frozen planet. Furthermore, we study the effect of a superimposed static electric field component, which, on the grounds of classical considerations, is expected to stabilize the real 3D dynamics against large (and possibly ionizing) deviations from collinearity. Received 7 November 2002 / Received in final form 2 December 2002 Published online 28 January 2003     

Messung von Lebensdauern und relativen Besetzungszahlen angeregter Zustände des Wasserstoffatoms mit der Methode der Ionenstrahlspektroskopie

H ⁺₂ ions or protons, accelerated to energies between 80 and 300 keV, are sent through a thin carbon target. Light emitted by the excited atoms moving out of the foil with a definite velocity is analyzed using interference filters and a cooled photomultiplier. Intensities of theH _α,H _β, andH _γ lines are measured as functions of the distance from the foil. The data is analyzed with a computer program, which yields (in agreement with theory) mean lives of the hydrogen atom excited states involved, and the relative initial state populations of states belonging to the same principal quantum number. There is no indication of cascading.

     

Resonance-assisted tunneling in near-integrable systems

Dynamical tunneling between symmetry related invariant tori is studied in the near-integrable regime. Using the kicked Harper model as an illustration, we show that the exponential decay of the wave functions in the classically forbidden region is modified due to coupling processes that are mediated by classical resonances. This mechanism leads to a substantial deviation of the splitting between quasidegenerate eigenvalues from the purely exponential decrease with 1/Planck's over 2pi obtained for the integrable system. A simple semiclassical framework, which takes into account the effect of the resonance substructure on the invariant tori, allows one to quantitatively reproduce the behavior of the eigenvalue splittings.     

Doubly excited states in the beam-foil Li spectrum

Beam-foil excited spectra of Li atoms and ions have been investigated in the wavelength range 2000–6000 Å. Line widths and lifetimes are given for the lines at 3155 Å, 3144 Å, and 5539 Å, the two latter representing transitions between doubly excited states.     

The significance of quadrupole-electric coefficients for the optical properties of nonmagnetic crystals

The physical significance of the quadrupole-electric tensor for the optical properties of nonmagnetic crystals is discussed. Some general symmetry properties between the optical material tensors are derived. As an example it is shown that the quadrupole-electric tensor induces the ellipticity of light which is reflected from crystals of the non centrosymmetric cubic classes 23 and ¯43m.     

Lifetimes of (2p53s1,3P) levels of Na II

Lifetimes of the resonance levels (2p⁵3s^1,3P) of Na II have been measured by the beam-foil method. The experimental accuracy is better than that of available theoretical predictions.     

Resonance-assisted decay of nondispersive wave packets

We present a quantitative semiclassical theory for the decay of nondispersive electronic wave packets in driven, ionizing Rydberg systems. Statistically robust quantities are extracted combining resonance-assisted tunneling with subsequent transport across chaotic phase space and a final ionization step.     

Coherent backscattering of Bose-Einstein condensates in two-dimensional disorder potentials

We study quantum transport of an interacting Bose-Einstein condensate in a two-dimensional disorder potential. In the limit of a vanishing atom-atom interaction, a sharp cone in the angle-resolved density of the scattered matter wave is observed, arising from constructive interference between amplitudes propagating along reversed scattering paths. Weak interaction transforms this coherent backscattering peak into a pronounced dip, indicating destructive instead of constructive interference. We reproduce this result, obtained from the numerical integration of the Gross-Pitaevskii equation, by a diagrammatic theory of weak localization in the presence of nonlinearity.