P. Lambropoulos

FLUORESCENCE INTO FLAT AND STRUCTURED RADIATION CONTINUA : AN ATOMIC DENSITY MATRIX WITHOUT A MASTER EQUATION

We investigate an atomic

Above-threshold ionization spectrum of hydrogen using B-spline functions

Lambda

Multichannel theory of two-photon single and double ionization of helium

-system with one transition coupled to a laser field and a flat continuum of vacuum modes and the other transition coupled to field modes near the edge of a photonic band gap. The system requires simultaneous treatment of Markovian and non-Markovian dissipation processes, but the photonic band gap continuum can not be eliminated within a density matrix treatment. Instead we propose a formalism based on Monte-Carlo wavefunctions, and we present results relevant to an experimental characterization of a structured continuum.

Population transfer through an autoionizing state by pulse delay

We present a method for the accurate calculation of above-threshold ionization (ATI) spectra of single active electron systems. The technique involves the expansion of the radial wavefunction onto a set of B-spline functions. Results of ATI in hydrogen are presented and particular features of the spectra are discussed. A detailed study of the sensitivity of the results to the critical parameters is given to emphasize the importance of the necessity of a high degree of convergence in such calculations. Finally, we discuss and quantitatively justify the use of methods in investigations of ATI.

Phase control of photoabsorption in optically dense media

We report on ab initio calculations of two-photon, single- and double-electron ejection cross sections in helium, using multichannel wavefunctions constructed within the framework of discretized bases involving linear combinations of B-splines. Within the lowest non-vanishing order of perturbation theory, two-photon double ionization, for photon energies from 42.5 to 46.5 eV, is found to be about 10% that of single ionization. It is also found that electron correlation in the final state tends to lower the amount of double ionization in comparison to predictions involving incomplete or no correlation. The potential of the approach in the context of further open problems is also discussed.

Photoelectron energy spectrum in `direct' two-photon double ionization of helium

We obtain the conditions for complete population transfer through an autoionizing state in terms of asymmetry parametersq, which characterize the coupling strength between a discrete state and a continuum. In some limits, thus obtained conditions correspond to those for population transfer through a smooth continuum as well as a bound state.

Strongly non-Lorentzian emission spectra in a radiative cascade

We present a self-consistent theory, as well as an illustrative application to a realistic system, of phase control of photoabsorption in an optically dense medium. We demonstrate that, when propagation effects are taken into consideration, the impact on phase control is significant. Independent of the value of the initial phase difference between the two fields, over a short scaled distance of propagation, the medium tends to settle the relative phase so that it cancels the atomic excitation. In addition, we find some rather unusual behavior for an optically thin layer.

Phase control through channels involving multiple continua and multiple thresholds

We study the photoelectron energy spectrum in direct two-photon double ionization of He, and show that in certain ranges of photon frequency it competes favourably with sequential double ionization. When the photon frequency allows two-photon double ejection in both the sequential and direct channels, the latter appears as a background dominated by the peaks of the sequential channel. However, in a different frequency regime, where the sequential channel requires three photons, the spectrum of the direct channel is separated in energy and dominates in magnitude. Our results have been obtained for a range of radiation intensity and pulse duration reasonably expected in high-order harmonic generation sources.

Effect of vibration and internuclear axis orientation on multiphoton ionization of H2

We investigate the spectrum for an atomic ladder system where one transition is coupled near-resonantly to the edge of a photonic band gap and the other transition is coupled to a flat background of radiation modes. The corresponding emission spectra are strongly non-Lorentzian.

Effect of a strong dressing field on the polarizability of atomic helium at harmonic frequencies

We present a theory of phase control which governs the time-dependent behaviour of a system involving one bound and one decaying discrete state embedded in several continua belonging to several ionization thresholds. Representative numerical results are shown, focusing on the behaviour of the branching ratio as a function of the relative phase of two lasers, detuning, and Rabi frequencies (or intensities). Specific quantitative results on the Ca atom are also reported.