Excitation of hydrogen atom by positron using hylleraas time-dependent approach

The time-dependent treatment of positron-hydrogen scattering for a zero total angular momentum has been presented. The initial wavefunction of the positron-hydrogen scattering system has been expanded in terms of three dimensional dynamical wave functions to include all higher angular momenta by solving a set of three coupled differential equations. This wavefunction is then time evolved using Taylor series expansion of the evolution operator. The excitation probabilities are monitored as the wavefunction propagates until there is no more change in the probabilities. The positron impact excitation cross-sections extracted from the final wavefunction are compared with the available results of converged close coupling approach. Received 23 July 2001 and Received in final form 25 November 2001     

Jaynes-Cummings-model with damping at resonance

The time evolution of a damped two-level atom coupled to a damped field mode at resonance is calculated analytically as well as numerically in the basis of dressed states. We study the dynamics of the density matrix and of observables, e.g. the number of field quanta. For the initial states we consider the field to be in a Fock or Glauber state and the atom in the ground or excited state. We show the significant influence of the damping on the well-known phenomena of this model, e.g. collapse and revival. Received 22 January 2001 and Received in final form 16 May 2001     

Static magneto-polarizability of cylindrical nanostructures

The static polarizability of cylindrical systems is shown to have a strong dependence on a uniform magnetic field applied parallel to the tube axis. This dependence is demonstrated by performing exact numerical diagonalizations of simple cylinders (rolled square lattices), armchair and zig-zag carbon nanotubes (rolled honeycomb lattices) for different electron-fillings. At low temperature, the polarizability as function of the magnetic field has a discontinuous character where plateau-like region are separated by sudden jumps or peaks. A one to one correspondence is pointed out between each discontinuity of the polarizability and the magnetic-field induced cross-over between the ground state and the first excited state. Our results suggest the possibility to use measurements of the static polarizability under magnetic field to get important informations about excited states of cylindrical systems such as carbon nanotubes. Received 29 March 2001 and Received in final form 8 August 2001     

Ultrafast dynamics in the excited hydrogen atom transfer states of ammonia clusters

Neutral ammonia clusters (NH₃)_m are photo-excited to the electronic state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH₃)_m-2NH₄(3s)NH₂ with the subunit NH₄ in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH₃)_m-2NH₄(3p)NH₂ states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation. Received 22 September 2001 and Received in final form 31 January 2002     

Ionization of iridium ions in the Dresden EBIT studied by X-ray spectroscopy of direct excitation and radiative recombination processes

Ir^q+ ( 41≤q≤64) ions with open-shell configurations have been produced in the electron beam of the room-temperature Dresden Electron Beam Ion Trap (Dresden EBIT) at electron excitation energies from 2 keV to 13 keV. X-ray emission from direct excitation processes and radiative capture in krypton-like to aluminium-like iridium ions is measured with an energy dispersive Si(Li) detector. The detected X-ray lines are analyzed and compared with results from multiconfigurational Dirac-Fock (MCDF) atomic structure calculations. This allows to determine dominant produced ion charge states at different electron energies. The analysis shows that at the realized working gas pressure of 5×10^-9mbar for higher charged ions the maximum ion charge state is not preferently determined by the chosen electron beam energy needed for ionization of certain atomic substates, but by the balance between ionization and charge state reducing processes as charge exchange and radiative recombination. This behaviour is also discussed on the basis of model calculations for the resulting ion charge state distribution. Received 12 July 2001 and Received in final form 10 September 2001     

A minimal model for vorticity and gradient banding in complex fluids

A general phenomenological reaction-diffusion model for flow-induced phase transitions in complex fluids is presented. The model consists of an equation of motion for a nonconserved composition variable, coupled to a Newtonian stress relation for the reactant and product species. Multivalued reaction terms allow for different homogeneous phases to coexist with each other, resulting in banded composition and shear rate profiles. The one-dimensional equation of motion is evolved from a random initial state to its final steady state. We find that the system chooses banded states over homogeneous states, depending on the shape of the stress constitutive curve and the magnitude of the diffusion coefficient. Banding in the flow gradient direction under shear rate control is observed for shear-thinning transitions, while banding in the vorticity direction under stress control is observed for shear-thickening transitions. Received 1 April 2001 and Received in final form 16 June 2001     

Scattering of light exotic atoms in excited states

The differential and total cross-sections for the scattering of muonic, pionic, kaonic and antiprotonic hydrogen in excited states from atomic hydrogen have been calculated for the purpose of atomic cascade calculations. The scattering problem is treated in a fully quantum mechanical framework which takes the energy shifts and, in the case of the hadronic atoms, the widths of the ns states into account. The validity of semiclassical approximations is critically examined. Received 4 December 2001 and Received in final form 4 February 2002     

III. A stochastic two-stage reaction model for heavy-ion collisions

We propose a two-stage, stochastic model of heavy-ion reactions. Nucleons becoming participants by mean-field effects or by nucleon-nucleon interactions are transferred to definite final states, creating a PLF, a TLF, clusters, or escaping to continuum. Nucleon transfer probabilities are governed by state densities. In this way different hot particle sources are created which afterwards decay by particle emission. Received: 12 March 2001 / Accepted: 20 June 2001     

Room temperature stable single-photon source

We report on the realization of a stable solid state room temperature source for single photons. It is based on the fluorescence of a single nitrogen-vacancy (NV) color center in a diamond nanocrystal. Antibunching has been observed in the fluorescence light under both continuous and pulsed excitation. Our source delivers 2×10⁴ s^-1 single-photon pulses at an excitation repetition rate of 10 MHz. The number of two-photon pulses is reduced by a factor of five compared to strongly attenuated coherent sources. Received 1st August 2001 and Received in final form 2 October 2001     

Electron temperature measurement in a surface-wave-produced argon plasma at intermediate pressures

The main objective of this work is to obtain the electron temperature in an argon surface-wave-produced plasma column at intermediate gas pressures. After proving that argon upper excited states remain in Excitation Saturation Balance, the value of electron temperature along the plasma column has been obtained using a modified Saha equation and a corrected Boltzmann-plot. Moreover, the electron energy distribution function has been verified to be nearly Maxwellian in a 0.8-2.8 torr intermediate pressure range. Received 24 July 2000 and Received in final form 19 January 2001     

Influence of laser pulse width on absolute EUV-yield from Xe-clusters

Using 50 fs ( ∼ 2×10¹⁸ W/cm²) and 2 ps ( ∼ 5×10¹⁶ W/cm²) pulses from a Ti:Sa multi-TW laser at 800 nm wavelength large Xe-clusters ( 10⁵...10⁶ atoms per cluster) have been excited. Absolute yield measurements of EUV-emission in a wavelength range between 10 nm and 15 nm in combination with cluster target variation were carried out. The ps-laser pulse has resulted in about 30% enhanced and spatially more uniform EUV-emission compared to fs-laser excitation. Circularly polarized laser light instead of linear polarization results in enhanced emission which is probably caused by electrons gaining higher energies by the polarization dependent optical field ionization process. An absolute emission efficiency at 13.4 nm of up to 0.8% in 2π sr and 2.2% bandwidth has been obtained. Received 11 January 2001 and Received in final form 27 March 2001     

DFT studies of ionic vibrations in Na clusters

We use time-dependent density functional theory coupled to molecular dynamics for ionic motion to compute the spectra of ionic vibrations in small Na clusters. Comparison with results from the distance dependent tight-binding approach shows good agreement between these two very different methods. We discuss the evolution of the spectra with cluster size and charge and the impact of ionic vibrations on the optical response. Received 23 July 2001 / Received in final form 5 July 2002 Published online 8 October 2002 RID="a" ID="a"e-mail: suraud@irsamc.ups-tlse.fr     

Ab initio calculations of the second dynamic hyperpolarizability of LiH by means of Floquet theory approach

Floquet theory is used to describe the response of a molecule to applied radiation electric field. The method of ab initio calculation of frequency dependent (hyper)polarizabilities based on combination of perturbation theory with the finite field method has been developed. Electron correlation is taken into account by means of the CIPSI algorithm. The total wave function expansion involves spectral, pseudo-spectral states and polynomial terms. The developed approach is applied to the calculation of the second hyperpolarizability of the lithium hydride molecule subjected to the superposition of harmonic and static electric fields. The method can be used to calculate the higher order nonlinear properties of molecules. Received 23 November 2000 and Received in final form 25 March 2001     

Carrier scattering by Auger mechanism in a single quantum wire

We report on time-resolved microphotoluminescence experiments in a single GaAs/GaAlAs V-shaped quantum wire as a function of optical excitation intensity. At low pump power we observe that excitons are localized in quantum boxes formed by the local potential minima existing along the wire axis. As the pump power is increased, state filling of the lowest lying levels of the boxes appears. When two carriers occupy the first excited level of the box, a very efficient Auger scattering occurs, leading to a transfer of carriers from one box to another neighbouring one. The intradot Auger scattering time has been measured and is of the same order of magnitude as the LA-phonon emission rate. Received 5 February 2001     

Charge carrier dynamics in CdSe nanocrystals: implications for the use of quantum dots in novel photovoltaics

The dynamics of photo-generated electrons and holes in CdSe quantum dots have been studied using the femtosecond fluorescence upconversion technique, permitting an unambiguous examination of the excited state. The band edge emission shows an expected size dependence on the decay rate. We find that the deep trap emission is coupled to the band edge fluorescence, implicating surface states as important factors in the excited state lifetime of the hole. As a factor of the overall efficiency of solar cells, the rate of charge separation and the fate of the exciton are important considerations in the design of nanocrystal-based photovoltaic devices. Received 30 November 2000     

What does an observed quantum system reveal to its observer?

The evolution of a quantum system under observation becomes retarded or even impeded. We review this “quantum Zeno effect” in the light of the criticism that has been raised upon a previous attempt to demonstrate it, of later reexaminations of both the projection postulate and the significance of the observations, and of the results of a recent experiment on an individual cold atom. Here, the micro-state of the quantum system gets unveiled with the observation, and the effect of measurement is no longer mixed up with dephasing the object's wave function by the reactive effect of the detection. A procedure is outlined that promises to provide, by observation, an upper limit for the delay of even an exponential decay. Received 11 January 2001 and Received in final form 28 February 2001     

Resonance interaction between one excited and one ground state atom

The nature of the resonance interaction between two isotropic atoms in an excited configuration is reinvestigated. The currently accepted oscillatory form for the long-range retarded resonance interaction is shown to be a subtle artefact that arises due to too drastic approximations. Formulation of the resonance interaction energy problem in terms of the interacting system leads to a form that it is ∝ r ^-4 in the retarded limit. We also demonstrate that the resonance interaction energy at any finite temperature goes over to purely classical long-range asymptote. This manifestation of the correspondence principle is due to thermal excitation of the electromagnetic field. We finally discuss why the textbook result for the F?rster energy transfer between two atoms is incorrect for the same reasons. Received 31 January 2002 / Received in final form 25 July 2002 Published online 29 October 2002 RID="a" ID="a"e-mail: mtb110@rsphysse.anu.edu.au     

Peculiarities in fragment mass distribution in the 238U + 40Ar (243 MeV) reaction

A pronounced fine structure (FS) in the form of distinct peaks was observed in neutron gated mass spectra from the decay of the ²⁷⁸110 composite system produced in the reaction ²³⁸U + ⁴⁰Ar (243 MeV) at an initial excitation energy E ^* > 70 MeV. The FS peaks are located in the vicinity of mass numbers 70-80, 100, and 130, which correspond to those of magic nuclei (clusters). In the data there is also evidence for a new type of decay -- collinear cluster tripartition of an excited nucleus. Received: 8 August 2000 / Accepted: 2 February 2001     

Laser-induced fluorescence measurements of a 3 Π metastable carbon monoxide in a high enthalpy flow

Excitation of carbon monoxide molecules has been carried out in a cold cell and in a low-pressure plasma jet using an ArF narrow-band excimer laser. The different excitation models are discussed and the relevance of atomic carbon absorption into the laser cavities is pointed out. Excitation spectra of Cameron bands have been obtained in a room-temperature cell and compared with calculated spectra. A value of the constant σ related to the interaction strength between a ³ Π( v = 2) state and its neighbouring singlet states is derived: 0≤σ≤0.05. The fluorescence spectrum following broad band excitation of CO has been observed both in UV and visible. Similar experiments carried out in a high enthalpy flow have allowed to point out the presence of a ³ Π metastable carbon monoxide. A method for relative measurements of this species concentration is proposed. Received 9 April 2001 and Received in final form 13 June 2001