A focussing funnel for metastable helium

We have constructed a magneto-optical funnel for He atoms and studied its properties using a laser cooled, highly mono-energetic atomic beam. A simple model of its action allows us to quantitatively understand the observed spot size and “focal length”. We show that for a fast beam, the velocity damping coefficient plays an important role in determining the focal length of the device. The observed spot size is limited mainly by transverse heating processes which impose a transverse velocity spread. The device also permits easy scanning of the focussed spot. Received 30 October 1998 and Received in final form 27 January 1999     

White light transverse cooling of a helium beam

We report a study of transverse laser cooling on a metastable helium beam using spectrally broadened diode lasers (“white light") to increase its flux. For this purpose, beam profile and atomic flux versus laser power and other parameters have been characterized. We have performed experiments to compare this technique with other transverse cooling methods using monochromatic light. Best results are obtained with a “ziz-zag" configuration using “white light". Received 21 December 1998 and Received in final form 27 May 1999     

Saturation of the weak probe amplification in a strongly driven cold and dense atomic cloud

We present an experimental and theoretical investigation of the weak probe amplification in a cold and optically thick atomic cloud that is highly driven by a strong pump laser. We find that for high optical densities the probe amplification is strongly saturated. We compare our saturation measurements with a model based on dressed-atom population equalization due to re-scattering of spontaneous emission. Good agreement between theory and experiment is obtained only when corrections due to multiple scattering are included. Received 3 November 1998 and Received in final form 5 March 1999     

Quantum motion of three trapped ions in one dimension

The hyperspherical-coordinate approach is employed to a one-dimensional model of three ions in a Paul trap. It is shown that the eigen wave functions have well-defined nodal structure indicating a near separability in the hyperspherical coordinates, then two approximate good quantum numbers are introduced to classify the eigenstates. Three important classical periodic motions, including the breathing motion and the (distorted-)symmetric or anti-symmetric stretching motion, are found to dominate the wave function distribution. Received: 10 February 1999 / Received in final form: 25 March 1999     

Electrodynamic trap for neutral atoms

An electrodynamic trap is proposed that stores cold neutral atoms or nonpolar molecules in their ground state as well as in excited states by means of the quadratic Stark effect. The trap uses an oscillating hexapole field and a superposed static homogeneous field. The dynamics of an atom in this trap can be described as a harmonic oscillation in a static pseudopotential. Stability criteria and sample parameters for a number of atomic species are given. Received: 7 August 1998 / Received in final form: 7 January 1999     

Near threshold photoionization of excited alkali atoms Ak(np) (Ak = Na, K, Rb, Cs; n = 3-6)

The effect of the polarization of the atomic core by the outer electron on near threshold photoionization of excited alkali atoms Ak(np) (Ak = Na-Cs; n=3-6) is investigated. Partial and total cross-sections for photo-ionization of the np-electron were computed utilizing the configuration interaction technique with Pauli-Fock atomic orbitals (CIPF) and including the long range core polarization potential (CP). To calculate the core polarization potential the variational principle is applied. Comparison with previous theoretical results and with available experimental data is made for the total cross-section , for the electron angular distribution parameter , for the ratio of the reduced electric dipole matrix elements and for the phase shift difference , associated with the d-wave and s-wave continua, respectively. In the comparison, new experimental results for , , and , measured for laser-excited, polarized ³⁹K(4p _3/2) atoms, have been included. Received 21 July 1999 and Received in final form 14 October 1999     

Semi-classical theory of laser cooling in two dimensions

We present a semi-classical theory of the light pressure force for atoms interacting with a two-dimensional laser field. Unlike previous 2D theory, ours is valid for general atomic level and laser field configurations. We show that striking new features appear in the velocity-dependent force arising from the multi-dimensionality. Finally, we describe in detail the novel numerical technique used in the calculation. Received 4 December 1998 and Received in final form 13 February 1999     

Evaporative cooling of an atomic beam

We present a theoretical analysis of the evaporative cooling of an atomic beam propagating in a magnetic guide. Cooling is provided by transverse evaporation. The atomic dynamics inside the guide is analyzed by solving the Boltzmann equation with two different approaches: an approximate analytical ansatz and a Monte-Carlo simulation. Within their domain of validity, these two methods are found to be in very good agreement with each other. They allow us to determine how the phase-space density and the flux of the beam vary along its direction of propagation. We find a significant increase for the phase-space density along the guide for realistic experimental parameters. By extrapolation, we estimate the length of the beam needed to reach quantum degeneracy. Received 24 September 1999     

3D dissipative motion of atoms in a strongly coupled driven cavity

We develop quantum models for the combined external and internal motion of atoms in a strongly coupled driven cavity mode including the transverse degrees of freedom. Using a simplified Gaussian mode function we determine the parameter regimes and prospects of 3D cooling and confinement of one or two atoms in the cavity field. Analysing the field dynamics for slow atoms traversing the cavity, we show that the spectrum of the transmitted and spontaneously scattered light contains ample information on the motional dynamics of the atom and can be nicely used to investigate the cooling properties of the system. Including several atoms in the dynamics we show how motional correlations build up by the common interaction with the cavity field. This can be looked upon as collisions at far distance and can be monitored via the transmitted field dynamics. Received 5 March 1999 and Received in final form 4 May 1999     

Flux enhancement model for cold cesium fine-structure changing collisions

The measurements of fine-structure changing collisions in a cesium magneto-optical trap, reported in a previous work [A. Fioretti et al., Phys. Rev. A 55, R3999 (1997)], are reanalyzed within a model based on the flux enhancement effect, which takes place in cold atomic collisions. In the present analysis, we consider the cooperative effect of the long-range and the shorter-range excitation by the strong trap laser. We evidence also the important role of the hyperfine structure of the Cs₂ molecular levels asymptotically connected to the ground-state and excited-state dissociation limits. Received 22 July 1999 and Received in final form 4 May 2000     

ac Stark effect for a two-level atom in a squeezed vacuum via a two-photon process

The nondegenerate two-photon interaction of a two-level atom with a broadband multimode squeezed vacuum is investigated. We find that in the two-photon process the squeezed vacuum has a driving effect on the atom which can lead to an ac Stark effect when the average photon number of the squeezed vacuum is larger than a critical value. Received: 2 February 1999 / Received in final form: 20 April 1999     

Condensation of laser cooled gases

We study a dynamical scheme for condensation of bosonic trapped gases beyond the Lamb-Dicke limit, when the photon-recoil energy is larger than the energy spacing of the trap. Using quantum master equation formalism we demonstrate that dark-state cooling methods similar to those designed for a single trapped atom allow for the condensation of a collection of bosons into a single state of the trap, either the ground, or an excited state. By means of Monte-Carlo simulations we analyse the condensation dynamics for different dimensions, and for different cooling schemes. Received 30 November 1998 and Received in final form 20 March 1999     

Fragmentation of CO+ molecular ion through dissociative excited states produced by swift multicharged heavy ion impact on CO molecules

We report the observation of resonances in the intensity correlation spectra of a 3D rubidium optical lattice, which we attribute to light scattering from propagating atomic density fluctuations in the lattice. This process is the spontaneous analog of the stimulated scattering mechanism recently described by Courtois et al.. We investigate the dependence of the new resonances on the lattice angle and show that they disappear for large angles, thus resolving previous discrepancies on the subject. Received: 30 January 1997 / Accepted: 9 October 1997     

Reordering kinetics of ion-disordered Ni Al

A study of the reordering kinetics of ion-irradiated Ni₃Al is presented. The development of long-range order during annealing treatments is measured by quantitative electron diffractometry. According to the temperature dependence of the observed kinetics, the reaction is dominated by non-equilibrium vacancies. As a consequence, the kinetics can be calibrated versus the absolute number of atomic jumps necessary to establish the observed degree of order. In order to analyse the experimental data, Monte Carlo simulations are performed. It is shown that, beside the temperature dependence of the driving force, the ordering efficiency of the vacancy jumps itself is temperature dependent due to different mobilities of the atomic species. Received 6 December 1999 and Received in final form 23 June 2000     

Triply excited three-electron systems--semiclassical model

We calculate rovibronic intrashell spectra of the triply highly excited atomic hydrogen dianion, helium anion and lithium atom, within a simple semiclassical model. Zero-order electronic energy levels and half-lives are calculated for a number of principal quantum numbers and approximate thresholds for the appearance of vibronic modes are estimated. Since no quantum-mechanical and experimental data are available for the highly excited levels (N > 5), where the semiclassical models apply, no comparison with other results are possible at present. The problem of comparing semiclassical and quantum-mechanical calculations for moderately large quantum numbers, which seem attainable by the present day experimental technique, has been discussed. Received: 16 September 1998 / Received in final form: 16 March 1999     

A continuous cold atomic beam from a magneto-optical trap

We have developed and characterized a new method to produce a continuous beam of cold atoms from a standard vapour-cell magneto-optical trap (MOT). The experimental apparatus is very simple. Using a single laser beam it is possible to hollow out in the source MOT a direction of unbalanced radiation pressure along which cold atoms can be accelerated out of the trap. The transverse cooling process that takes place during the extraction reduces the beam divergence. The atomic beam is used to load a magneto-optical trap operating in an ultra-high vacuum environment. At a vapour pressure of 10^-8mbar in the loading cell, we have produced a continuous flux of 7×10⁷atoms/s at the recapture cell with a mean velocity of 14 m/s. A comparison of this method with a pulsed transfer scheme is presented. Received 19 February 2001     

Vacuum ultraviolet photoionization of the 5d elements in the region of the 5p and 4f excitation

The results of recent VUV photoionization experiments of the 5d elements Ta, W, Re, Ir and Pt using atomic beam technique, excitation with monochromatized synchrotron radiation and detection of singly and doubly charged photoions are discussed. Special attention is given to the resonances caused by discrete transitions of the 5p and 4f electrons into the unfilled 5d subshells. As there is a crossover of the 5p and 4f levels along the series of the 5d elements the result is a complicated structure of the corresponding interacting resonances. The photoion spectra are compared with theoretical calculations carried out within the relativistic time dependent local density and the relativistic Hartree-Fock approximation. Received: 9 October 1997 / Accepted: 3 November 1997     

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