Theoretical study of molecular hydrogen clusters

Hydrogen clusters are formed by packing H₂ molecules. A structural characterization of (H₂)_N clusters up to N=35 has been carried out at zero temperature by using density functional theory. The binding between the hydrogen molecules is very weak and the cluster growth reminds that of the inert gas clusters. An icosahedron is obtained for (H₂)₁₃. For clusters larger than (H₂)₁₃ several growth models have been compared. The binding energy indicates specially stable clusters for some particular sizes. The magic numbers can be related to Raman spectroscopy experiments, where the intensity of the Raman signal serves to assign enhanced abundance to clusters with N≈13,32,55, which coincide with some of the most stable clusters obtained in the present study. In addition, comparison of theory and experiment suggests that clusters with N smaller than 27 are liquid. The photoabsorption spectra have been calculated using time-dependent density functional theory. Those spectra can be interpreted as a widening of the absorption peaks of the H₂ molecule due to the various environments experienced by different molecules in the same cluster.     

Atomic Compton profiles within different exchange-only theories

The impulse Compton Profiles (CPs) J(q) and the expectation values for some inert gas atoms (He-Kr) are computed and compared within the Harbola-Sahni (HS), Hartree-Fock (HF) theories and a Self-Interaction-Corrected (SIC) density functional model. The Compton profiles for excited states of helium atom are also calculated. While the calculated CPs are found to generally agree, they differ slightly from one another for small values of the Compton parameter q and are in good agreement for large q values. The expectation values within the three theories are also found to be comparable. The HS formalism is seen to mimic HF reasonably well in the momentum space, establishing the logical consistency of the former. Received: 11 December 1998 / Received in final form: 20 April 1999     

On the continuity in energy of the free-bound one-photon cross-sections

For a charged particle in a central field of force we discuss the continuity in energy of the cross-sections for the related processes of ground state photoeffect and radiative capture. This continuity follows from general arguments, while for a Coulomb potential, where exact analytic expressions are available, the continuity can also be traced explicitly through the calculation. Our analysis allows us to identify an error in papers on radiative muon capture in the ground state which claim the existence of a discontinuity in the cross-section, occurring in hydrogen for muon energies in the neighbourhood of 2.8 keV. As a discontinuity in radiative capture would imply a discontinuity in atomic photoeffect, we note that in the latter case there are extensive experimental results in good qualitative agreement with the usual continuous result (Stobbe formula) for the cross-section. Received: 13 February 1998 / Accepted: 14 May 1998     

Photo-atomic effect: Temperature dependence of the photodesorption of Na and from polymer surfaces

Alkali metals adsorbed to surface films of the polymer poydimethylsiloxane (PDMS) have been shown to exhibit a unique photodesorption behavior, characterized by a frequency threshold and high efficiency. In this work, the temperature dependence of the photodesorption yields of Na and Na ₂ from PDMS surfaces were measured between room temperature and 183 K. Over most of the temperature range, the yields exhibited an Arrhenius behavior characterized by thermal activation energies of 0.36 eV and 0.34 eV for Na and Na ₂ , respectively. These values are suggestive of a surface diffusion as one of the elementary steps in the photodesorption mechanism. Moreover, the similarity of the two values indicates that the same elementary step applies to the desorption of both Na and Na ₂ . Received 23 April 1999 and Received in final form 15 July 1999     

Pulsed standing wave deflection of sodium atoms

We study the deflection of sodium atoms by a resonantly tuned pulsed standing wave of high field intensity. The effects of the phase fluctuations of the pulsed laser field on the momentum distribution of the deflected atoms are experimentally determined. The results are explained using a theoretical model based on the generalized density matrix formalism of two-level atoms. Received 23 November 1998 and Received in final form 27 January 1999     

Reflection of thermal atoms by a pulsed standing wave

Reflection of thermal atoms by a pulsed standing wave with a duration in the nanosecond range is studied. The momentum distribution of the reflected atoms is determined by calculations based on the adiabatic atom-photon interactions. It is shown that with a proper choice of the field intensity and the pulse duration the standing-wave pattern functions as a row of independent atom mirrors. At an optimum choice of the parameter values, the fraction of the elastically reflected atoms is more than 20%. Furthermore, we show that the pulsed standing-wave mirror can be used to manipulate their final momentum distribution. When using laser pulses with an intensity of several tens of MW/cm², tens of thousands of atoms can be reflected by a single laser pulse. Received 3 December 1999 and Received in final form 25 April 2000     

Electron angular distributions in double photoionization: Use of effective Sommerfeld parameters

We present calculations of the fivefold differential cross-section (FDCS) for double photoionization of helium at excess energies of 6 and 20 eV above threshold. Our results are obtained using for the final double-continuum state a product of three Coulomb wave functions, with the Sommerfeld parameters modified to describe the strength of interaction of any two particles affected by the third particle. Our calculations are compared with recent absolute measurements by D?rner et al. (Phys. Rev. A 57, 1074 (1998)), both in coplanar and non-coplanar geometries. Very good agreement is obtained for the shape of the angular distributions, and differences in the absolute magnitude exist in comparison with the standard choice of Sommerfeld parameters. Received: 17 July 1998 / Received in final form and Accepted: 23 October 1998     

A new class of photo-induced phenomena in siloxane films

The light induced atomic desorption effect, known as LIAD, is observed whenever Pyrex cells, coated with siloxane films and containing alkali atoms, are illuminated. LIAD is a non-thermal phenomenon and it can be observed even with very weak light intensities. We show that the simultaneous contribution to the photo-emission of atoms adsorbed both at the film surface and within the film must be taken into account in order to fit the experimental data. We demonstrate that both the desorption efficiency and the diffusion coefficient of the alkali atoms embedded in the dielectric film depend on the desorbing light intensity. These features characterize a new class of photo-induced phenomena whose analysis gives new insights in the comprehension of the atom-surface interaction and of the atom-bulk diffusion and opens interesting perspectives for applications. Received 27 April 2000 and Received in final form 15 September 2000     

A non-perturbative open-shell theory for atomic and molecular systems: Application to transbutadiene

A non-perturbative theory is proposed in this article in which an energy independent effective Hamiltonian is obtained for open-shell systems. We have given a diagrammatic version of theory to facilitate the analysis of the problem. The theory has been applied to a model 4-π electron problem, for calculating the lowestπ-π* singlet and triplet energy levels of transbutadiene. Comparison with full Cl calculation indicates the excellent workability of the theory.     

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     

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     

Energy pooling to the level arising from collisions between pairs of metastable atoms

The energy-pooling rate coefficient for the process has been measured. The barium atoms were excited by a cw diode laser tuned to the frequency of the 791.3 nm intercombination line and the metastable atoms in the 6s5d state were produced due to radiative and collisional depopulation of the laser-excited 6s6p state. The measurements were performed at number densities of about and at 30 mbar argon as the buffer gas. Most of the barium ground state atoms in the excitation zone were transferred to the trip let metastable state at the laser pump power applied. The energy pooling rate coefficient was determined by comparing the fluorescence intensity of the barium 553.6 nm resonance line and the fluorescence intensity of the intercombination line 791.3 nm. In addition, the populations of the metastable atoms were probed with low intensity laser radiation from a single mode ring dye laser. The rate coefficient was found to be at . Received: 11 April 1997 / Revised: 24 September 1997 / Accepted: 10 December 1997     

Energy pooling in caesium vapour:

The total rate coefficient k for the energy pooling process has been measured in dependence on the ratio of the number densities of the Cs 6P fine-structure sublevels. The measurements were performed applying the laser fluorescence method. From the set of the obtained data for k, the rate coefficients for the particular collisions were derived. The obtained values for the and collisions at T = 600 K are , and , respectively. The value for k₁ and the estimate for k₃ are consistent with the data previously published by other authors. The rate coefficient k₂ is reported for the first time. Received: 10 June 1997 / Revised: 13 October 1997 / Accepted: 5 January 1998     

Collision induced dissociation of the heteronuclear alkali ion .Comparison with the and dissociations

An experimental study of the dissociation of the heteronuclear Na ion by collisions with He and H₂ targets at a collision energy of 2.4 keV is presented. The use of a technique based on the measurement of the velocity vectors of the fragments allows investigating in detail the two basic dissociation mechanisms: impulsive and electronic. In the present asymmetric case, one is able to assess the relative role of close encounters with each of the Na⁺ and atomic cores of the molecule. Electronic transitions at a surface crossing are proposed to explain processes which happens in close collisions between the core and the He target. Received: 3 October 1997 / Revised: 1 December 1997 / Accepted: 16 January 1998     

Fringe contrast in three grating Mach-Zehnder atomic interferometers

Several three-grating Mach-Zehnder atomic interferometers have been built and operated in recent years but no general theory of the contrast of the fringes produced by these apparatus is available. The purpose of this paper is to develop this theory, based on the Fresnel-Kirchoff approximate treatment of diffraction. Such a theory has been developed by Turchette et al. [JOSA B 9, 1601 (1992)] but because the necessary multiple integrals were evaluated in a purely numerical way, this treatment was not fully general. We show here how to reduce the computation by analytic means and we are thus able to calculate the contrast with a modest numerical effort. Moreover, we get a simple insight of the contrast reduction related to several defects of a real apparatus. We apply our calculations to existing interferometers as well as to an apparatus working with lithium which is under construction in our laboratory. Received: 24 April 1998 / Revised: 25 October 1998 / Accepted: 11 December 1998     

Renormalization of the two-photon vacuum polarization and the self energy vacuum polarization for a tightly bound electron

The renormalization method of Bogoljubov-Parasiuk-Hepp-Zimmermann (BPHZ) is used in order to derive the renormalized energy shift due to the gauge invariant K?llén-Sabry diagram of the two-photon vacuum polarization (VPVP) as well as the self energy vacuum polarization S(VP)E beyond the Uehling approximation. It is outlined, that no outer renormalization is required for the two-photon vacuum polarization and that only the inner renormalization has to be accomplished. It is shown that the so-called nongauge invariant spurious term is absent for a wide class of vacuum polarization (VP) diagrams if one applies the widely used spherical expansion of bound and free-electron propagator. This simplifies significantly calculations in bound state quantum electrodynamics. As one result of our paper the use of the BPHZ-approach in bound state QED is established. Received 28 September 2001     

Evidence for N1/3 dependence of the sticking cross-section of atoms on small and medium-size van der Waals clusters

This paper introduces a simple model describing the cluster growth in supersonic expansions. The predicted terminal mean cluster size is compared to the available data in the case of argon. The agreement between the model and the experimental results requires that the cross-section describing the sticking of an atom on a cluster of size N scales like with in the range 0.34-0.44, well below the predicted by the simplest geometrical scaling argument. We explain this unexpected result in two steps. First, using Monte Carlo simulations, we check that the potential between an atom and a cluster is accurately represented by the Gspann and Vollmar potential, even at finite temperature. Then, using Langevin's approximation, we show that the sticking cross-section scales like N ^1/3 for small to moderate N values and switches to the geometric scaling N ^2/3 for very large N values. The crossover between these two scalings occurs when for argon, but the mean exponent over the size range 1-10⁴ is 0.46. This N scaling of the sticking cross-section should play an important role whenever condensation is important as it modifies the kinetics of the early stages. Received 20 May 1999 and Revised in final form 22 July 1999     

Vibrational excitation by positron collisions with molecular gases: a study of O and NO targets

The scattering of slow positrons from and NO molecules is treated using exact static interactions and a model potential for correlation-polarisation forces. The quantum coupled equations for the elastic scattering are extended to vibrationally inelastic processes and the different excitation probabilities are evaluated. Comparison with existing experiments for the NO target indicates that the present calculations provide a realistic treatment of positron scattering below Ps formation and give computational estimates on the efficiency of such projectiles in producing vibrationally excited molecules in the ambient gas. Received: 23 April 1999 / Received in final form: 3 June 1999     

Cooling mechanisms in potassium magneto-optical traps

We report on a theoretical and experimental investigation of ³⁹K magneto-optical trapping. The small hyperfine splitting characterizing the upper level of the cooling transition affects the cooling mechanism. In order to model the atom-laser interaction, the whole level structure of the D₂ line has to be taken into account. Two different regimes have been recognized, one optimizing the loading of the trap, the second minimizing the temperature of the atoms. We investigated these two regimes experimentally and found results in agreement with the theoretical predictions. Received: 6 March 1998 / Received in final form: 13 May 1998 / Accepted: 13 May 1998