Elastic and inelastic evanescent-wave mirrors for cold atoms

We report on experiments on an evanescent-wave mirror for cold ⁸⁷Rb atoms. Measurements of the bouncing fraction show the importance of the Van der Waals attraction to the surface. We have directly observed radiation pressure parallel to the surface, exerted on the atoms by the evanescent-wave mirror. We analyze the radiation pressure by imaging the motion of the atom cloud after the bounce. The number of photon recoils ranges from 2 to 31. This is independent of laser power, inversely proportional to the detuning and proportional to the evanescent-wave decay length. By operating the mirror on an open transition, we have also observed atoms that bounce inelastically due to a spontaneous Raman transition. The observed distributions consist of a dense peak at the minimum velocity and a long tail of faster atoms, showing that the transition is a stochastic process with a strong preference to occur near the turning point of the bounce.     

Electric and magnetic mirrors and gratings for slowly moving neutral atoms and molecules

Those atoms or molecules which happen to have positive Stark or Zeeman energy shift (by virtue of their particular internal quantum state) are repelled by regions of high electrostatic or magnetostatic energy density, respectively. Using electrostatic or magnetostatic fields, which are periodic in a plane, it is possible to construct mirrors and gratings for slowly moving atoms and molecules. The theory of such devices is presented, together with some ideas for their fabrication.     

Reactions of excited atoms and molecules with atoms and molecules

From a He-beam excited by electron impact we eliminated the He(2¹ S) component to better than 0.5% by irradiating light from a He discharge. The quenching process ishv(2¹ P→2¹ S)+He(2¹ S)→He(2¹ P)→He(1¹ S +hv) (2¹ P 1)¹ S. By measuring the ions produced in collisions of the He-metastables with various target gases in a mass spectrometer, singlet to triplet Penning-cross section ratios were obtained. These ratios are without exception close to one, which is taken as evidence for the previously proposed electron exchange mechanism of the Penning ionization. In the case that more ions are produced in the collision of He (2¹ S) and He(2³ S) with a target gas, separate relative production cross sections are obtained for the two metastables. For the rare gases the measurements are performed at two temperatures of the He-beam, 320 and 90 °K. It is found that the cross section ratio of associative — to Penning ionization increases considerably as the temperature is decreased for both, He(2¹ S) and He(2³ S), the effect being much more pronounced for He(2¹ S). The results of this work are found to confirm conclusions drawn from measured energy distributions of the electrons ejected in the Penning process.     

Reactions of excited atoms and molecules with atoms and molecules

Ionizing collisions of long lived excited particles with atoms and molecules are studied by a cross beam technique. For the first time reactions of atoms in high Rydberg states are included in the investigation. In this paper we report relative cross sections for the production of the ions RH⁺, RH ₂ ⁺ , and H ₂ ⁺ by collisions of excited rare gas atoms R^* with H₂. With HD as the target molecule the isotope effect for the production of RD⁺ and RH⁺ has been determined. In the case of argon and krypton, ions are produced only by the high Rydberg states, whereas in the case of helium and neon only the metastable states contribute to a measurable extent. The data indicate, that the reaction mechanism is different in principle for metastable and highly excited atoms. Simple models are proposed to explain the experimental results.     

Reactions of excited atoms and molecules with atoms and molecules

Penning electron distributions arising from the ionization of Na and K by He (1s 2s ^1,3 S)-metastables in thermal collisions, as well as the absolute cross section for Penning ionization of Na by He (2³ S) and relative cross sections for ionization of Na and K by He(2¹ S) and He(2³ S) are measured. It is shown that under fairly general conditions the well depth ε* of the interaction potential between the metastable and the target particle can be obtained directly from the measured electron distributions. ε*-values are reported for the moleules He(1s 2s ^1,3 S)-Na(² S), K(² S) (^2,2Ω), and for He(1s 2s ^1,3 S)-Hg(¹ S)(^1,3Ω). These latter values are obtained from previously published measurements and are to be considered preliminary. Further, additional evidence is given, that Penning ionization with metastables is an electron exchange process.     

Tunable Wigner states with dipolar atoms and molecules

We study the few-body physics of trapped atoms or molecules with electric or magnetic dipole moments aligned by an external field. Using exact numerical diagonalization appropriate for the strongly correlated regime, as well as a classical analysis, we show how Wigner localization emerges with increasing coupling strength. The Wigner states exhibit nontrivial geometries due to the anisotropy of the interaction. This leads to transitions between different Wigner states as the tilt angle of the dipoles with the confining plane is changed. Intriguingly, while the individual Wigner states are well described by a classical analysis, the transitions between different Wigner states are strongly affected by quantum statistics. This can be understood by considering the interplay between quantum-mechanical and spatial symmetry properties. Finally, we demonstrate that our results are relevant to experimentally realistic systems.     

Plasmonic atoms and plasmonic molecules

The proposed paradigm of plasmonic atoms and plasmonic molecules allows one to describe and predict the strongly localized plasmonic oscillations in the clusters of nanoparticles and some other nanostructures in uniform way. Strongly localized plasmonic molecules near the contacting surfaces might become the fundamental elements (by analogy with Lego bricks) for the construction of fully integrated opto-electronic nanodevices of any complexity and scale of integration. PACS 78.67.-n; 73.20.Mf; 32.50.+d     

Improving the specularity of magnetic mirrors for atoms

An array of anti-parallel current-carrying wires creates an inhomogeneous magnetic field capable of reflecting neutral atoms. We present analytical and numerical analyses of the magnetic field produced by such an array, and describe methods for reducing the resulting rms angular deviation from specular reflection to less than 0.1 mrad. Careful choice of cross-sectional wire profiles is shown to dramatically improve specularity of reflection close to the surface. Additionally, we find that the specularity depends on whether the number of wires in the mirror is even or odd, and that there exists an optimal turning height above the surface that maximizes the specularity of reflection from the mirror. Received 16 November 1998     

Highly spin-polarized atoms and molecules from rotationally state-selected molecules

It is shown that large nuclear polarizations in isolated molecules may be created via the hyperfine interaction following excitation to selected rotational states |JM>. Explicit time-dependent nuclear polarization expressions for pulsed rotational excitation are presented for the case of one nuclear spin I, and for the case of two nuclear spins I1 and I2 in the hierarchical coupling limit. Photodissociation of the polarized molecules allows the production of polarized atoms, on short time scales if pulsed, at densities close to the parent molecule density.     

Electron scattering by atoms and molecules

Recent advances in the scattering of electrons by free atoms and molecules are considered with an emphasis on the interface between experiment and theory. A survey is made of the theory and methods of measurement and their trend towards the perfect scattering experiment. For atomic targets a detailed coverage is given of the various theoretical models and the results compared with experimental data. For elastic and excitation collisions, the low and high energy regions are reasonably well understood. In the medium energy region, extending from a few times the ionization threshold down to about the ionization threshold there are still difficulties arising from the infinite number of open channels. For ionizing collisions, the three body Coulomb interaction still presents unanswered theoretical problems but has inspired some elegant experimental studies. For molecular targets the additional complexities arising from the nuclear motion, the lack of a centre of symmetry and the evaluation of multicentre integrals are considered. A brief discussion of some current problems and possible future developments is given.     

Isolated atoms,molecules and clusters

The basic constituents of matter, namely, atoms molecules and small clusters can be isolated in a solid rare-gas-matrix (RGM) and be characterized by Mössbauer Spectroscopy (MS). Experiments can be conducted both by the absorption method using a co-evaporation method of Mössbauer isotopes or by emission spectroscopy where source nuclei are isolated in the RGM. Experimental aspects of both techniques are outlined. Examples of applications of RGMI-MS are described, namely,⁵⁷Fe clustering with noble-metals (Cu, Ag, Au); unusual electronic states of Fe following⁵⁷Co e.c. decay; species formed by the bonding of Fe⁰, in its ground state, to small molecules are compared with those formed by Fe in its various excited states; and finally, the characterization of atomic I⁰ and Te⁰ and Te^?1 embedded in a solid argon matrix.     

Optomechanical control of atoms and molecules

We briefly review some of our recent and ongoing work on nanoscale optomechanics, an emerging area at the confluence of atomic, condensed matter and gravitational wave physics. A central tenet of optomechanics is the laser cooling of a moving mirror, typically an end mirror of a Fabry-Perot resonator, to a point near its quantum-mechanical ground state of vibration. Following a general introduction we discuss how the motion of such a macroscopic quantum oscillator can be squeezed, and then show how the placement of a ferroelectric tip on the oscillator allows the coherent manipulation and control of the center-of-mass motion of ultracold polar molecules.     

Deexcitation of Cd 53P1 atoms in collisions with ground state atoms and molecules

Collisions of excited Cd 5³P₁ atoms were investigated using atomic fluorescence spectroscopy. Cadmium vapor, together with a quenching gas, was irradiated in a quartz fluorescence vessel with Cd 3261 Å resonance radiation and the intensity of the resulting resonance fluorescence was monitored in relation to the gas pressures. The experiments yielded the following cross sections Q₁₀ (in A²) for collisional transfer 5³P₁→5³P₀: CdAr=2×10^?3, CdN₂=8.0, CdH₂=7.0, CdCO=15.6. The cross sections Q for collisional deexcitation to the ground state (quenching) in A² are CdN₂ = 2.6×10^?2, CdH₂ = 11.0, CdCO = 3.4, CdCO₂ = 26.     

Quantum theory of electromagnetic fields interacting with atoms and molecules

This paper reviews the recent achievements in nonrelativistic quantum electrodynamics, especially nonlinear and coherent phenomena. The general properties of coupled radiation and matter are presented within simple models in section 1. The following sections treat in some detail three main aspects of the system and can be read independently of each other. Section 2 discusses some experiments with long-wave-length radiation (r.f.) and atoms. Section 3 presents the quantum theory of a laser and the ensuing photon distributions. Section 4 treats the case of strongly correlated emission of radiation called superradiance. The use of statistical ensembles is briefly discussed in Appendix A, whereas Appendices B, C and D present some technical details of the text.     

Green's functions for open-shell atoms and molecules

Green's functions for atoms and molecules with a degenerate ground state are evaluated by means of tensorial analysis. The final results obtained are of simple character allowing the usual diagrammatic approach via Wick's theorem for non-degenerate ground states. As an example the ionization of a σ orbital of O₂ is considered and an equivalent approach to the frozen orbital approximation is discussed.     

Adiabatic method for positronic atoms and molecules

The adiabatic separation of electronic and positronic degrees of freedom is proposed. The positronic charge in expressions for the potential energy is partitioned between the electronic and positronic Hamiltonians. This partitioning assures an acceptable electron density distribution around the positron. Our adiabatic method gives good values of binding energies and annihilation rates for model systems, in which the positron is weakly bound. On the other hand, it does not work well if the wave function contains a significant contribution of the positronium cluster.     

Detecting electron motion in atoms and molecules

The detection of spatial and temporal electronic motion by scattering of subfemtosecond pulses of 10 keV electrons from coherent superpositions of electronic states of both H and T2(+) is investigated. For the H atom, we predict changes in the diffraction images that reflect the time-dependent effective radius of the electronic charge density. For an aligned T2(+) molecule, the diffraction image changes reflect the time-dependent localization or delocalization of the electronic charge density.     

Monte Carlo calculations of atoms and molecules

The variational and Green's function Monte Carlo (GFMC) methods can treat many interesting atomic and molecular problems. These methods can give chemical accuracy for up to 10 or so electrons. The various implementations of the GFMC method, including the domain Green's function method and the short-time approximation, are discussed. Results are presented for several representative atoms and molecules.