Dense Atomic Gas Research Articles

Producing negative permittivity via quantum coherence and local field effect in dense media

A proposal to produce negative permittivity based on the refractive index enhancement and Lorentz–Lorenz local field effect in dense atomic gases is presented. In strongly driving dense two-level atom system, the negative permittivity can be achieved by controlling atom coherence and atomic density. This method will significantly reduce the loss for the propagated optical wave and can be applied to the realization of isotropic single-negative materials in the atomic scale in the optical regime.

Erratum: Electromagnetically induced left-handedness in a dense gas of three-level atoms [Phys. Rev. A70, 053806 (2004)

Erratum: Electromagnetically induced left-handedness in a dense gas of three-level atoms [Phys. Rev. A<b>70</b>, 053806 (2004)

Mesoscopic Electromagnetic Wave Dynamics in Ultracold Atomic Gases

Ultracold atomic gases present a unique medium for studies of classical and quantum optical phenomena. Recent research on coherent multiple light scattering in such media shows a range of surprising effects, and suggests possibilities for strong localization of electromagnetic radiation in a dense and ultracold atomic gas sample.

Threshold of molecular bound state and BCS transition in dense ultracold Fermi gases with Feshbach resonance

We consider the normal state of a dense ultracold atomic Fermi gas in the presence of a Feshbach resonance. We study the BCS and the molecular instabilities and their interplay, within the framework of a recent many-body approach. We find surprisingly that, in the temperature domain where the BCS phase is present, there is a non zero lower bound for the binding energy of molecules at rest. This could give an experimental mean to show the existence of the BCS phase without observing it directly.

Electromagnetically induced left-handedness in a dense gas of three-level atoms

We discuss how a three-level system can be used to change the frequency-dependent magnetic permeability of an atomic gas to be significantly different from 1. We derive the conditions for such a scheme to be successful and briefly discuss the resulting macroscopic electrodynamics. We find that it may be possible to obtain left-handed electrodynamics for an atomic gas using three atomic levels.

Crossover to 2D in a double-evanescent wave trap

We report the preparation of a dense gas of cesium atoms at the crossover to two-dimensionality in a highly anisotropic surface trap that is realized with two evanescent light waves. Temperatures as low as 100 nK are reached with 20.000 atoms at a phase-space density close to 0.1. The lowest quantum state in the tightly confined direction is populated by more than 60%. The system offers intriguing prospects for future experiments on degenerate quantum gases in two dimensions.

Molecular Cloud Formation in Disk Galaxies: The Case of M33

We present results from the BIMA survey of 12CO(1 → 0) emission from M33. The survey resolves this emission into individual objects making this study the first unbiased, complete census of giant molecular clouds (GMCs) in a Spiral galaxy. The GMCs are clearly associated with dense atomic gas, suggesting formation from the HI. Moreover, high resolution observations of these clouds show that they appear the same as Milky Way GMCs when viewed through the same observational filters. The GMCs show significantly less angular momentum than predicted by simple formation theories, suggesting the importance of magnetohydrodynamic effects in their formation.

Long-range molecular resonances in a cold Rydberg gas.

We present evidence for molecular resonances in a cold dense gas of rubidium Rydberg atoms. Single UV photon excitation from the 5s ground state to np Rydberg states (n=50-90) reveals resonances at energies corresponding to excited atom pairs (n-1)d+ns. We attribute these normally forbidden transitions to avoided crossings between the long-range molecular potentials of two Rydberg atoms. These strong van der Waals interactions result in avoided crossings at extremely long range, e.g., approximately 58 000 times the Bohr radius (a(0)) for n=70.

On the origin of diffuse clouds

We explore the possibility that observational differences might exist between diffuse clouds which have been formed from the dissipation of unbound molecular clouds and those which have been formed from less dense atomic gas. Using single-point chemical models, we show that molecular-rich initial conditions will significantly enrich the gas-phase chemistry of diffuse clouds in the former scenario. An injection of hydrogenated grain mantles through photoevaporation will also enrich this chemistry further. If the population of interstellar diffuse clouds contains some members that are in contraction and others that are in expansion, then wide variations in atomic and molecular abundances are to be expected in this population. We predict several signatures of this effect.

Evanescent-Wave Trapping and Evaporative Cooling of an Atomic Gas at the Crossover to Two Dimensions

A dense gas of cesium atoms at the crossover to two dimensions is prepared in a highly anisotropic surface trap that is realized with two evanescent light waves. Temperatures as low as 100 nK are reached with 20,000 atoms at a phase-space density close to 0.1. The lowest quantum state in the tightly confined direction is populated by more than 60%. The system provides atoms at a mean distance from the surface as low as 1 microm, and offers intriguing prospects for future experiments on degenerate quantum gases in two dimensions.

Positronium molecule formation, Bose–Einstein condensation and stimulated annihilation

Low energy positron production and storage may be used to make and observe some interesting many-positron, many-electron systems both in vacuum and in the presence of ordinary matter. One of the most interesting possibilities is to make an annihilation photon laser. One might start on the path to this laser by creating a high density positron burst and forming a dense gas of positronium atoms within a cavity in a solid. This will pave the way for subsequently demonstrating the near room temperature Bose–Einstein condensation of a dense gas of positronium and establishing conditions under which stimulated emission of annihilation photons can be observed.

Spontaneous emission and level shifts in absorbing disordered dielectrics and dense atomic gases: A Green’s-function approach

Spontaneous emission and Lamb shift of atoms in absorbing dielectrics are discussed. A Green's-function approach is used based on the multipolar interaction Hamiltonian of a collection of atomic dipoles with the quantised radiation field. The rate of decay and level shifts are determined by the retarded Green's-function of the interacting electric displacement field, which is calculated from a Dyson equation describing multiple scattering. The positions of the atomic dipoles forming the dielectrics are assumed to be uncorrelated and a continuum approximation is used. The associated unphysical interactions between different atoms at the same location is eliminated by removing the point-interaction term from the free-space Green's-function (local field correction). For the case of an atom in a purely dispersive medium the spontaneous emission rate is altered by the well-known Lorentz local-field factor. In the presence of absorption a result different from previously suggested expressions is found and nearest-neighbour interactions are shown to be important.

Many-Body Effects in a Frozen Rydberg Gas

We studied the properties of a cold ( $\ensuremath{\sim}100\ensuremath{\mu}\mathrm{K}$) and dense ( $\ensuremath{\sim}{10}^{8--10}{\mathrm{cm}}^{\ensuremath{-}3}$) atomic Rydberg Cs gas, and found that the observed widths and shapes of resonances in population transfers cannot be explained in the framework of a usual gas model. We propose a ``frozen Rydberg gas'' model, where the interplay between two-body and many-body phenomena affects in an unexpected way the width and the shape of spectral lines.

Dipole-Dipole Broadened Line Shape in a Partially Excited Dense Atomic Gas.

We have experimentally investigated the resonance line shape of a partially excited dense atomic rubidium vapor at a density where the binary collision approximation starts to break down. The overall spectral shape is well represented by a simple expression for the dielectric function that includes an excitation-dependent linewidth and shift. The results for the excitation dependence of the linewidth agree with an estimate based on a quasistatic picture of the collisions.

Laser-dressed molecular interactions at long range

Dense and ultra-cold atomic gases are now commonly produced by laser cooling and trapping techniques. In these gases, the long-range interactions which are usually negligible become very important, and light-shifts also play an important role. In the present paper, we investigate the dominant cross-terms of long range interactions and light-shifts, and we discuss the consequences of these terms for collisions or optical lattices.

Hubble Space Telescope Observations of Planetary Nebulae in the Magellanic Clouds. IV. [O iii] Images and Evolutionary Ages

view Abstract Citations (42) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope Observations of Planetary Nebulae in the Magellanic Clouds. IV. [O iii] Images and Evolutionary Ages Dopita, M. A. ; Vassiliadis, E. ; Meatheringham, S. J. ; Bohlin, R. C. ; Ford, H. C. ; Harrington, J. P. ; Wood, P. R. ; Stecher, T. P. ; Maran, S. P. Abstract The Planetary Camera of the Hubble Space Telescope has been used to obtain images in the [O III] λ5007 line for a sample of 15 planetary nebulae in the Magellanic Clouds chosen to eliminate any selection bias in either excitation class or in flux [for log F(Hβ) > -13.7 ergs cm^-2^ s^-1^]. These images are used to derive the physical dimensions, the spatial structure, and the kinematic ages of the nebulae. The raw images were deconvolved using the Richardson-Lucy image restoration algorithm, and based on extensive tests of model images, a limit of 100 iterations of the algorithm was adopted. The restored images show clear evidence for size evolution across the H-R diagram. The younger, low-excitation, compact planetary nebulae tend to be systematically smaller than photoionization models based on ground- based data would predict, suggesting that these planetary nebulae have a central reservoir of dense atomic and molecular gas. This gas lies close to the central star and is undergoing ionization and being accelerated into outflow. Planetary nebulae previously classified as nitrogen-rich objects with massive central stars (Peimbert type I) show the bipolar "butterfly" symmetry that is also a characteristic of their Galactic counterparts. The derived kinematic ages range from less than 1000 yr up to almost 5000 yr but show little sign of systematic increase along the evolutionary tracks. The true ages of the larger objects are systematically underestimated because of acceleration of the nebular shell during its lifetime. Using both the empirical fit that we had previously derived for the expansion velocity as a function of the position on the H-R diagram, and the theoretical evolutionary tracks of the central star, we have derived two semiempirical estimates for the evolutionary timescales based upon the nebular size and the measured dynamical age. If these evolutionary timescales are to be consistent with the evolutionary age derived from theory, then He burners outnumber H burners in the approximate ratio 2:1. Publication: The Astrophysical Journal Pub Date: March 1996 DOI: 10.1086/176972 Bibcode: 1996ApJ...460..320D Keywords: GALAXIES: MAGELLANIC CLOUDS; ISM: PLANETARY NEBULAE: GENERAL; ISM: STRUCTURE; STARS: EVOLUTION full text sources ADS | data products SIMBAD (17) NED (2) MAST (1) ESA (1) Related Materials (6) Part 1: 1993ApJ...418..804D Part 2: 1994ApJ...426..150D Part 3: 1996ApJS..105..375V Part 5: 1997ApJ...474..188D Part 6: 1998ApJS..114..237V Part 7: 1998ApJ...503..253V

How Important is Neutral Carbon to an Understanding of the Dense Interstellar Medium?

In virtually all of the dense interstellar medium, H2 is the most abundant form of hydrogen, but it is not directly observable in the bulk of the gas. As a result, we are forced to use trace constituents of the gas as surrogates when we want to know the distribution of material in the dense ISM. Most commonly, we employ the lowest few rotational transitions of CO and its isotopes as the trace species. One of the most hotly debated issues in the study of the molecular ISM is the extent to which one can trust CO or isotopic CO lines to reflect reliably the underlying H2 distribution (see Shier, Rieke, &amp; Rieke (1994), Sodroski et al. (1995) for recent comments on the I(12CO)/N(H2) ratio and Lada et al. (1994) for a recent analysis of the relationship between isotopic CO and total cloud column densities). CO becomes increasingly unreliable as a tracer of H2 as the average column density between cloud surfaces exposed to ultraviolet photons and the shielded centers of clouds becomes smaller. Young stars in the galactic plane perfuse atomic and molecular clouds with far–UV (λ &gt; 91 nm) radiation. This radiation tends to dissociate CO more readily than it dissociates H2 (Van Dishoeck &amp; Black 1988). The differences in susceptibility of H2 and CO to photodissociation may lead to the existence of significant portions of the molecular medium where the usual trace species are underabundant or even absent. In addition, there is dense H I at the cloud boundaries, immediately outside the molecular material. In the UV–illuminated cloud surfaces, the gas-phase carbon is in the form of C I or C II. It is important, therefore, to determine the amount and location of large-scale C I emission if one hopes to know how much molecular and dense atomic gas is missing from studies using CO as a tracer and to what extent photodissociation is responsible for the absence of this CO. We discuss here some of the relevant theoretical and observational work on the relationship between C II, C I, CO and H2. Our principal aim is to see if and how observations of C I might help us to improve our knowledge of the distribution of dense neutral gas in the Milky Way.

Onset of a collisional modification of the Faraday effect in a high-density atomic gas.

We investigate, theoretically and experimentally, the onset of modified electronic precession due to quasimolecular behavior in dense atomic gases. The close connection between the angular coupling of collision pairs and a modification of the Faraday effect is pointed out. We have observed the onset of this quasimolecular modification of the atomic Faraday effect with high-resolution laser measurements of the absorption and Faraday spectra near the ${\mathit{D}}_{2}$ line of Rb atoms immersed in high-density buffer gases. The atomic Faraday effect itself is also affected by the presence of nuclear spin; this modifies the electronic precession through angular-momentum coupling. We describe how to separate the effect of nuclear spin from the effects due to quasimolecular behavior. In this way we establish the existence of angular coupling of the Rb valence electrons to neighboring buffer-gas atoms at densities above \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ in Xe buffer gas. In He buffer gas, quasimolecular modifications of the Faraday effect are approximately two times smaller than for Xe.

Near-infrared spectroscopic imaging of the circumnuclear environment of NGC 1068

view Abstract Citations (78) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Near-Infrared Spectroscopic Imaging of the Circumnuclear Environment of NGC 1068 Blietz, M. ; Cameron, M. ; Drapatz, S. ; Genzel, R. ; Krabbe, A. ; van der Werf, P. ; Sternberg, A. ; Ward, M. Abstract We report <= 1" resolution imaging of the 1.6435 micron [Fe II] and 2.1212 micron H_2_ ν = 1-0S(1) lines toward the nucleus of the Seyfert 2 galaxy NGC 1068. Our observations suggest that the near-infrared [Fe II] and H_2_ emission, as well as the radio continuum and optical line emission observed toward the narrow-line region (NLR), arises at the interface between the nuclear outflow/radiation and dense (n(H_2_)> 10^4^ cm^-3^) circumnuclear molecular clouds. The [Fe II] emission extends over >= 8", is elongated along P.A.~35^deg^ and tracks the NLR and the central collimated part of the twin radio jet. The [Fe II] emission may be produced in gas exposed to nuclear X-ray radiation or in gas which is interacting with the outflow/jet from the active nucleus. We also find that the abundance of iron appears to be enhanced along the jet axis. We propose that this enhancement may be due to efficient grain destruction by the hard nuclear radiation or fast J-shocks produced in the outflow and/or due to an unusually high value of the total fractional abundance of iron compared to solar. When leaving the molecular disk, the outflow/jet enters less dense atomic gas at a much lower pressure which causes the jet to widen, seen both in the [Fe II] and radio emission. Alternatively, and less likely, the [Fe II] emission may be emitted in gas shocked by supernova remnants. In contrast, the H_2_ emission is concentrated at the nucleus and extends ~5" (340 pc) east-west, with several embedded knots which probably represent concentrations of dense, massive molecular clouds. The high signal-to-noise quality of these new data have enabled us to ascertain that the warm molecular gas is not symmetrically distributed about the near-infrared continuum peak as was previously suspected. Rather, the brightest H_2_ knot is centered 0.2" +/- 0.3" southwest of the near-infrared continuum peak and, due to a total molecular hydrogen column density likely exceeding 10^23^ cm^-2^, may contribute significantly to the obscuration of the broad-line region in the nucleus of NGC 1068. Publication: The Astrophysical Journal Pub Date: January 1994 DOI: 10.1086/173628 Bibcode: 1994ApJ...421...92B Keywords: Astronomical Spectroscopy; Infrared Spectroscopy; Near Infrared Radiation; Nuclear Radiation; Radio Emission; Radio Jets (Astronomy); Seyfert Galaxies; Stellar Radiation; Stellar Spectra; Hydrogen; Iron; Stellar Physics; X Rays; Astrophysics; GALAXIES: ACTIVE; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1068; GALAXIES: ISM; GALAXIES: NUCLEI; GALAXIES: SEYFERT; INFRARED: GALAXIES full text sources ADS | data products SIMBAD (1) NED (1)

Dissociation energy of molecules in dense gases.

A general approach is presented for calculating the reduction of the dissociation energy of diatomic molecules immersed in a dense (n = less than 10 exp 22/cu cm) gas of molecules and atoms. The dissociation energy of a molecule in a dense gas differs from that of the molecule in vacuum because the intermolecular forces change the intramolecular dynamics of the molecule, and, consequently, the energy of the molecular bond.