Unexcited Atoms Research Articles

Dicke model: quantum nonlinear dynamics and collective phenomena

We study the resonant interaction of A initially unexcited two-level atoms with single-mode Fock, coherent and thermal cavity fields of arbitrary intensity. Two mechanisms lead to a spread of the eigenfrequencies: one is due to the photon-number distribution in the initial state and the other reflects the cooperativity of the system. Both affect the evolution of the photon number and photon statistics. We present approximate expressions for the eigenvalues and eigenvectors of the system. Its regular dynamical regions are found and the difference in shape of the collapses and revivals connected with the above mechanisms is discussed.

Determination of Ionization Cross Section for Argon Metastable-Metastable Collision by Means of Afterglow Technique

Subtracting the energy loss rates of electrons from the decay rate of electron temperature in an argon afterglow at pressure 133 Pa (Ar), we have separated the energy gain of electrons, which was assumed to be due to superelastic collisions and energetic electrons created by the metastable-metastable ionizing collisions. The cross section obtained has a value between (1.30±0.11)×10-17 m2 and (1.40±0.12)×10-17 m2, which is greater than the cross section for collisions between unexcited argon atoms.

Stimulated resonance scattering of light waves in laser crystals with a population inversion

A new type of stimulated scattering of light is discovered in a Nd:YAG laser crystal with a population inversion. The scattering occurs for light at the resonance frequency of the laser transition, with a small anti-Stokes shift. It is shown that the effect is due to the appearance of a wave of population of the metastable level and the associated traveling index grating in a laser medium with different polarizability of the excited and unexcited atoms.

Satellites on Lyman Alpha due to H-H and H-H[TSUP]+[/TSUP] Collisions

We have experimentally observed satellites on Lyman-α arising from radiative collisions of excited atomic hydrogen with unexcited neutral hydrogen atoms or protons. The wavelength, relative intensity, and shape of features in the Lyman-α wing emitted by a laser-produced plasma in pure hydrogen agree with new theoretical calculations based on ab initio H2+ and H2 potentials. These results confirm the identification of satellites observed with IUE and HST in the ultraviolet spectra of hydrogen-rich white dwarfs, and verify the accuracy of the spectral line shape theory used to determine their effective temperatures and surface gravities.

Spontaneous emission near the edge of a photonic band gap.

We study spontaneous emission near the edge of a photonic band gap. Instead of a simple exponential decay in the vacuum, spontaneous emission displays an oscillatory behavior. A single photon-atom bound dressed state exhibits a fractional steady-state atomic population on the excited state. For a three-level atom we evaluate the spectral splitting and subnatural linewidth of spontaneous emission. In the presence of N-1 unexcited atoms we show that the collective time scale factor is equal to ${\mathit{N}}^{\mathrm{\ensuremath{\varphi}}}$, where \ensuremath{\varphi}=2/3 for an isotropic band gap and \ensuremath{\varphi}=1 or 2 for anisotropic two-dimensional or three-dimensional band edges, respectively.

Analytical approach to the photon statistics in the thermal Jaynes-Cummings model with an initially unexcited atom.

Simple analytical expressions for the time evolution of the mean photon number and its normally ordered variance for the case of an initially thermal cavity field coupled to an initially unexcited two-level atom are proposed. The possibility of appearance of sub-Poissonian field statistics is discussed.

Interaction of a System of Initially Unexcited Two-level Atoms with a Weak Cavity Field

Abstract Using a perturbation method, constructed in terms of SU(2) group representations, the interaction of N initially unexcited two-level atoms and a weak single-mode cavity field is studied. The field is assumed to be initially either in a Fock state with a number of photons equal to n or in a coherent state. In the case of the photon-number state with n  3, the pure phenomenon of collective collapses and revivals manifests itself. For the initially coherent field the phenomenon of collapses and revivals arising from the photon number distribution mechanism is additionally modulated by this collective mechanism. The problem of the interaction of excited atoms with an initially coherent field has already been solved numerically by Barnett and Knight. For n=1 2 and 3 the approximate solution is compared with the exact solutions also given in this paper and the limit of applicability of our approach is established.

One- and two-photon Jaynes-Cummings models as sources of sub-Poissonian fields by chaotic pumping.

The multiphoton Jaynes-Cummings model with an initially unexcited two-level atom is considered in the context of its ability to produce a sub-Poissonian field in interaction with an initially chaotic (thermal) cavity field.

Interference of reactive components of an electromagnetic field

This article discusses the formation of an interference energy flux as a result of changing the phase shift between the oscillations of the reactive components of the vectors of the electric and magnetic field intensities of radiation when spatially superposed. On the basis of a model of the interference of the reactive components of the electromagnetic field from a common physical standpoint, the passage of light through a transparent plane-parallel plate at an angle of incidence exceeding the critical angle of total internal reflection, the formation of a refracted wave in the region of total internal reflection from a semiinfinite medium, and the radiationless transport of energy between excited and unexcited atoms are described.

Optical activity induced by rotation of atomic spin

Physical systems or processes invariant under mirror reflection are not expected to exhibit intrinsically chiral asymmetries. Thus, it is ordinarily the case that unbounded and unexcited atoms (in the absence of weak nuclear interactions) should not manifest circular birefringence. It is shown, however, that in a rotating reference frame the coupling of atomic spin angular momentum to the angular velocity of the rest frame gives to optical activity even in the absence of contributions from nuclear interactions and the classical Coriolis force. Ground-state atomic hydrogen is predicted to be optically active in the microwave region near the hyperfine splitting frequency of 1420 MHz. Under appropriate circumstances, this spin-associated rotational circular birefringence can be some ten orders of magnitude larger than that previously estimated for virtual electronic transitions falling within the visible and UV portions of the spectrum.

Dynamics of an excited two-level atom in the presence of N-1 unexcited atoms in the free space.

The dynamics of the system of N two-level atoms in the free space is studied, when at the initial moment just one of the atoms is excited. It is shown that though the number of the field modes is sufficiently larger than the number of the atoms and all energy is transferred into radiation field for t\ensuremath{\rightarrow}\ensuremath{\infty}, the radiation suppression (in a broader sense) can be still observed.

Slip of a gas in a field of optical radiation

A new mechanism of surface light-induced drift associated with the difference between the collision frequencies of excited and unexcited atoms is predicted.

On spontaneous emission

The problem of the spontaneous emission from a single excited two-level atom in the presence of N unexcited similar atoms is discussed. A formalism is presented for evaluation of the time-development operator (and thus the density matrix) which has certain desirable features for time-dependent problems in the area, e.g. of atom-field interactions, when the off-diagonal elements are large compared to the diagonal elements of the Hamiltonian. By use of this formalism, it is shown that the spontaneous emission from a single atom is often drastically altered in the presence of unexcited atoms with similar resonances. When the number of atoms present becomes very large relative to the number of modes of the electromagnetic field, the excited atom becomes trapped in its excited state. >

Spontaneous emission from a single two-level atom in the presence of N initially unexcited identical atoms.

The quantum-electrodynamical problem of N identical two-level atoms interacting with M field modes is considered, and an exact solution for the case of spontaneous emission from a single atom is obtained. The behavior of this system is shown to be quite different from the exponential decay of a single excited atom radiating into free space. A ``ringing'' behavior at the enigmatic frequency \ensuremath{\sim} \ensuremath{\surd}N which occurs when only one mode is accessible to the field persists when many modes are present. When a continuum of modes is accessible, the long-time limit shows that the energy of the originally excited atom is equally likely to be shared by the N-1 originally unexcited atoms and the electromagnetic field when M/N\ensuremath{\rightarrow}1. ``Radiation suppression'' occurs whenever N\ensuremath{\gg}1 and M\ensuremath{\ll}N, when the originally excited atom does not emit its energy. When M=N\ensuremath{\gg}1, the single atom decays monotonically to zero (and approximately exponentially) but with a decay rate that is \ensuremath{\surd}N times the Rabi frequency. Such unexpected effects are presently within experimental range.

Spontaneous emission from an excited atom in the presence of N atoms and M modes.

The long-time behavior of the number of photons in a mode of the electromagnetic field is given in closed form in terms of elementary functions when there is initially one (two-level) atom excited in the presence of $N$ unexcited identical atoms and $M$ field modes. The radiation trapping which occurs with only one mode disappears when the number of modes $M$ approaches the total number of atoms in the continuum mode approximation.

ELECTRON-ATOM-PHOTON INTERACTIONS IN A LASER FIELD

We are currently studying the scattering of low and intermediate energy electrons by sodium atoms in a laser field. The experimental method is discussed, with particular attention to atomic after resonant photon absorption. Total scattering cross sections by 3 2 ~ sodium atoms are presented. In this paper we will discuss the experiments being conducted in the New York University Atomic Beams Laboratory, which involve the scattering of low and intermediate energ-j electrons by alkali atoms in ground and laser-excited states. The principal goals of this research are, 1) to effect as a comparison of ab-initio calculations for simple few-body systems with experiment as possible, 2) to reduce the collision experiment to a determination of separate observables, e.g., direct and exchange scattering amplitudes, selected angular momentum orientations, relative phases of scattering matrix elements, etc., using state selection and analysis, 3) to obtain reliable absolute cross sections for these, without requiring normalization to theory or other experiments, 4) to achieve sufficient redundancy in these state-sensitive cross section measurements to overdetermine scattering amplitudes, permitting detailed scrutiny of timeindependent collision theory, 5) to use the laser as a tool in state-preparation and analysis for both fine and hyperfine states, as well as short-lived electronically excited states, 6) to look for direct intense-field effects on electron-atom collisions, and finally, 7) to study the atom-laser interaction itself. From the point of view of this work, therefore, electron-atom scattering in the presence of laser interactions can be considered to be a natural extension of field-free scattering, which makes it possible to perform better collision experiments, but which do not add any intrinsically new physical processes to the two-body scattering problem. In 6) above, this of course changes; however generally speaking very strong fields are required. On the other hand, even when the laser field is not strong, it is necessary to understand the laser-atom interaction in order to obtain absolute excited-state cross sections. This last point will be discussed more fully later. We will touch on all six items noted above in the present paper, concentrating of course on items 6, 7 and we will attempt to show how they relate to one another. We consider one-electron atoms, such as the alkalis or atomic hydrogen, and refer the reader to the literature for discussing of collision theory and details of previous work1. For electron scattering by alkali atoms, close coupling calculations have been extremely successful in predicting behavior of ground-state Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985123 C1-242 JOURNAL DE PHYSIQUE csllision cross sections. This is in part due, of course, to the relatively simple nature of the interaction, in which electron-correlation only plays a role through (cooperative) core polarization effects. In addition, because of the large (and spherical) core shielding, close-coupling convergence is extremely rapid, particularly with respect to optically-connected electric dipole-type matrix elements. Accordingly these systems can serve as stringent test cases for comparison of theory and experiment. The experimental work described here has been performed using variations of the atomic beam recoil technique2, wherein observation of the scattering is made on the heavy partner, i.e. the atom, rather than the electron. The method possesses the intrinsic advantages that (1) it is possible to directly observe change in the atomic magnetic quantum number, caused either by spin-exchange or by angular momentum projection changes; (2) absolute values of cross sections can be obtained, without need for knowledge of the atomic beam density; ( 3 ) laser-excited atoms can be spatially separated, using photon-recoil, from unexcited atoms enabling determination of absolute excited-state cross sections. The work discussed herein was performed on a new atomic beams apparatus, which is described briefly below. Mutually orthogonal rectangular electron, atom and laser beams intersect in an interaction volume; the (ground-state) atom beam is velocityand hyperfine-state-selected by a tunable hexapole electromagnet. It is recoil-scattered by both electron and laser beams. After travelling through a field-free drift regionu 3 m long its spatial dispersion is analyzed by a surface ionization detector of square cross sectional area, capable of displacement in two dimensions. Fig. 1 shows a schematic view of the experimental setup. SINGLE-MODE RING DYE

Comparison of ionization potentials of unexcited atoms and ions calculated by different methods

A model differential equation relating the vertical ionization potentials of unexcited atoms and ions of elements is obtained; its solution establishes the relation between these quantities and the spectroscopic index of the electron expressed by an exponential. On the basis of this relation, the ionization potentials calculated in various works are compared. Using the model equation proposed here, it is possible to calculate unknown and refine known ionization potential values.

On coherence in spontaneous emission

The case of a single excited two-level atom emitting spontaneously in the presence of N unexcited atoms is solved exactly for emission into a single electromagnetic mode. The two-level atoms are in inequivalent mode positions.

Reactions of excited I*(2P1/2) and unexcited I(2P3/2) iodine atoms in perfluoroalkyl radicals

A method of investigating reactions of excited and unexcited atoms is discussed. It is based on pulsed photolysis of molecules with simultaneous passage of laser radiation through the working medium. The method proposed is used to investigate the reactions that accompany the photolysis of the molecules RI(CF3I, n-C3F7I, i-C3F7I). The rate constants of the recombination of iodine atoms into I2 in the presence of RI molecules are calculated for the atoms I(2P3/2) and I*(2P1/2), as are the recombination constants of the radicals R into R2 and with the atoms I*(2P1/2) and I(2P3/2) into the RI molecule. It is shown that the I(2P3/2) atoms are much more active in the recombination into Ia and RI than the I*(2P1/2) atoms. The role of the investigated reactions in the kinetics of a photodissociation iodine laser (PDIL) is discussed. The results are compared with the published data.

Laboratory study of the PDX/PLT laser blow-off trace element injectors

A laboratory study of the beams produced by laser blow-off trace element injectors used on the PDX and PLT tokamaks has been conducted. Eight target materials were studied, in the form of thin films on glass substrates: Al, Si, Sc, Cr, Fe, Ni, Cu, and Mo. Beam detectors included a quadrupole mass spectrometer, gridded retarding field analyzer, and beam collector plates. The collector plates were later analyzed by optical and electron microscopy and Auger spectroscopy. The results show that the injection produces the following species, in the usual order of decreasing velocity: free electrons, plasma, free ions, highly excited neutral atoms, unexcited neutral atoms, and metal clusters having diameters ranging up to several microns. The percentages of the various species, as well as their distributions of velocities, were found to be strong functions of the metal used, the film thickness, and the laser power density. Monoatomic species are generally contained within a half angle of ∠12°, while clusters are within ∠5°. Models of the production mechanism and implications for the use of the device as a tokamak diagnostic are discussed.