Unexcited Atoms Research Articles

Control of the Purcell effect via unexcited atoms and exceptional points

Control of the Purcell effect via unexcited atoms and exceptional points

A hybrid mechanism of 3D optical trapping of particles immersed in a cold buffer gas

A novel mechanism of the formation of a deep optical superlattice (OSL) for three-dimensional (3D) trapping of resonant impurity atoms immersed in a transparent cold buffer gas is proposed and investigated theoretically. This mechanism is associated with the joint action of two factors of different physical nature: rectification effect of the gradient force of light pressure and effect of the light induced drift (LID) of atoms (a consequence of the difference in gas-kinetic cross sections of excited and unexcited atoms). Such a situation can be realised when a gas mixture is irradiated by a bichromatic combined light beam which is a special coaxial superposition of cosine-Gaussian optical beams. Here, the RcGF creates a periodic 1D array of deep potential wells (OSL) along the beam axes, and an effective force, causing LID, pulls resonant particles into the combined light beam and provides their transverse (with respect to the beam axes) confinement in OSL. As a result, there can occur a giant accumulation of impurity particles in the OSL cells, accompanied by a strong spatial particle localization far away from the walls of the reservoir, in which the gas mixture is located.

Magnetic-field control of subradiance states of a system of two atoms

A method is proposed for the creation of an entangled metastable (subradiance) excited state in a system of two closely spaced identical atoms. The system of unexcited atoms is first placed in a magnetic field that is directed at a magic angle of $${\alpha _0} = {\text{arccos}}\left( {1/\sqrt 3 } \right) \approx 54.7^\circ $$ to the line connecting the atoms and has a transverse gradient. The gradient of the field results in the detuning of frequencies of an optical transition of the atoms. Then, the resonant laser excitation of an atom with a higher transition frequency is performed with the subsequent adiabatic switching-off of the gradient of the magnetic field. It is shown that the excited atomic system in this case transits with overwhelming probability to an entangled subradiance state. Requirements on the spectroscopic parameters of the transitions and on the rate of varying the gradient of the magnetic field necessary for the implementation of this effect are analyzed.

Off-resonance regimes in nonlinear quantum Rabi models

We study the nonlinear quantum Rabi model—both in the two-photon and the intensity-dependent coupling scheme—by utilizing the perturbation method in the off- and near-resonance regimes characterized by the field and the atomic transition frequencies. We consider the weak-coupling condition, the perturbation parameter being the ratio between the coupling constant and the atomic frequency. For both models we determine the first- and second-order corrections to energy eigenvalues and eigenstates, disclosing the doublet structure characterizing the energy spectrum. Then we apply our findings to both the coupling schemes in order to calculate the time evolution of a general initial state and the atomic population inversion for an initial state composed by a mixture of excited and unexcited atoms. [Phys. Rev. A 93 (2016) 043814]

Development of multiple-wavelength oscillation in plasma of a pulse-periodic He + Ne + Sr laser

Mechanisms of multiple-wavelength oscillation at the lines of He I, Ne I, Sr I, and Sr II ions in the active medium of a high-voltage pulse-periodic laser based on a He + Ne + Sr mixture at a total pressure of ∼200 Torr and duration of the current pulse of about 150–300 ns are analyzed. An important role played by collisional thermal mixing of the laser levels of Ne I and Sr I in multiplets, as well as by collisions of the second kind between metastable and unexcited atoms, in populating and depopulating atomic levels is demonstrated. Absolute populations of the upper and lower levels of the laser transitions in Ne I are found. It is discovered that the degree of ionization nonequilibrium of plasma, both during and after the pulse of current, determines the mechanism of population inversion in laser transitions.

Illuminance of FL Tubes Controlled by Depths of Gap Between Positive Column and Phosphor Screen

Since the opaque phosphor screens cover up the inside of the FL tubes, the presence of the depths of the gap between positive column and phosphor screen has ever reported in the study on the lighted FL tubes since the invention of the FL tube. It has found that the depths of the gap in the lighted FL tubes conceal the unrivaled advantage of the FL tubes over the solid lighting devices. The invisible depths of the gap are non-distractively and quantitatively determined by the measurements of the build-up curve of the light intensities from the lighted FL tubes in the Ulbricht Sphere. The determined depth of the gap of the selected commercial 40W-HCFL tube as the best one was 3 x 10-3 m. The depths of the gap are markedly changed with the FL producers, indicating no quality control of the gaps. The unexcited Hg atoms in the gap act as the optical filter for the UV lights from the positive column before reaching to the phosphor screen. The results suggest us that if the depth of the gap reduces to 3 x 10-4 m, the illuminance (lm m-2) of the present 40W-HCFL tube will go up to 2.5 times. The systematic study on the gaps of the lighted FL tube remains for a future study.

Special Arrangement of Phosphor Particles in Screen for Optimization of Illuminance (lm m<sup>-2</sup>) of FL Tubes

The performance of lighted FL tubes is severely influenced by the depth of the gap between phosphor screen on inner glass wall and positive column which is defined by Fvect ≥ Fphos. Fphos is vertical electric field of the surface bound electrons (SBE) on electric insulator in vacuum. The SBE on phosphor particles in the screen of the commercial FL tubes pushes back approaching electrons from phosphor screen to positive column. Naturally, there is the gap between positive column and phosphor screen. The depths of the gap ever study on the lighted FL tubes quantitatively. The depth by the gap by SBE is 3 x 10-3 m that gives rise to the slow build - up curve of illuminance from FL tube. Unexcited Hg atoms in the gap severely control the illuminance (lm m-2) of FL tubes. The reliable FL tubes should have the depth of the gap less than 2 x 10-4 m. The formation of the narrow gap requires the special arrangement of (a) the low voltage CL phosphor particles and (b) PL phosphor particles side by side. The coil-EEFL tubes in the narrow gap allow the Ar gas pressures (>7 x 103 Pa) for the high illuminance (>103 lm m-2) with nearly zero power consumption by the DC operation.

Flame Atomic Absorption Spectrometry Based on Self-absorption in the Flame and Using the Flame as a Light Emission Source

A method of flame ato mic absorption analysis has been developed which does not need a light source, such as hollow-cathode lamp, wh ich to produce the radiation absorbed by the analyte atoms. Light emission fro m the analyzed atoms in the flame can serve as a light source because this emission has the same wave length as the resonance absorption line of the unexcited analyte atoms. At certain conditions, in the flame can take place an absorption process known as self-absorption. In the work self-absorption occurring in the flame is used to determine absorbance. Absorbance is calcu- lated fro m the flame emission intensity signal of the analyte atoms. A co mputer program is specially created to calculate absorbance, to draw the calibration curve and to co mpute the analyte concentration. For safety purposes ethyl alcohol is used as a flame fuel and is applied as an alcohol vapor / air flame. Because this flame has low temperature, only lithiu m, potassium, sodium, calciu m, bariu m is able to be analyzed.

UV dominant optical emission newly detected from radioisotopes and XRF sources

The current paper reports first and definite experimental evidence for γ-, X-, or β radiation causing UV dominant optical radiation from (1) radiochemicals such as 131I; (2) XRF sources such as Rb XRF source present as salts; and (3) metal sources such as 57Co, and Cu XRF sources. Due to low quantum yield a need arose to develop two techniques with narrow band optical filters, and sheet polarizers that helped in the successful detection of optical radiation. The metal 57Co spectrum observed at room temperature hinted that it could be optical emission from excited 57Co atoms by a previously unknown phenomenon. In order to explain UV emission, it was predicted that some eV energies higher than that of UV, termed temporarily as Bharat radiation are generated within the excited atom, while γ-, X-, or β radiation passes through core-Coulomb field. In turn, the Bharat energy internally produced within the excited atom causes UV dominant high-energy spectrum by valence excitation. As excited atoms become free from surrounding unexcited atoms by valence excitation, room temperature atomic spectra of solid radioisotopes and XRF sources became a possibility. It implies existence of temporary atomic state of solids. The experimental evidence that γ-, X-, and β radiations causing UV dominant optical emission from within excited atoms of radioisotopes suggests the possibility for solar γ-, X-, and β radiations causing EUV by the atomic phenomenon described here.

Discrete photodetection of quantum jumps on the V configuration of atomic levels

A theory of the discrete photodetection of quantum jumps on the V configuration of atomic levels has been developed. A three-level source atom is placed in a cavity excited by a resonance fluorescence field. The cavity is tuned to exact resonance with an atomic transition. The cavity mode state is tested by a flux of unexcited (at the entrance) probe atoms passing through the cavity. The energy states of the outgoing probe atoms are detected by ionization chambers, which are assumed ideal. This a posteriori statistical information is indirectly related to the numerical characteristics of a measured quantum system consisting of the source atom and cavity mode. The “tuning” conditions for a discrete photodetector, i.e., the rules for choosing the parameters and durations of the interactions of the cavity mode with the probe and source atoms, intensities of the pump and probe fields that are necessary for observing quantum jumps from the “bright” state to the “dark” one and vice versa, have been determined. A two-state model that describes the dynamics of a quantum jump has been analyzed. The formulas have been obtained for the observable characteristics of quantum jumps: the mean residence time of the quantum system in quasistationary states (durations of the bright and dark periods), probabilities of quantum jumps, mean excitation levels of the quantized cavity mode, etc.

Collisional effects in the far red wing of Lyman-$\sf \alpha$

Context. An accurate determination of the line broadening of the Lyman series of atomic hydrogen has been shown to be fundamental to interpretating UV and FUV spectra of DA white dwarfs. Quasi-molecular lines have been detected in the red wing of Lyman-α, Lyman-β, and Lyman-γ. They arise from radiative collisions of excited atomic hydrogen with unexcited neutral hydrogen atoms or protons. Aims. The aim of this paper is twofold. First, we examine the range of validity of the one-perturber approximation widely used to calculate the line wing. Second, we study the relative contributions of the two main transitions contributing to the far wing of the Lyman-α line profile according to the effective temperature and perturber density. Methods. In cool white dwarfs, the perturber density is so high that the one-perturber approximation breaks down and the collisional effects must be treated by using the autocorrelation formalism in order to take into account simultaneous collisions with more than one perturbing atom. Results. We show that, at the low temperatures of cool white dwarfs, the contribution of the singlet X 1 Σ + → B 1 Σ + transition cannot be neglected in the calculation of Lyman-α profile perturbed by neutral hydrogen. A comparison with experimental laboratory spectra shows that the effects of multiple H-perturbers appear in the far wing. Conclusions. A reliable determination of the line profiles for the physical conditions of cool white dwarfs requires a unified theory that takes account of both the singlet and triplet transitions contributing to Lyman-α using accurate interaction potentials and radiative dipole transition moments. Multiple perturber effects must be considered and the autocorrelation formalism permits calculations for the densities of the atmospheres of cool white dwarfs.

Measuring statistical characteristics of a quantized mode in various photodetection regimes

We develop a theory of the discrete photodetection model where individual unexcited atoms from the atomic beam passing through a cavity are used as pointers. Atomic energy state selective detectors (ionization chambers) measure the escape statistics of ground-or excited-state atoms. Mandelstam’s method of successive indirect quantum measurements underlies the theory being developed. In contrast to the available works, the suggested approach takes into account the atomic beam statistics, the operation nonideality (quantum efficiencies) of ionization chambers, the detector recovery time, the back action of the probe on the quantized mode, and the relaxation process in the time interval when the cavity is empty (without atoms). We derive formulas that relate the a posteriori probabilities of photodetector clicks to the elements of the main diagonal of the initial density matrix for the cavity mode. Our numerical simulations confirm that the inverse problem of reconstructing the initial photon number distribution in a cavity can be solved using the statistics of detector clicks measured during the transient process.

Nonradiative transition to the ground state and superelastic scattering of exited atoms upon impact on the surface of wide-bandgap dielectrics

The impact of excited cesium atoms on sapphire and glass surfaces have been experimentally studied. It is established that the probability of electron excitation quenching upon impact of an atom on the dielectric surface is close to unity. The velocity distribution of unexcited atoms upon scattering from the surface has been determined using the time-of-flight technique. The kinetic energies of most of these atoms are several tens of times greater than the energy of thermal motion of the excited atoms impinging on the surface. Conversion of the internal energy of atoms into their kinetic energy is explained in terms of nonradiative electron transitions with simultaneous excitation of lattice vibrations in the dielectric crystal. This mechanism of atomic excitation quenching near the dielectric surface explains the significant difference between the energies of atoms upon superelastic scattering and upon photodesorption from an adsorbed state.

On the mechanism of formation of helium molecules in a low-temperature plasma

The glow of He2 molecules in a low-pressure (P ≤ 2 Torr) helium plasma excited by a monochromatic electron beam is detected and studied. The experimental data point to the existence of two basic mechanisms of formation of an excited He2 molecule, namely, the collision of an excited helium atom with two unexcited atoms and the recombination of a molecular ion.

Time evolution of the Rabi Hamiltonian from the unexcited vacuum

The Rabi Hamiltonian describes a single mode of electromagnetic radiation interacting with a two-level atom. Using the coupled cluster method, we investigate the time evolution of this system from an initially empty field mode and an unexcited atom. We give results for the atomic inversion and field occupation, and find that the virtual processes cause the field to be squeezed. No anti-bunching occurs.

Electron energy distribution function in a moderately ionized argonplasma

A kinetic model is proposed, allowing us to describe the kineticsof the electronic components of a non-equilibrium low-temperature argonplasma over a wide range of values of the degrees of the ionization. Themodel incorporates a large number of electronic states of argon atoms (upto n~30) and takes into account the processes of electron elasticscattering from neutral argon atoms and ions, electronic excitation anddeexcitation by electron impact, the processes of ionization of atoms incollisions with electrons and unexcited atoms and mutually reverseprocesses of electron-ion recombination, reactions of dissociativerecombination and electron-impact dissociation of molecular ions. Theresults of calculations performed in the framework of the proposed modelhave shown that the electron energy distribution in a high-pressure plasmamay conserve a non-Maxwellian character up to rather high values of thedegree of ionization (ne/N~3×10-3), principally followingthe ionization and recombination processes initiated by atomic impact.

Spontaneous emission from Bloch states of a two-level atom in a periodic field of the biparabolic shape

By analogy with the Wigner-Weisskopf model, spontaneous emission of an atom is considered in a spatially periodic field of the resonance counter-propagating waves. The Bloch state obtained by the interaction of the initially excited (unexcited) atom with a field of the counter-propagating waves plays the role of the excited (ground) state of the free atom. The spontaneous emission probability averaged over polarizations is shown to become anisotropic, with the symmetry axis directed along the wave propagation path. The anisotropy is caused by the spatially periodic distribution of the wave function of the translational motion of an atom in the external field. The degree of anisotropy depends on the position of Bloch energy levels in the allowed bands of the energy spectrum.

High-efficiency Zn isotope separation in a photochemical reaction induced by two-photon excitation

High-efficiency laser isotope separation was accomplished for zinc atoms in the situation when isotope shifts are much smaller than the Doppler width. The atoms were excited via two-photon absorption of two counter-propagating waves slightly detuned from the intermediate state, so that all “0requisite” atoms were excited. The isotope separation results from a photochemical reaction between the selectively excited zinc isotope and the CO2 molecules. The reaction rate is 3–5 orders of magnitude higher than for the unexcited atoms. The mechanisms of two-photon resonance line broadening are considered both theoretically and experimentally, and the possibilities of the method for obtaining isotopes in quantitative yields are estimated.

Laser detection of optical signals

A system of unexcited three-level atoms, located in a nonresonant laser field, was examined. One of the excited levels in the system becomes populated under the influence of a weak optical signal, as a result of which the atoms resonate with the laser field. A consequence of this is intense scattering of the laser radiation and particularly excitation of the hitherto unexcited modes of the cavity in which the system of atoms is located. The system considered thus possesses states which are specially sensitive to external agencies and therefore sensitive optical-radiation detectors can be based on these states.

Anomalous light-induced drift of lithium atoms in a mixture of noble gases

This paper is a theoretical study of the spectral features of the velocity of light-induced drift (LID) of lithium atoms (7Li and 6Li) in a binary mixture of noble gases: Ne + Ar, Ne + Kr, and Ne + Xe. The spectral shape of the LID signal is predicted to depend strongly on the fraction ξ of neon in the buffer mixture in the range ξ≈0.8–0.9 (ξ=NNe/Nb, where NNe is the neon concentration, and Nb is the total concentration of the buffer particles). When the velocity of anomalous LID is treated as a function of the radiation frequency, it is found to have one, three, five, or seven zeros and to differ substantially from the dispersion-curve-like behavior with one zero predicted by the standard LID theory with velocity-independent transport collision rates. The reason for these additional zeros of the drift velocity is the alternating-sign dependence on the lithium-atom velocity of the relative difference of transport rates of collisions between buffer particles and excited and unexcited atoms. What is also established is that the anomalous LID of lithium atoms can be observed at almost all temperatures, depending on the value of ξ. At a fixed temperature, anomalous LID can be observed only in a narrow range of values of the fraction of neon in the buffer mixture, Δξ≈0.02. The results make possible highly precise testing in the LID experiments of the interatomic potentials used in calculations of the velocity spectrum of anomalous LID.