Resonance Radiation Trapping Research Articles

Transfer and Trapping of Resonance Radiation in a Two-Level System

The population kinetics of a two-level system has been theoretically investigated taking into account the transfer of spontaneous radiation under continuous optical pumping. It is shown that, when the photon mean free path in a resonantly absorbing medium is smaller than the characteristic size of this medium or comparable to it, the spontaneous radiation may become an additional pump source. The contribution of spontaneous photons to the kinetics of level has been considered within two approaches. The first approach consists of solution of the radiation transfer equation within the diffusion approximation. The second approach is related to solution of the Biberman–Holstein equation.

Doppler Frequency Redistribution upon Coherent Photon Emission by Atoms in an Optically Dense Medium

The problem of transfer of line radiation in sodium vapor under photoexcitation by laser radiation of a resonance line with wavelength λ = 589.6 nm is numerically solved. The influence of Doppler frequency shifts on photon emission coherent in the atomic reference system is taken into account in the formation of an emission line profile using the model of partial frequency redistribution. In the case of a small optical thickness of a medium τ0 ≤ 1, the Doppler frequency redistribution in the laboratory reference system yields higher intensity values in the emission line profile core in comparison with the model of complete frequency redistribution. For an optically dense medium, thermal motion of atoms leads to an increase of reabsorption in the emission line core, and intensity increases in the line wings as compared to the complete redistribution model. The dependence of the Holstein trapping factor on optical thickness is closer to the analytic dependence than the complete redistribution model. This is explained by the fact that the frequencies of emitted photons in the laboratory reference system fall into the spectral profile core, which increases the effects of resonance radiation trapping in a dense medium.

Role of resonance radiation trapping in the mechanisms of constriction of the glow discharge. Theory and experiment.

The paper presents latest results of constricted glow discharge simulations. Detailed self-consistent plasma model includes a large set of plasma chemical reactions, considering inhomogeneous gas heating and accurate solution of radiation transport equation. Results of the computer simulations were compared with an experimental data of positive column constriction in neon and argon. Experimental plasma diagnostics was performed using classic methods of emission and absorption spectroscopy and a line ratios method. Comparison of the obtained results allows to reveal the influence of resonance radiation trapping on the plasma parameters of a constricted discharge.

Radial structure of the constricted positive column: Modeling and experiment.

We present a detailed self-consistent model of a positive column in argon glow discharge at moderate pressures and currents. This model describes the discharge transition between diffuse and constricted states. The model includes an extensive set of plasma chemical reactions and equation for inhomogeneous gas heating. The nonequilibrium behavior of an electron distribution function is also considered. One of the main features of the model is an accurate treatment of radiation trapping by solving the Holstein-Biberman equation directly. Influence of the radiation trapping on macroscopic parameters of the constricted positive column is studied. We propose a method for solving a boundary-value problem, including particle and energy balance equations for electrons, ground state atoms, atomic and molecular ions, and excited species. Unlike traditional solution approaches for similar systems, the method provides continuous Z- and S-shaped characteristics of discharge parameters, describing hysteresis in transition between diffuse and constricted discharge regimes. Performed experiments include measurements of volt-ampere characteristics and spectroscopic study of radial density profiles of excited atoms by measuring line emission and absorption, and electrons by measuring bremsstrahlung intensity. The role of resonance radiation trapping in spatial redistribution of 1s and 2pstates of argon is demonstrated. Results of modeling are compared to the experimental data.

Thermal and Chemical Nonequilibrium Effects in Free-Burning Arcs

Effects caused by thermal and chemical nonequilibrium in free-burning arcs in argon are presented and discussed. The results cover a range of arc currents between 100 and 200 A, interelectrode distance of 5–10 mm, and a variation of the electrode material and the shape of the cathode tip. The results obtained show that nonequilibrium in the near-electrode regions has a strong impact on the distributions of temperatures, electric conductivity and current density, and the arc voltage. Departure from chemical nonequilibrium in the vicinity of the cathode as a result of transport processes is indicated. Moreover, overpopulation of the atomic ground state close to the arc axis, under population of the excited states in the arc periphery with respect to the equilibrium values, axial dependence of the off-axis maximum of the radial emission coefficient of Ar I at 696.5 nm, and enhanced population of the Ar (1s4) resonance level due to trapping of resonance radiation in the arc periphery are considered.

Effect of trapping of resonance radiation in a free-burning Ar arc

A modified method for the solution of the Holstein-Biberman transport equation is proposed for cylindrical geometry, arbitrary absorption coefficient accounting for plasma inhomogeneity and a Lorentz spectral lineshape. The method is applied to the plasma generated by a free-burning arc in argon at atmospheric pressure in order to study the effect of trapping of resonance radiation on the population of the resonance state Ar(1s4). The results obtained are compared with those of a self-consistent nonequilibrium model of the arc including excited atomic levels. They show an enhanced (compared to the results of the nonequilibrium model) population of the resonance state outside of the arc core as an effect of the trapping of resonance radiation and almost negligible collisional de-excitation. In the arc core, the effect is negligible so that the calculated population of the Ar(1s4) state confirms the results obtained with the nonequilibrium model.

Impact of the radiation trapping on lifetime measurement of Er 3+ doped glasses

We report on the interplay between experimental arrangement, resonant radiation trapping and decay measurements of infra-red emission of erbium doped glasses. The impact of the relative position between the exciting and detecting positions of the bulk samples is investigated. We show that the radiation trapping leads to long rise time dynamics evenly distributed outside the pulsed pumped region.

Precursor Luminescence near the Collapse of Laser-Induced Bubbles in Alkali-Salt Solutions

A precursor luminescence pulse consisting of atomic line emission is observed as much as 150 nanoseconds prior to the collapse point of laser-created bubbles in alkali-metal solutions. The timing of the emission from neutral Na, Li, and K atoms is strongly dependent on the salt concentration, which appears to result from resonant radiation trapping by the alkali atoms in the bubble. The alkali emission ends at the onset of the blackbody luminescence pulse at the bubble collapse point, and the duration of the blackbody pulse is found to be reduced by up to 30% as the alkali-salt concentration is increased.

Photonic superdiffusive motion in resonance radiation trapping

In this work we consider the relation between the jump length probability density function and the line shape function in resonance radiation trapping in atomic vapors. The two-sided jump length probability density function suitable for a unidimensional formulation of radiative transfer is also derived. As a side result, a procedure to obtain the Maxwell distribution of velocities from the Maxwell-Boltzmann distribution of speeds was obtained. General relations that give the asymptotic jump length behavior and the Levy flight parameter mu for any line shape are obtained. The results are applied to generalized Doppler, generalized Lorentz, and Voigt line shape functions. It is concluded that the lighter the tail of the line shape function, the less heavy the tail of the jump length probability density function, although this tail is always heavy, with mu < or =1.

Resonance radiation trapping in alkaline-earth metal atomic lasers

The resonance radiation trapping in alkaline-earth metal atomic lasers were investigated according to Holstein's theory. The threshold temperature, ground-state density and resonant level effective lifetime of the resonance radiation trapping were calculated for Ca, Sr, and Ba atomic vapor lasers with different tube radii. The calculated threshold temperature is found to be in good agreement with the experimental laser-starting temperature, which indicates the resonance radiation trapping is a significant factor in establishing the population inversion of the resonance to metastable transition lasers.

One GHz linewidth, 33 line per mm, wide angle imaging filter at the potassium resonant line

This paper presents a narrow linewidth, high resolution, and high quantum efficiency imaging transmission filter based on optical trapping of resonance radiation in potassium vapor. The filter can be used to image radiation over a bandwidth narrow enough to fall within a Fraunhofer dark zone in the solar spectrum, and it can be applied to the imaging of flames, plumes or discharges containing potassium. It may also be applicable to the imaging of Raman scattering from a tunable laser. The spectral and imaging properties of the filter are demonstrated with a 1 cm aperture optically thick potassium cell illuminated by a narrow linewidth tunable laser. The spectral width at the potassium D2 line wavelength, 766.5 nm, is shown to be 1 to 2 GHz (.002 nm). At the line center, the quantum efficiency is better than 60% and the imaging resolution is better than 30 line pairs per mm. By employing a 200 micron "thin" potassium vapor cell, it is also shown that the filter maintains the high quantum efficiency (~50%) and good imaging capability (~20 lines per mm) across the 2 GHz spectral bandwidth of the cell. The "thin" cell has an out-of-band rejection of better than 1000. Its operation is demonstrated with a tunable laser as well as with broad band light from a potassium lamp and from a potassium chloride seeded flame.

Resonant Laser Ablation of Metals Detected by Atomic Emission in a Microwave Plasma and by Inductively Coupled Plasma Mass Spectrometry

It has been shown that an increase in sensitivity and selectivity of detection of an analyte can be achieved by tuning the ablation laser wavelength to match that of a resonant gas-phase transition of that analyte. This has been termed resonant laser ablation (RLA). For a pulsed tunable nanosecond laser, the data presented here illustrate the resonant enhancement effect in pure copper and aluminum samples, chromium oxide thin films, and for trace molybdenum in stainless steel samples, and indicate two main characteristics of the RLA phenomenon. The first is that there is an increase in the number of atoms ablated from the surface. The second is that the bandwidth of the wavelength dependence of the ablation is on the order of 1 nm. The effect was found to be virtually identical whether the atoms were detected by use of a microwave-induced plasma with atomic emission detection, by an inductively coupled plasma with mass spectrometric detection, or by observation of the number of laser pulses required to penetrate through thin films. The data indicate that a distinct ablation laser wavelength dependence exists, probably initiated via resonant radiation trapping, and accompanied by collisional broadening. Desorption contributions through radiation trapping are substantiated by changes in crater morphology as a function of wavelength and by the relatively broad linewidth of the ablation laser wavelength scans, compared to gas-phase excitation spectra. Also, other experiments with thin films demonstrate the existence of a distinct laser-material interaction and suggest that a combination of desorption induced by electronic transition (DIET) with resonant radiation trapping could assist in the enhancement of desorption yields. These results were obtained by a detailed inspection of the effect of the wavelength of the ablation laser over a narrow range of energy densities that lie between the threshold of laser-induced desorption of species and the usual analytical ablation regime. Normal ablation employs high-power lasers in an attempt to create a vapor plume without selective vaporization, and with a stoichiometry that accurately represents the stoichiometry of species in the solid sample. RLA, as a method of selective vaporization, appears to provide an opportunity to exploit selective vaporization in new ways.

Radiation trapping of the Hg 254 nm resonance line

The decay rate of Hg 6 3P1 atoms, due primarily to the escape of trapped 254 nm resonance radiation, was measured as a function of both Hg and Ar density in cylindrical, sealed fused silica cells. Time-resolved laser-induced fluorescence at 254 nm was used to obtain the decay rates for Hg densities from 5 × 1013 to 7 × 1015 cm−3 (22–101°C cold spot temperatures) and Ar densities from 0 to 9.7 × 1017 cm−3 in a cell of inner radius 1.05 cm. These new experimental data are compared to Monte Carlo results from a highly realistic code for simulating radiation trapping. This code includes Voigt profiles from a combination of Doppler, resonance, buffer gas and radiative line broadening, as well as hyperfine and isotopic structure with proper collisional redistribution. Upper limits on the rate constants for the quenching of Hg 6 3 P1 atoms by collisions with ground level Ar and Hg atoms were derived. Additional Monte Carlo simulations covering cell radii from 0.1 to 3.0 cm were performed. A broadly applicable engineering formula is derived from this study, which predicts the Hg 6 3P1 decay rate from 254 nm resonance radiation trapping as a function of Hg and Ar densities and cell radius.

Three-dimensional self-consistent radiation transport model for the fluid simulation of plasma display panel cell

In plasma display panels (PDPs), the resonance radiation trapping is one of the important processes. In order to incorporate this effect in a PDP cell, a three-dimensional radiation transport model is self-consistently coupled with a fluid simulation. This model is compared with the conventional trapping factor method in gas mixtures of neon and xenon. It shows the differences in the time evolutions of spatial profile and the total number of resonant excited states, especially in the afterglow. The generation rates of UV light are also compared for the two methods. The visible photon flux reaching the output window from the phosphor layers as well as the total UV photon flux arriving at the phosphor layer from the plasma region are calculated for resonant and nonresonant excited species. From these calculations, the time-averaged spatial profiles of the UV flux on the phosphor layers and the visible photon flux through the output window are obtained. Finally, the diagram of the energy efficiency and the contribution of each UV light are shown.

Two-dimensional self-consistent radiation transport model for plasma display panels

A two-dimensional radiation transport model is coupled with a fluid simulation to incorporate the resonance radiation trapping effect in a plasma display panel cell. Compared with the conventional trapping factor approach, this model has an advantage in describing the spatial evolution of the radiative excited-state density. Compared with a Monte Carlo model, it also takes advantage of its fast computation to couple the radiation transport self-consistently with the time-dependent fluid model. The effect of the spatial evolution of the resonant excited state on the light emission is investigated for the variations of system sizes and the gas mixture ratio, and the results are compared with those of the conventional trapping factor approach. The discrepancy between the two methods increases as the gap size between the dielectrics increases, but does not change significantly for the variation of the gas mixture ratio.

Diffusion of resonance radiation in atomic vapor imaging

The effect of resonance radiation diffusion due to radiation trapping has been studied in an atomic vapor imaging filter. Using a cesium resonance fluorescence imaging monochromator, the spatial distortions due to resonance radiation trapping by Cs atoms in the pumping/imaging region have been investigated. It was found that the spatial distortions were dependent on the number density of the Cs atoms, as well as the irradiance of the signal photons at 852.12 nm (6 2S 1/2→6 2P o 3/2). The pump laser (917.23 nm, 6 2P o 3/2→6 2D 5/2) did not influence the radiation diffusion to the same degree as the signal beam. It is shown that there is a compromise between maximum optical density and spatial resolution. The number density of the Cs resonance fluorescence imaging monochromator was optimized to obtain the highest spatial resolution and signal-to-background ratio.

Resonance radiation transport in plasma display panels

In fluid models of the gas discharges in plasma display panels, the trapping of resonance radiation is usually accounted for by a trapping factor. In this work, we present a Monte Carlo model for resonance photons, which gives a much more accurate description. First, we compare the results of this Monte Carlo model with the results of the fluid model trapping factor approach. Although the trapping factor approach does not yield the same spatial distribution for the density of the resonant state atoms, the spatially integrated density is in good agreement with the results of the Monte Carlo model. Next, we compare the results of the Monte Carlo model with measured spectra of emitted resonance radiation. The agreement is very good. Thus we provide, via the Monte Carlo model, experimental support for the widely used trapping factor approach.

Radiation trapping of the3P1−1S0resonant transitions of xenon and krypton in Xe-Kr, Xe-Ar, and Kr-Ar mixtures: Kinetic analysis and determination of the van der Waals broadening coefficients

This work concerns radiation trapping of the 123-nm krypton and 147-nm xenon lines, in the presence of a lighter rare gas, as well as the study of the decay processes of the relevant resonant states due to inelastic collisions. It deals with Kr-Ar, Xe-Kr, and Xe-Ar mixtures. Pulsed, brief selective excitation of the $\mathrm{Xe}{(}^{3}{P}_{1})$ or $\mathrm{Kr}{(}^{3}{P}_{1})$ resonant states by three-photon absorption was achieved by means of a tunable dye laser. Spectral and temporal analysis was then performed. The time resolved luminescence, filtered in the vacuum ultraviolet region, obeys a decay law described by the sum of two exponential terms which are attributed to the deexcitation of the ${}^{3}{P}_{1}$ and ${}^{3}{P}_{2}$ states. The decay constants are estimated by the maximum likelihood method applied to a Poisson statistical law. In order to correctly determine the collision rate constants, it is important to account for variations of the apparent lifetime according to the gas concentration. Analysis of both systematic experimental errors and statistical errors leads to a good estimation of the accuracy of the results obtained. For each mixture, the variation of the time constants as a function of partial pressures allows a determination of the resonance broadening coefficient and van der Waals broadening coefficient of the transition studied, as well as the two- and three-body inelastic collision rates relative to the ${}^{3}{P}_{1}$ and ${}^{3}{P}_{2}$ states. There exists no energy transfer between the two gases. For binary rare-gas mixtures, where only the heavier gas is excited, homonuclear and heteronuclear three-body reactions account for the decay of the ${}^{3}{P}_{1}$ state. Nevertheless, for the ${}^{3}{P}_{2}$ state we observe both two- and three-body collisions. In order to simulate resonance radiation trapping, a numerical method based upon Monte Carlo techniques was used. Calculations were first performed and validated for an infinite cylinder. The difference between calculation and experiment was less than 1%. Then the program was adapted to our real experimental conditions and applied to the binary mixtures studied. A good agreement was found between experiments and calculations. Furthermore, our program allows us to obtain information not easily obtained experimentally.

VUV lasing on puff metal vapor of neon-like aluminum ions produced by Z-pinch plasma

A collisional-radiative equilibrium model was developed to investigate the population inversion on metal vaporaluminum plasma. The population inversion has been establishedbetween lower3s(3P1 °, 1P1°) and upper 3p(3P0, 1S°) levels. The vacuumultra-violet lasing on neon-like aluminum ions provideswavelengths of 111.9, 143.2, and 157.3 nm. The rate of relativedensity population, between lower 3s1P1° and upper 3p3P0 levels, reaches its highest valueof 7.16 at the electron density of 1019 cm-3. Themodified expression of the optical escape factor, in the case of the Doppler profile, was introduced in the calculation of the opacityeffect on the gain magnitudes. The Holstein expressionunderestimates the optical escape factor in the region ofrelatively small thickness. The resonance radiation trapping by the 3dconfiguration drastically alters the gain values. A gain greater than 1 cm-1 was found between lasing levels 3s 3P1 ° and3p3P0. The enhancement of gain was due to the pumping of heupper level by a dielectronic recombination process from the groundstate of fluorine-like and by the inner-shell ionization processfrom excited levels of sodium-like ions.PACS No.: 52.00

VUV lasing on puff metal vapor of neon-like aluminum ions produced by Z-pinch plasma

A collisional-radiative equilibrium model was developed to investigate the population inversion on metal vaporaluminum plasma. The population inversion has been establishedbetween lower3s(3P1 °, 1P1°) and upper 3p(3P0, 1S°) levels. The vacuumultra-violet lasing on neon-like aluminum ions provideswavelengths of 111.9, 143.2, and 157.3 nm. The rate of relativedensity population, between lower 3s1P1° and upper 3p3P0 levels, reaches its highest valueof 7.16 at the electron density of 1019 cm-3. Themodified expression of the optical escape factor, in the case of the Doppler profile, was introduced in the calculation of the opacityeffect on the gain magnitudes. The Holstein expressionunderestimates the optical escape factor in the region ofrelatively small thickness. The resonance radiation trapping by the 3dconfiguration drastically alters the gain values. A gain greater than 1 cm-1 was found between lasing levels 3s 3P1 ° and3p3P0. The enhancement of gain was due to the pumping of heupper level by a dielectro...