Effect Of Radiation Trapping Research Articles

A two-step collisional-radiative model for interpreting spectra originating from autoionizing levels

A two-step collisional-radiative model is proposed for describing spectra which arise from radiative decays from autoionizing levels. In this procedure only two adjacent ionization states are considered at a time, and the ratio of the ionic populations is taken to be a free parameter. The other free parameters are the electron temperature and density. In the first step of the model, the populations of the ground and singly excited levels of the two ionization states are determined by solving two separate sets of collisional radiative rate equations; one set for each ionization state. In the second step, the populations of the doubly excited levels are calculated using the results of the first step. Radiative trapping effects are taken into account using the mean escape factor approximation. The results of the model obtained using atomic coefficients generated by the HULLAC atomic code are compared with experimental spectra emitted from laser-produced plasmas of highly charged xenon, barium, and cerium, and are used for interpreting the observed spectra, as well as for electron temperature and density estimates.

Spectroscopic Properties and Laser Performance Assessment of Yb3+ in Borophosphate Glasses

Absorption and emission properties of Yb3+ in borophosphate glasses of the (55‐x)P2O5·xB2O3·10ZnO·10BaO·20SrO·5Nb2O5·1Yb2O3 series (x= 0, 11, 22, 33, 44, and 55) have been investigated. To avoid the problem arising from the radiation trapping effect, the reciprocity method is employed to calculate the emission cross section. It is found that the spectroscopic properties of Yb3+ in our borophosphate glasses are sensitive to the Yb3+ site environments. In addition, the laser performance of the Yb3+‐doped borophosphate glasses has been assessed. And the borophosphate glasses with values of x= 33 and 55 appear to be superior to the phosphate glasses as Yb‐grain media.

Semiclassical interaction of moving two-level atoms with a cavity field: from integrability to Hamiltonian chaos.

The dynamics of an ensemble of two-level atoms moving through a single-mode lossless cavity is investigated in the semiclassical and rotating-wave approximations. The dynamical system for the expectation values of the atomic and field observables is considered as a perturbation to one of the following integrable versions: (i) a model with atoms moving through a spatially inhomogeneous resonant field, and (ii) a model with atoms interacting with a nonresonant eigenmode which is assumed to be homogeneous on the cavity size. We find the general exact solutions for both the models and show that they contain special solutions describing a coherent effect of population and radiation trapping. Using the Melnikov method, we prove analytically transverse intersections of stable and unstable manifolds of a hyperbolic fixed point under a small modulation of the vacuum Rabi frequency. These intersections are believed to provide the Smale horseshoe mechanism of Hamiltonian chaos. The analytical results are accompanied with direct computation of topographical maps of maximal Lyapunov exponents that give a representative image of regularity and chaos in the atom-field system in different ranges of its control parameters--the frequency detuning, the number, and the velocity of atoms.

Determination of fluorine atom density in reactive plasmas by vacuum ultraviolet absorption spectroscopy at 95.85 nm

Vacuum ultraviolet absorption spectroscopy was developed for the measurement of absolute fluorine (F) atom density in reactive plasmas. In order to minimize the influence of radiation trapping (self-absorption) in the light source, fluorescence at a wavelength of 95.85 nm from the F atoms in an electron–cyclotron resonance (ECR) CF4 plasma, which was operated with a low microwave power (0.1 kW) and a low gas pressure (1 mTorr), was employed as the probe emission. A windowless transmission system for the probe emission was constructed by connecting the ECR light source with the target plasma and the detection system using vacuum tubes having small slits. The connection tubes were differentially evacuated with turbomolecular pumps to prevent neutral particles from passing through between the ECR and target plasmas. The present method was applied to high-density CF4 and C4F8 plasmas produced by helicon-wave discharges. The accuracy of the measurement was examined carefully by evaluating various sources of error. In the present article, we have emphasized the evaluation of the radiation trapping effect in the light source plasma.

Concentration Dependence of the 1.5 μm Emission Lifetime of Er3+ in LiNbO3 by Radiation Trapping

The lifetime of the 1.5 μm transition of Er3+ doped LiNbO3 has been measured for different doping concentrations and geometries of luminescence collection. It has been found that this emission suffers of a strong radiation trapping effect, which affects the measured lifetime depending on the concentration and the geometry adopted.

Concentration Dependence of the 1.5 μm Emission Lifetime of Er3+ in LiNbO3 by Radiation Trapping

The lifetime of the 1.5 μm transition of Er3+ doped LiNbO3 has been measured for different doping concentrations and geometries of luminescence collection. It has been found that this emission suffers of a strong radiation trapping effect, which affects the measured lifetime depending on the concentration and the geometry adopted.

The Effect of Radiation Trapping in Gain for Ne-like Kr

The effect of radiation trapping in the atomic transitions of Ne-like Kr plasma has been investigated theoretically. First, the relative sublevel populations and gains for some specific 3s–3p transitions in the 27 levels of 2p6 and 2p5 3l configurations were calculated in the range of electron density 1018–1023 cm-3 for the electron temperature of 1 and 3 keV. Next, the influence of resonance radiation trapping was approximated by introducing the effective rates for some spontaneous emissions. Then, we recalculated the gains with these modified rates. It is noted that the effect of radiation trapping becomes significant at high electron densities and even increases the gain for some lasin transitions

Determination of collision site pressures - a Monte Carlo approach

A Monte Carlo simulation of radiative transfer in an atomic helium beam is carried out to investigate the effects of radiation trapping on electron-atom collision experiments. By comparing the simulated and measured radiation intensity distributions the pressure at the collision site can be determined, while such information has been hitherto otherwise inaccessible.

Measurement of electron-impact excitation into the3p54plevels of argon using Fourier-transform spectroscopy

To experimentally determine electron-impact excitation cross sections with the optical method, it is necessary to measure all transitions out of a level (the apparent cross sections), as well as the cascades into the level. In the case of the ten ${3p}^{5}4p$ levels of argon, the emissions to lower levels lie in the visible and near infrared (660\char21{}1150 nm) and are hence observable with a monochromator\char21{}photomultiplier-tube (PMT) system. A Fourier-transform spectrometer (FTS) allows us to measure the previously uninvestigated cascades that lie in the infrared. For the incident electron energy range between onset and 300 eV, we have measured the apparent cross sections with a monochromator-PMT system, and the cascade cross sections with a weak emission FTS system. The magnitude of both the apparent and cascade cross sections increases with target gas pressure due to radiation trapping effects. By subtracting the cascade contributions from the apparent cross sections, we have determined the direct cross sections and verified that they do not vary with pressure in the 0.5\char21{}4-mTorr pressure range considered here.

Experimental study of caesium energy pooling collisions and modelling of the excited atom density in the presence of optical pumping and radiation trapping

An experimental study of caesium energy pooling collisions, , at thermal energies, has been carried out in a capillary cell using diode laser excitation. Use of the capillary cell minimizes the effects of radiation trapping, but nonetheless, such effects still play a significant role in the analysis. Consequently, a rate equation model, which treats simultaneous effects of saturation, optical pumping, and radiation trapping, has been developed and is used to determine the atom density under these experimental conditions. The excited atom densities are combined with measured fluorescence ratios to determine rate coefficients for the caesium energy pooling process. Our values for these rate coefficients are in agreement, within combined error bars, with values we have recently obtained under very different experimental conditions.

Determination of emission cross section of Yb 3+ in glasses by the reciprocity method

The reciprocity method is applied to calculate the emission cross section of Yb 3+ in glasses, the error in σ em being less than ± 15%. The effect of radiation trapping is quantitatively evaluated by the parameter [( I p I S ) c − ( I p I s ) m]/( I p I s ) c, and fluorescence lifetime is determined by the equation τ f = τ m( I p I s ) m ( I p I s ) c .

A Monte Carlo Study of Radiation Trapping Effects

A Monte Carlo simulation of radiative transfer in an atomic beam is carried out to investigate the effects of radiation trapping on electron–atom collision experiments. The collisionally excited atom is represented by a simple electric dipole, for which the emission intensity distribution is well known. The spatial distribution, frequency and free path of this and the sequential dipoles were determined by a computer random generator according to the probabilities given by quantum theory. By altering the atomic number density at the target site, the pressure dependence of the observed atomic lifetime, the angular intensity distribution and polarisation of the radiation field is studied.

Multipole treatment of radiation trapping in a stepwise-excitation, electron-photon coincidence experiment.

A theory is presented of radiation trapping effects in time-resolved, stepwise-excited, electron-photon coincidence signals. The theory is based on a detailed analysis of laser-induced optical pumping effects in the stepwise excitation process combined with a multipole treatment of the radiation trapping processes. New data from a stepwise excitation experiment on the inelastic excitation of the 6${\mathrm{}}^{1}$${\mathit{P}}_{1}$ state in ${\mathit{e}}^{\mathrm{\ensuremath{-}}}$-Hg collisions and previously reported data are compared with the predictions of the theory. Good agreement is found between experiment and theory, including confirmation that under some experimental conditions, the ${\mathit{P}}_{10}$ Stokes parameter of the coincidentally detected stepwise excited fluorescence increases in magnitude with increasing target gas density. \textcopyright{} 1996 The American Physical Society.

Spatial effects of radiation trapping in an optically thick atomic vapor excited by a laser beam

We describe the influence of radiation trapping on the spatial distribution of the excited-state population in sodium vapor, interacting with a linearly polarized Gaussian laser beam. Counterintuitively the excitation volume experiences a strong spatial shrinking at high optical depth. The interpretation of this nonlocal effect is based on the numerical solution of the stationary Holstein equation, which takes into account the full frequency dependence of the reabsorption probability within the process of diffusion of radiation. The calculations performed are in good qualitative agreement with the spatial effects observed experimentally. \textcopyright{} 1996 The American Physical Society.

Modeling and efficient computation of nonlinear radiation trapping in three-level atomic vapors.

In three-level atoms where one level is the ground state and one is a metastable state, even low-intensity resonance radiation can lead to bleaching. This effect causes the radiation trapping to become nonlinear and thus greatly complicates the simulation of such a system. We set up equations to model an optically excited three-level atomic vapor in a two- or three-dimensional cylindrical cell; the model includes the effects of radiation trapping, particle diffusion, and quenching. We then develop a fast and efficient approximate method for the solution of these equations. Results are compared to Monte Carlo simulations; agreement within 15% is achieved.

1.047-μm Yb:Sr/sub 5/(PO/sub 4/)/sub 3/F energy storage optical amplifier

The pumping and gain properties of Yb/sup 3+/-doped Sr/sub 5/(PO/sub 4/)/sub 3/F (Yb:S-FAP) are reported. Using a tunable, free running 900-nm Cr:LiSAF oscillator as a pump source for a Yb:S-FAP rod, the saturation fluence for pumping was measured to be 2.2 J/cm/sup 2/ based on either the spatial, temporal, or energy transmission properties of the Yb:S-FAP rod. The emission peak of Yb:S-FAP (1047.5 nm in air) is shown to overlap with that of Nd:YLiF/sub 4/ (Nd:YLF) to within 0.1 nm, rendering Yb:S-FAP suitable as an effective power amplifier for Nd:YLF oscillators. The small signal gain, under varying pumping conditions, was measured with a cw Nd:YLF probe laser. These measurements implied emission cross sections of 6.0/spl times/10/sup -20/ and 1.5/spl times/10/sup -20/ cm/sup 2/ for /spl pi/ and /spl sigma/ polarized light. Respectively, which fall within the error limits of the previously reported values of 7.3/spl times/10/sup -20/ and 1.4/spl times/10/sup -20/ cm/sup 2/ for /spl pi/ and /spl sigma/ polarized light, obtained from purely spectroscopic techniques. The effects of radiation trapping on the emission lifetime have been quantified and have been shown to lead to emission lifetimes as long as 1.7 ms, for large optically dense crystals. This is substantially larger than the measured intrinsic lifetime of 1.10 ms. Yb:S-FAP crystal boules up to 25/spl times/25/spl times/175 mm in size, which were grown for the above experiments and were found to have acceptable loss characteristics (>

Limitation of the Polarization by Radiation Trapping in a Helium Afterglow Electron Source

A polarized electron source using an optically pumped helium afterglow was built at Orsay. Unfortunately the spin polarization decreases at high metastable densities. Calculations of the radiation trapping effects in a weak magnetic field are presented using the Anderson formalism. Comparison with experimental data leads to the conclusion that these trapping effects are one explanation of this polarization decrease. Effects of the main parameters are studied. Some deductions for a new design can be made

Effect of radiation trapping on fluorescence lifetime and emission cross section measurements in solid-state laser media

Radiation trapping causes significant lengthening of the measured fluorescence lifetime in optically thin solid-state laser gain media, which leads to underestimates of the stimulated emission cross section by as much as 30%. A measurement technique is demonstrated that greatly reduces radiation trapping in Yb:YAG, a prototypical quasi-three-level laser medium. The radiative lifetime of the (2)F(5/2) manifold was measured to be 0.951 ms at 300 K, which is more than 10% lower than any previous measurement. This more-accurate radiative lifetime gives a 1.03-microm peak effective stimulated emission cross section of 2.3 x 10(-20) cm(2). Our measurement technique is expected to be most relevant for three-level and quasi-three-level laser media.

Photon angular correlations and pressure-dependent effects of helium 21P and 31P excitation by electrons

The orientation and alignment parameters of the 21P and 31P excited states of helium at an incident electron energy of 81.6 eV have been remeasured for electron scattering angles from 25 degrees to 5 degrees and extended down to 2 degrees . The studies investigate a wide range of experimental conditions and measurement techniques, including beam alignment, electron and photon angular distribution asymmetries in both the singles and coincidence detection modes, in order to establish better precision in the measurements. Nevertheless the present coherence lambda , chi and gamma a, parameters for He(21P) and He(31P) state excitation for scattering angles in the range theta e=2 degrees to 25 degrees are collectively not in good agreement with any single distorted wave, first Born or close coupling model predictions, although there are ranges of scattering angles where the individual measured parameters are in good agreement with a particular theory. Radiation trapping effects on the 31P state lifetime enabled the relative relaxation rates for the population, orientation and alignment, i.e. rank 0, 1 and 2 multipoles, of the excited state to be measured. The effects of these relaxation rates on the measured intensities and the coherence parameters are quantified. The lifetime, which can be measured an order of magnitude more quickly than the state parameters, reflects these relaxation rates and provides a reliable guide to radiation trapping effects. Radiation trapping influences the state parameters down to the lowest background operating pressure of 10-7 Torr at which the measurements have been made.

Nonlinear effects of radiation trapping in ground-state oriented sodium vapor.

Effects of radiation transport in the interaction of a laser beam with sodium vapor in an argon atmosphere are reported. Investigations of the temporal and spatial evolution of ground-state orientation, which is produced by optical pumping with a circularly polarized near-resonant laser beam, reveal a strong deviation from the expected pure diffusive behavior. Transverse profiles are narrowed considerably with increasing sodium density as well as with increasing pump intensity. The temporal decay of orientation is also drastically altered. A simplified nonlinear and nonlocal theoretical model of radiation trapping gives excellent qualitative agreement with the observations and allows an intuitive interpretation.