Self-absorption Process Research Articles

An optical emission spectroscopy method for determining the electron temperature and density in low-temperature xenon plasma by using a collisional-radiative model considering the hyperfine structure of emission line into metastable state

Optical emission spectroscopy (OES) is an important technique for the diagnostics of plasma parameters in the development of plasma devices and material processing technology. OES depends on the collisional-radiative model which can accurately describe the kinetics of excited species in plasma. Metastable species usually have higher density (2–4 orders of magnitude) and longer lifetime (3–6 orders of magnitude) compared to other excited species, thus play important roles in the reaction kinetics. However, the kinetic of metastable species can be complex due to the transport and self-absorption processes, which may lead to coupling of the plasma reaction kinetics in the adjacent areas. Moreover, the line splitting of some elements make the description of metastable kinetics more complex. This work presents an OES method based on a collisional-radiative model considering the hyperfine splitting of emission lines into metastable levels. The density distribution of metastable state is determined by the self-absorption effect. It is then taken into the rate balance equations of other excited species to make an accurate description of metastable related kinetics. The electron density and temperature are determined based on this model. The OES method is demonstrated on a miniaturized electron-cyclotron-resonance ion thruster. The metastable density, electron density and electron temperature are determined along the radial direction of the ion thruster, which can be useful for revealing the physics of discharge phenomenon and optimizing the performance of the plasma source. This method can be used to determine electron parameters in a large pressure range.

Avoiding Misunderstanding Self-Absorption in Laser-Induced Breakdown Spectroscopy (LIBS) Analysis

In a recent article, I discussed seven common errors that may affect the results of laser-induced breakdown spectroscopy (LIBS) analysis. Among these errors, one of the recurring ones is related to misunderstanding the process of self-absorption in LIBS plasmas. Because many people working in LIBS still consider self-absorption as a problem yet to be solved, I am dedicating a full article to this topic. The physics and chemistry of the phenomenon have been well known for many years, and this knowledge can tell us how self-absorption can be not only “corrected,” but also tuned to our advantage in analytical applications of LIBS.

Improvements in the radiochemical method for separating 226Ra in solid samples through coprecipitation with BaSO4

The aim of this work was to improve the commonly used method for 226Ra determination in water and to establish its application in solid samples. This method is based on the coprecipitation of Ra with BaSO4 and gross alpha counting of the precipitate. An exhaustive study of the coprecipitation behaviour of the most abundant cations present in solid samples was performed to avoid incorrect radiochemical yields. As a result, it was considered necessary to introduce two new purification steps into the conventional method. Likewise, two nuclides, 241Am and 226Ra, were compared to obtain the mass efficiency curve given their different behaviour in the coprecipitation process. While Ra behaves similarly to Ba, Am coprecipitates, forming mixed crystals that may behave differently in the self-absorption process. The influence of the cations on the chemical yield with no precipitate purification was: Sr2+≫Fe3+>Mg2+≈Ca2+>K+≈Na+. The method was successfully applied to soil, sediment, and plant ash samples.

Emulsion-templated construction of enzyme-nanozyme integrated hierarchically porous hydrogels for smartphone-assisted pesticide biosensing

Due to their great threat to human health and the environment, it is always of significance to develop efficient tools to monitor pesticide residues. Although the enzyme inhibition-based strategy is considered as a potential method and has been intensively explored for pesticide analysis, both vulnerable enzymatic activity and tedious operation greatly hinder its practical applications. Developing robust, easy-to-use and efficient devices for pesticide sensing is still challenging. Herein, we recommend an enzyme-nanozyme integrated hierarchically porous hydrogel (AChE-MnO2@HPH) for the convenient detection of pesticides. MnO2 microspheres were first employed as an emulsion stabilizer to fabricate MnO2@HPH by thermal polymerization, and AChE was then immobilized into the MnO2@HPH via a fast self-absorption process. With the unique porous structure and hydrophilic microenvironment, the constructed AChE-MnO2@HPH exhibited excellent enzymatic activity and stability as well as rapid response toward pesticides through inhibiting the activity of AChE. By applying a smartphone to read the color information originating from the chromogenic reaction of 3,3′,5,5′-tetramethylbenzidine (TMB) catalyzed by oxidase-mimicking MnO2, highly sensitive and convenient determination of fenthion, an organophosphorus pesticide model, was achieved with a detection limit down to 0.63 ng mL−1. Excellent performance robustness over 30 storage days and reliable detection of the target in real food samples were also demonstrated. With the advantages of easy operability, free of equipment, and good detection performance and long-term stability, the integrated AChE-MnO2@HPH will find promising use for pesticide residue in-field analysis.

Optical activity induced by rare-earth ions in As2S3 glasses and KCl crystals

Optical transmission spectra of As2S3 glass-like plates and nanolayers, undoped and doped with Sm and Pr ions have been investigated. The spectral dependences of the refractive index and glass transmission spectra, registered in the configuration of crossed polarizers, were studied. The concentration of rare-earth ions in the matrix influences on the magnitude of refractive indices (n−, n+) of polarized light. The emission spectra of As2S3 glasses, doped with rare-earth ions (Sm and Pr) and KCl crystals doped by Sm ions have been investigated for different temperatures in the range 300–10 K. A broad luminescence band associated with recombination transitions of charge carriers from 5D0 levels to 7F0-6 levels or from 4G5/2 levels to 6H5/2…15/2 levels of samarium ions (Sm2+ and Sm3+, respectively) was registered. A number of narrow absorption bands were registered on this broad photoluminescence band, which can be attributed to self-absorption process, determined by electronic transitions between the levels (7F0-6 ⟶ 5D0 or 6H5/2…15/2 ⟶ 4G5/2) of samarium ions.

Synchrotron self-absorption in GRB afterglows: the effects of a thermal electron population

In the standard synchrotron afterglow model, a power law of electrons is responsible for all aspects of photon production and absorption. Recent numerical work has shown that the vast majority of particles in the downstream medium are actually "thermal" particles, which were shock-heated but did not enter the Fermi acceleration process (the name stands in contrast to the nonthermal high-energy tail, rather than connoting a Maxwellian distribution). There are substantial differences at optical and higher energies when these thermal electrons participate in the afterglow, but early work along these lines ignored the radio end of the electromagnetic spectrum. We report here on an extension of previous Monte Carlo simulations of gamma-ray burst afterglows. The model now includes the synchrotron self-absorption (SSA) process and so can simulate afterglows across the entire EM spectrum, and several orders of magnitude in time. In keeping with earlier work, inclusion of the thermal electrons increases the SSA frequency by a factor of 30, and the radio intensity by a factor of 100. Furthermore, these changes happen with no modification to the late optical or X-ray afterglow. Our results provide very strong evidence that thermal electrons must be considered in any multiwavelength model for afterglows.

Concentration- and temperature-dependent photoluminescence of CsPbBr3 perovskite quantum dots

The concentration and temperature dependence of photoluminescence (PL) performance was investigated, which indicated that the main emission in CsPbBr3 perovskite quantum dots was related to the band-edge mechanism. As the concentration increased from 0.1 to 10.0mol/L at 293K, the center of emission band exhibited a regular redshift, as well as the peak intensity increased initially and then declined, which could be attributed to the self-absorption processes. Along with the variation of temperature (273–353K), peak wavelength, full width of half maximum (FWHM), and peak intensity of PL spectra presented a corresponding linear changes. In addition, the temperature reversibility revealed that CsPbBr3 QDs were reliable candidates for temperature sensor and optoelectronic devices.

Compton scattering of self-absorbed synchrotron emission

Synchrotron self-Compton (SSC) scattering is an important emission mechanism in many astronomical sources, such as gamma-ray bursts (GRBs) and active galactic nuclei (AGNs). We give a complete presentation of the analytical approximations for the Compton scattering of synchrotron emission with both weak and strong synchrotron self-absorption. All possible orders of the characteristic synchrotron spectral breaks ($\nu_{\rm a}$, $\nu_{\rm m}$, and $\nu_{\rm c}$) are studied. In the weak self-absorption regime, i.e., $\nu_{\rm a} < \nu_c$, the electron energy distribution is not modified by the self-absorption process. The shape of the SSC component broadly resembles that of synchrotron, but with the following features: The SSC flux increases linearly with frequency up to the SSC break frequency corresponding to the self-absorption frequency $\nu_{\rm a}$; and the presence of a logarithmic term in the high-frequency range of the SSC spectra makes it harder than the power-law approximation. In the strong absorption regime, i.e. $\nu_{\rm a} > \nu_{\rm c}$, heating of low energy electrons due to synchrotron absorption leads to pile-up of electrons, and form a thermal component besides the broken power-law component. This leads to two-component (thermal + non-thermal) spectra for both the synchrotron and SSC spectral components. For $\nu_{\rm c} < \nu_{\rm a} < \nu_{\rm m}$, the spectrum is thermal (non-thermal) -dominated if $\nu_a > \sqrt{\nu_m \nu_c}$ ($\nu_a < \sqrt{\nu_m \nu_c}$). Similar to the weak-absorption regime, the SSC spectral component is broader than the simple broken power law approximation. We derive the critical condition for strong absorption (electron pile-up), and discuss a case of GRB reverse shock emission in a wind medium, which invokes $\nu_{\rm a} > {\rm max} (\nu_{\rm m}, \nu_{\rm c})$.

The water-ice librational band: radiative transfer model for AFGL 961

On the basis of radiative transfer models we have investigated the librational band of amorphous water-ice (H2O-ice), which might be expected to occur in the spectra of young stellar objects with dust shells, producing an absorption feature at about 13 μm. Models have been run with only bare silicate grains present, together with models including water-ice mantles. We find that the 13-μm feature may be substantially explained without the use of water-ice if laboratory silicates are employed and the 10-μm silicate feature is in self-absorption, a phenomenon which may arise for shells of intermediate optical depth (). The self-absorption process may lead to a variation in the shape of the local minimum between the 10- and 20-μm silicate features, the local minimum occurring at about 13 μm, close to the position of the water-ice librational band. This local minimum could easily be mistaken for the librational water-ice band itself. Nevertheless, the librational band undoubtedly does contribute to the finer details of the shape of the 13-μm absorption feature. We find that for shells of small optical depth the water-ice feature is not distinguishable from the local minimum between the 10- and 20-μm silicate features, while for shells of large optical depth it is washed out completely, so in neither of these two extreme cases should the feature be readily observable. The fact that relatively few objects exist in the intermediate state where the silicate feature is in self-absorption may account for the extremely small number of cases in which the librational band apparently appears as a distinct feature. Radiative transfer models have been produced for the massive pre-main-sequence object AFGL 961E, with the aim of investigating whether the 13-μm feature in its spectrum is due to the librational band of water-ice. Clearly there is water-ice somewhere in the line of sight to AFGL 961, as evidenced by the strong 3.1-μm water-ice band, and the case for the 13-μm feature being due to the librational band is uniquely strong amongst all astronomical objects. However, we present evidence that the grains may have properties similar to laboratory silicates rather than astronomical silicates and that the feature may largely be a manifestation of the 10-μm silicate feature being in self-absorption, with amorphous water-ice being a minor contributor. Neither bare astronomical silicates nor water-ice mantled astronomical silicates will reproduce the 13-μm feature.

Detection of chloride in reinforced concrete using a dualpulsed laser-induced breakdown spectrometer system: comparative study of the atomic transition lines of Cl I at 59485 and 83759 nm

The presence of chloride in reinforced concrete can cause severe damage to the strength and durability of buildings and bridges. The detection of chloride in concrete structures at early stages of the corrosion buildup process is, therefore, very important. However, detection of chlorine in trace amounts in concrete is not a simple matter. A dual-pulsed laser-induced breakdown spectrometer (LIBS) has been developed at our laboratory for the detection of chloride contents in reinforced concrete by using two atomic transition lines of neutral chlorine (Cl I) at 594.8 and 837.5 nm. A calibration curve was also established by using standard samples containing chloride in known concentration in the concrete. Our dual-pulsed LIBS system demonstrated a substantial improvement in the signal level at both wavelengths (594.8 and 837.5 nm). However, the new atomic transition line at 594.8 nm shows a significant improvement compared to the line at 837.5 nm in spite of the fact that the relative intensity of the former is 0.1% of the latter. This weak signal level of the 837.5 nm transition line of chlorine can be attributed to some kind of self-absorption process taking place in the case of the concrete sample.

Radio emission and nonlinear diffusive shock acceleration of cosmic rays in the supernova SN 1993J

The extensive observations of the supernova SN 1993J at radio wavelengths make this object a unique target for the study of particle acceleration in a supernova shock. To describe the radio synchrotron emission we use a model that couples a semianalytic description of nonlinear diffusive shock acceleration with self-similar solutions for the hydrodynamics of the supernova expansion. The synchrotron emission, which is assumed to be produced by relativistic electrons propagating in the postshock plasma, is worked out from radiative transfer calculations that include the process of synchrotron self-absorption. The model is applied to explain the morphology of the radio emission deduced from high-resolution VLBI imaging observations and the measured time evolution of the total flux density at six frequencies. Both the light curves and the morphology of the radio emission indicate that the magnetic field was strongly amplified in the blast wave region shortly after the explosion, possibly via the nonresonant regime of the cosmic-ray streaming instability operating in the shock precursor. The turbulent magnetic field was not damped behind the shock but carried along by the plasma flow in the downstream region. Cosmic-ray protons were efficiently produced by diffusive shock acceleration at the blast wave. We find that during the first ~8.5 years after the explosion, about 19% of the total energy processed by the forward shock was converted to cosmic-ray energy. However, the shock remained weakly modified by the cosmic-ray pressure. The high magnetic field amplification implies that protons were rapidly accelerated to energies well above 1 PeV. The results obtained for this supernova support the scenario that massive stars exploding into their former stellar wind are a major source of high-energy Galactic cosmic rays.

N-(2′,5′-di- t-butylphenyl)-4-ethoxy-1,8-naphthalimide: A new fluorophore highly efficient for fast neutrons/gamma-rays discrimination in liquid media

Discrimination between fast neutrons and gamma rays has been successfully achieved using a blue-fluorescing 4-substituted 1,8-naphthalimide in aromatic solvents. As previously stated in the literature, an important solvent effect was observed, but the substitution groups surrounding the core of the naphthalimide also appeared to be extremely crucial. Our system presents the advantages of being based on the use of a single highly soluble solute, exhibiting a large Stokes shift (to avoid self-absorption process) and presenting an emission spectrum fitting well to the photomultiplier (PM) sensitivity. More importantly, we observed that our scintillator was not affected by the concentration of oxygen dissolved in the medium. This observation has no equivalent in commercial liquid scintillators, which are known to be oxygen-sensitive. The discrimination efficiency of our system is described in terms of scintillation performances, figure of merit and the angle between neutrons and gamma lobes visualized in the bi-parametric discrimination spectrum.

Evolution of and High‐Energy Emission from GHz‐Peaked Spectrum Sources

Here we discuss evolution and broad-band emission of compact (< kpc) lobes in young radio sources. We propose a simple dynamical description for these objects, consisting of a relativistic jet propagating into a uniform gaseous medium in the central parts of an elliptical host. In the framework of the proposed model, we follow the evolution of ultrarelativistic electrons injected from a terminal hotspot of a jet to expanding lobes, taking into account their adiabatic energy losses as well as radiative cooling. This allows us to discuss the broad-band lobe emission of young radio sources. In particular, we argue that the observed spectral turnover in the radio synchrotron spectra of these objects cannot originate from the synchrotron self-absorption process but is most likely due to free-free absorption effects connected with neutral clouds of interstellar medium engulfed by the expanding lobes and photoionized by active centers. We also find a relatively strong and complex high-energy emission component produced by inverse-Compton up-scattering of various surrounding photon fields by the lobes electrons. We argue that such high energy radiation is strong enough to account for several observed properties of GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In addition, this emission is expected more » to extend up to GeV (or possibly even TeV) photon energies and can thus be probed by several modern {gamma}-ray instruments. In particular, we suggest that GPS radio galaxies should constitute a relatively numerous class of extragalactic sources detected by GLAST. « less

High-Resolution VUV Spectra of Carbon, Neon and Argon in a Wavelength Range of 250 to 2300 Å for Plasma Diagnostics Observed with a 3 m Normal Incidence Spectrometer in LHD

Intrinsic impurities have been much reduced in toroidal fusion devices through the development of several wall-conditioning techniques as well as by the use of carbon materials in the first wall and divertor plates. Impurity elements useful for passive plasma spectroscopy have been then extremely limited. At present, only carbon is a subject for spectroscopic diagnostics in most discharges except for fuel atoms. The use of rare gas as a brighter light source is a method to overcome the present difficulty in passive spectroscopy. Recently, rare gases have also been used for edge cooling to reduce the divertor heat flux. Therefore, high-resolution spectra (Δλ ∼ 0.2 A) from neon and argon in a 250 to 2300 A wavelength range have been measured using a 3 m normal incidence spectrometer in Large Helical Device (LHD) and the measured spectra were precisely analyzed. The VUV spectra of carbon, neon and argon are presented for spectroscopic use and their wavelengths are tabulated with their relative intensities. The spectral profiles of almost all the spectral lines measured here are formed by the Doppler broadening and self-absorption processes. The Doppler broadening of neon and argon spectra are plotted against the ionization energies and Doppler spectra from carbon lines are presented. The self-absorption spectra of the hydrogen Lyman-α line, which are found in the LHD high-density discharge, are also presented and the neutral density is analytically estimated.

Evaluation of self-absorption of manganese emission lines in Laser Induced Breakdown Spectroscopy measurements

This paper is part of a more general study aimed to the determination of the best experimental procedures for reliable quantitative measurements of Fe–Mn alloys by LIBS. In this work, attention is pointed on the self-absorption processes, whose effect deeply influences the LIBS measurements, reflecting in non-linear calibration curves. The effect of self-absorption on the line intensity can be quantified by defining a self-absorption coefficient, that measures the deviation of the line intensity from the linear extrapolation of the curve of growth in the optically thin regime. The authors demonstrated in a previous paper that self-absorption coefficients could be calculated once the electron density of the plasma is known and the Stark coefficients of the lines are available. However, when the Stark coefficients of the lines of interest are not known, a different approach is needed. In this work a new method for evaluation of self-absorption coefficients in LIBS measurements is presented, which does not require the knowledge of Stark coefficients. In order to understand the basic principles and setting out the theoretical tools that will be used for the analysis of the alloys, a preliminary study was done on pure Mn; LIBS spectra were acquired in different experimental conditions, at different laser energies and different delays after the laser irradiation of the sample. Moreover, collinear double pulse measurements were also performed. Analytical relations were derived and experimental procedures devised for evaluation of the self-absorption coefficients of several Mn lines, which are important for characterization and control of the experimental conditions in which the analysis is performed.

Depth-resolved optical studies of excitonic and phonon-assisted transitions in ZnO epilayers

The authors report depth-resolved optical properties of excitonic and phonon-assisted transitions in ZnO epilayers by photoluminescence (PL) and cathodoluminescence. A weaker free exciton (FX) emission than its first longitudinal optical phonon replica (FX-1LO) is observed at elevated temperatures (T&amp;gt;150K) for interior area, while a stronger FX than FX-1LO is seen at all temperatures for top surface area of the sample. The authors exclude out a possible self-absorption process by PL excited at back surface of the sample. Therefore, the authors conclude that the different intensity ratios of FX and FX-1LO depending on the sample depth are strongly associated with extrinsic features of ZnO.

Dependence of the vibronic emission on self-absorption and reemission processes in conjugated polymers

Photoluminescence spectra from the edge of thin films of a conjugated polymer were obtained by varying the self-absorption level at different temperatures. This technique together with the use of a Franck-Condon analysis enabled a clear identification of the vibrational relaxation processes, providing a way to recognize the participating vibrational modes. The higher order vibronic bands were observed to shift to lower energies than those predicted theoretically. This was interpreted as an effect of self-absorption processes, which lead to the loss of energy by intramolecular interactions due to the increase of thermal disorder.

The cyclo-synchrotron process and particle heating through the absorption of photons

We propose a new approximation for the cyclo-synchrotron emissivity of a single electron. In the second part of this work, we discuss a simple application for our approximation, and investigate the heating of electrons through the self-absorption process. Finally, we investigate the self-absorbed part of the spectrum produced by a power-law population of electrons. In comparison to earlier approximations, our formula provides a few significant advantages. Integration of the emissivity over the whole frequency range, starting from the proper minimal emitting frequency, gives the correct cooling rate for any energy particle. Further, the spectrum of the emission is well approximated over the whole frequency range, even for relatively low particle energies (beta << 0.1), where most of the power is emitted in the first harmonic. In order to test our continuous approximation, we compare it with a recently derived approximation of the first ten harmonics. Finally, our formula connects relatively smooth to the synchrotron emission at beta=0.9. We show that the self-absorption is a very efficient heating mechanism for low energy particles, independent of the shape of the particle distribution responsible for the self-absorbed synchrotron emission. We find that the energy gains for low energy particles are always higher than energy losses by cyclo-synchrotron emission. We show also that the spectral index of the self-absorbed part of the spectrum at very low frequencies differs significantly from the well known standard relation I(nu) ~ nu^(5/2).

An Atomic and Molecular Study of the Interstellar Medium Around the Supernova Remnant RCW 103

AbstractWe report on the detection of HCO+ and 12CO emission in the rotational transition J = 1–0 in the vicinity of the shock front at the southern border of the supernova remnant RCW 103, where previous infrared observations suggest an interaction with a molecular cloud. The observations were carried out with the Australian Millimeter Radiotelescope at Mopra. We observed a depletion of HCO+ behind the supernova shock front. In addition, we studied the interstellar medium over an extended region towards RCW 103 based on archival λ 21 cm Hı line observations from the Australia Telescope Compact Array (ATCA) and the Parkes Telescope. No atomic gas was observed in emission in coincidence with the molecular feature. This absence was interpreted in terms of self-absorption processes.

Hot gases: The transition from line spectra to thermal radiation

The question of how the line spectrum of atoms is transformed into the continuous spectrum of thermal radiation of a gas by repeated absorption and emission of photons is addressed. We consider a simple model of a hydrogen-like gas in thermal equilibrium where only the Lyman-α transition is allowed and calculate the spectra for increasing thickness of the gas. The transition from line spectra for optically thin samples to the spectra of thermal radiation for optically thick samples is demonstrated. We estimate the emissivities for hot gases and study the line width of spectral lines due to self-absorption processes.