Self-absorption Process Research Articles

Effect of dopant on the intrinsic properties of some multifunctional aromatic compounds films for target applications

UV–VIS transmission and fluorescence spectroscopy have been used to study some intrinsic properties of crystalline films of pure meta-dinitrobenzene (m-DNB) and, pure and doped benzil. We have evaluated the band gap energy, Eg=2.92eV, emphasising the near classical semiconductor behaviour of m-DNB. The two bands split of the (n, π*) level is assumed to be the origin the two edges absorption in benzil films with the energetic thresholds at Eg1=2.84eV and Eg2=3.55eV. The red shift of the emission peak in m-DNB films has been correlated with the process of self-absorption of the emitted radiation in the thicker film. The effect of the impurities on the shape and position of the emission peaks in benzil has been investigated and we have observed no significant shift of the absorption peak situated at 3.25eV. We also have identified a three steps process for the relaxation of the excited crystalline lattice of benzil involving the geometrical change of the (S1) excited state, the intersystem crossing and the radiative decay by phosphorescence with a peak at 2.30eV. The small blue shift of this emission peak in benzil highly doped with sodium has been attributed to the modification of the molecular geometry of benzil as a result of the change in the trans-planar molecular configuration induced by the chemical reaction between the atoms of alkali metal and the carbonyl groups.

Determination of ions in individual fluid inclusions by laser ablation optical emission spectroscopy: development and applications to natural fluid inclusions

Chemical data on the composition of individual fluid inclusions are required to model paleofluid-rock interactions, as these inclusions are the direct remains of the former fluid circulation in the earth's crust. The proposed technique of laser ablation coupled with optical emission spectrometry (LA-OES) determines ion ratios in individual fluid inclusions. The radiations of the elements present in the plasma are analysed with a spectrometer equipped with a pulsed and gated multichannel detector. The lateral resolution is around 6 µm and the depth of the quartz crater obtained for one laser shot is around 1 µm. For fluid inclusions, the laser beam is initially focused on the surface of the sample, to drill until the inclusion is reached, then the liquid is analysed. Plasma temperature studies showed that various kinds of standards (synthetic glasses, fluid inclusions and minerals) can be used for the establishment of calibration curves for Na/K, Na/Ca and Na/Li ratios. As the emission line intensity is a function of the ablated mass, only emission line ratios between two elements are used. In addition, a self-absorption process was considered for the calibration strategy. The relative standard deviations of the calibration curves vary from 5% (glasses) to 25% (fluid inclusions) and the detection limits are those required for the determination of ions in individual inclusions (Na and Li 10, Ca 20 and K 750 ppm). Natural fluid inclusions were analysed using the LA-OES technique and it is now possible to determine the ratios of major elements in individual fluid inclusions.

Self-absorption and phonon-assisted radiative energy transfer processes in a ruby laser rod

Systematic investigations of the effect of self-absorption on the dynamic and steady-state spectroscopic properties in ruby have been carried out at different temperatures (from approximately 130 to 500 K) using a long, pink laser rod. At about 140 K,the lengthening of decay times of the R lines reached a maximum and then decreased with increasing geometric trapping. Spectral diffusion as large as 10 cm-1 was observed in the complete-trapping regime. These observations suggest reexamination of the theoretical model for phonon-assisted radiative energy transfer in ruby.

Energy absorption by metal-vapor-dominated plasma during carbon dioxide laser welding of steels

During laser welding, the plasma plume affects the amount of energy reaching the weld surface and the composition and properties of the welds. Light emissions during welding were recorded by emission spectroscopy to understand the energy absorption and the nature of the plasma formed during welding of various grades of steels. The flow of gases and the concentrations of the various metal vapors were computed by solving the Navier Stokes equation and the equations of conservation of various species. The variables studied were shielding gas composition and flow rate and the base metal composition. Until now, self-absorption of emissions arising from species present at high concentrations within the plasma has kept researchers limited to either analyzing ideal situations that are unrelated to the welding process or not accounting for the attenuation of the emissions. It is demonstrated that during welding, the peaks in the emission spectra that are affected by the self-absorption process can be eliminated on the basis of the initial and the terminal energy levels for electronic transitions. By selectively eliminating the affected transitions and by using the numerically computed local concentrations of metal vapors, the absorption of the laser beam energy by the plasma can be accurately determined.

Influence of luminescence self-absorption on photoluminescence decay in GaAs

In direct-gap semiconductors, self-absorption processes are known to increase minority carrier lifetimes. In this paper, an original method is developed to solve the dynamic continuity equation with the inclusion of self-absorption phenomena. Also named photo recycling, this process is treated in a model which takes into account the experimental conditions, together with geometrical and optical characteristics of the sample. This method is used to analyze photoluminescence decay experiments on angle-lapped GaAs/Ga0.15Al0.85As double heterostructures. The inclusion of the reabsorption effects is shown to improve the global fit of calculated luminescence with experimental data. This leads to a more accurate determination of key electronic parameters such as diffusion coefficient, surface recombination velocities, and nonradiative lifetime of minority carriers.

Status of timing with plastic scintillation detectors

Timing properties of scintillators and photomultipliers as well as theoretical and experimental studies of time resolution of scintillation counters are reviewed. Predictions of the theory of the scintillation pulse generation process are compared with the data on the light pulse shape from small samples, in which the light pulse shape depends only on the composition of the scintillator. For larger samples the influence of the light collection process and the self-absorption process on the light pulse shape are discussed. The data on rise times, fwhm's, decay times and light yield of several commercial scintillators used in timing are collected. The next part of the paper deals with the properties of photomultipliers. The sources of time uncertainties in photomultipliers as a spread of the initial velocity of photoelectrons, emission of photoelectrons under different angles and from different points at the photocathode, the time spread and the gain dispersion introduced by electron multiplier are reviewed. The experimental data on the time jitter, single electron response and photoelectron yield of some fast photomultipliers are collected. As the time resolution of the timing systems with scintillation counters depends also on time pick-off units, a short presentation of the timing methods is given. The discussion of timing theories is followed by a review of experimental studies of the time resolution of scintillation counters. The paper is ended by an analysis of prospects on further progress of the subnanosecond timing with scintillation counters.

(Abstract) The Effects of Recombination Balmer Decrements of Collisional and Self-Absorption Processes

(Abstract) The Effects of Recombination Balmer Decrements of Collisional and Self-Absorption Processes

Light pulse shapes from plastic scintillators

A detailed study of the light pulse shape from the binary NE 111 and the ternary Pilot U, Naton 136, KL 236, NE 102A, NE 104 and NE 110 plastic scintillators was performed by the single photon method using XP 1021 and C 31024 photomultipliers. The analysis of the shape of the light pulses determined experimentally for several samples of different dimensions gave the following conclusions. The original light pulse shape from the binary NE 111 scintillator, as measured with a 5 mm thick polished sample, is described analytically by the convolution integral of a Gaussian and an exponential function. The Gaussian function may reflect a deexcitation of several higher levels of the solvent molecules excited by nuclear particles preceding an intermolecular energy transfer in the scintillator. It may introduce a rather important limitation of the speed of plastic scintillators as the standard deviation of the Gaussian function is equal to 0.2 ns. The light pulse shape from the ternary plastics is described by the convolution integral of a Gaussian and two exponential functions. The Gaussian function presents the rate of energy transfer from nuclear particles to the primary solute as in the binary plastics. The exponential functions describe the energy transfer from the primary solute to the wavelength shifter and the final emission of the light. The light pulse shapes from larger volume scintillators show a slowing-down effect due to light collection and self-absorption and reemission processes reflected in an increase of the fwhm and the effective decay time. The evidence for the self-absorption and reemission process is based on study of the light pulse shape from thin and thick samples of ternary Kl 236 plastic by selecting different wavelengths of the emitted light. Analytically both processes may be represented by subsequent exponential probability density functions. It is shown that the parameters determined for the thin polished samples are the basic ones to describe the light pulse from a given scintillator as those dependent only on the composition of the plastic. The effective decay times determined directly from the slope of the time distribution spectra of the light pulses measured with the thin polished samples are systematically shorter than those published in the literature which in turn agree well with the measured ones for the thick samples.

Inverse Compton Process in Quasars and Seyfert-Type Nuclei

The inverse Compton spectral power of the infrared source, which is optically thick to its own radiation due to the synchrotron self-absorption process, is calculated. A radio source of a single ensemble of relativistic electrons and magnetic fields is considered. These elec­ trons emit synchrotron radiation, the high frequency part of the high frequency cutoff of which is observed in the infrared frequency range. A small fraction of the synchrotron photons suffers the inverse Compton scattering by the same ensemble of relativistic electrons. The scattered radiation is observed in the optical range. On the basis of this interpretation, the physical parameters of the infrared and optical source of 3C 273 are determined. A pos­ sible role played by the inverse Compton process is discussed for Seyfert-type nuclei. § 1. Introduction and summary In recent years, much attention has been paid to the similarity between some of the quasars and some of the nuclei of Seyfert-type galaxies. A promi­ nent feature of these objects is the enormous energy release in the infrared frequency range. Their output energy so far observed is mainly in this frequency range. Therefore, in orde'r to clarify their structures, it is of prime importance to interpret the large infrared radiation. Though the emission mechanism of the infrared radiation of the quasars and the Seyfert-type nuclei is not yet clear, the synchrotron mechanism is one of likely candidates. The possible presence of polarization in the infrared of 3C 273/) though not confirmed, may support this mechanism. We assume the infrared radiation of quasars and Seyfert-type nuclei to be produced by the synchrotron mechanism. As for the size of infrared emitting region, the rapid time variation with the time scale of the order of one day observed for NGC 1068 and NGC

Temperature determination from a cloud of alkali vapour in the upper atmosphere

A new method is discussed for measuring the temperature in an artificial cloud of alkali vapour. This method overcomes the complications introduced by the process of self-absorption. Estimates of the intensities of the relevant emission lines to be observed are made. It is concluded that observations made on lithium vapour present the best means of measurement.

The efficiency tracing technique for eliminating self-absorption errors in 4π β-counting

This paper examines in detail the validity of a standardization technique recently described by Merritt et al. in which the advantages of the 4π β-γ coincidence method in eliminating self-absorption errors are extended to pure β-emitters by the addition of suitable β-γ “tracer” nuclides. Calculations based on several models for the self-absorption process together with further experimental results supporting the method are presented and compared. The use of these theoretical considerations in making corrections for complex disintegration schemes in coincidence counting is discussed. The application of this “tracer” technique to nuclides having complex disintegration schemes is demonstrated for Rb 86 and the result used to obtain a value of 8·7 ± 0·2 per cent for the fraction of disintegrations populating the 1·08 MeV level in Sr 86.