Atomic Absorption Coefficient Research Articles

Quartz tube atomizers for hydride generation atomic absorption spectrometry: mechanism of selenium hydride atomization and fate of free atoms

The influence of the purge gas type and its flow, as well as the oxygen supply and the temperature on the selenium sensitivity of a commercial, externally heated quartz tube atomizer, was investigated. The gas flow acts analogously as in the flame-in-tube atomizers, and the type of gas influences the atomic absorption coefficient. There is a synergic effect of oxygen supply and temperature on the sensitivity in a broad temperature range starting below 600°C. The higher the temperature, the lower oxygen supply required to achieve optimum sensitivity. A “hybrid” hydride atomizer, capable of working as either a flame-in-tube atomizer or an externally heated quartz tube atomizer, or in several combined modes, was employed to investigate hydride atomization and analyte transfer in quartz tube atomizers. The following conclusions have been drawn from the experiments: (i) There is no fundamental difference between both types of quartz tube atomizers, externally heated quartz atomizers being, in fact, oxygen-deficient, externally heated flame-in-tube atomizers. Hydride atomization takes place in a small cloud of hydrogen radicals located at the entrance to the heated portion of the cell. The decay of analyte free atoms takes place on the whole inner atomizer surface; (ii) The decay rate increases with temperature. This increase is much more pronounced if the surface is contaminated. The contamination does not affect the efficiency of atomization in the radical cloud; (iii) Typically, the sensitivity is controlled by the rate of free atom decay; and (iv) Decayed Se species are volatile-they can be completely reatomized in an additional flame downstream in the atomizer either externally heated or unheated. They cannot be atomized at all in a heated atomizer without a separate oxygen inlet.

Atomic absorption coefficient of the Se 196 nm line—theoretical calculation and experimental evaluation

A procedure to calculate the atomic absorption coefficient of the Se 196 nm line for conditions corresponding to various atomizers is described. The procedure employs an exact evaluation of the overlap integral of the emission and absorption line. Computed values of the atomic absorption coefficient of the Se 196 nm line for the flame-in-tube atomizer and for the graphite furnace have been assessed by comparison with experimental data from the literature. The agreement is good for flame-in-tube atomizers where the Lorentz broadening is predicted to dominate the absorption line. For atomization in Massmann-type graphite furnaces sensitivities reported by majority of authors are significantly lower than predicted.

Is there a “universal” MAC equation?

The search for an empirical absorption equation begins with the work of Siegbahn (1) in 1914. At that time Siegbahn showed that the value of (μ/ρ) for a given element could be expressed as a function of the wavelength (λ) of the x-ray photon by the following equationwhere C is a constant for a given material, which will have sudden jumps in value at critial absorption limits. Siegbahn found that n varied from 2.66 to 2.71 for various solids, and from 2.66 to 2.94 for various gases.Bragg and Pierce (2) , at this same time period, showed that their results on materials ranging from Al(13) to Au(79) could be represented by the followingwhere μa is the atomic absorption coefficient, Z the atomic number. Today equation (2) is known as the “Bragg-Pierce” Law. The exponent of 5/2(n) was questioned by many investigators, and that n should be closer to 3. The work of Wingardh (3) showed that the exponent of Z should be much lower, p = 2.95, however, this is much lower than that found by most investigators.

Theoretical profiles of balmer lines in the spectra of stars with moving envelopes with allowance for spectral dependence of the hydrogen absorption coefficient

A method for constructing theoretical profiles of emission lines in the spectra of stars with moving envelopes with allowance for a fre- quency dependence of the atomic absorption coefficient is described. The results of calculations for Voigt, Doppler, and rectangular pro- files of the absorption coefficient are compared. The example of the construction of the profile of H a for a disk envelope having differ- ent orientations relative to the observer is given. distribution of the radiating matter with respect to the radial velocities. The profiles were constructed using a rectangular profile of the hydrogen absorption coefficient in the lines. A series of models of envelopes with different sets of basic parameters describing the structure and kinematics, and also the main properties of the star, was considered. Comparison of the theoretical profiles of the lines H~_7 with the observed profiles showed that in a number of cases perfectly satisfactory agreement can be obtained. However, there are conditions under which the method becomes invalid or at least only partly valid. Three cases can be distinguished: i. The most general case is when a) an envelope is opaque in the continuum at the frequencies of the investigated lines, or b) the velocity gradient in an envelope is not sufficiently high for the two conditions described above, the basis of the calculations, to hold. Then the problem requires simultaneous solution of the stationarity equations for the energy levels and the equation of radiative transfer in the lines over the complete envelope. The problem is rather complicated and requires a special treatment that goes beyond the scope of the present paper. 2. The velocity gradient is not yet sufficiently large for one to be able to use Sobolev's method to calculate the level populations but is already too small for the real profile of the hydrogen absorption coefficient to have no influence on the form of the line profiles.

Absorption and emission line profile coefficients of multilevel atoms—IV. Velocity-averaged generalized redistribution functions for three-photon processes

The atomic generalized redistribution functions for three-photon processes, derived in the previous paper of this series, are formulated in terms of linear superpositions of newly introduced auxiliary functions q I − q VI , thus extending the traditional formalism of redistribution functions for two-photon processes. The corresponding velocity-averaged laboratory functions Q I − Q VI of these auxiliary functions are derived in both their angle-dependent and angle-averaged forms. Since the expressions found for Q I − Q VI are quite complicated, the so-called disentangled approximation is employed that uses the representative values of the generalized redistribution function at an orthogonal triad of photon directions rather than the angle-averaged function itself. This approximation yields relatively simple expressions which can be used in radiative transfer calculations.

Absorption and emission line profile coefficients of multilevel atoms—III. Generalized atomic redistribution functions for three-photon processes

A previously published semiclassical formulation of the atomic line profile coefficients in terms of generalized redistribution functions is made more explicit by deriving atomic redistribution functions for three-photon processes. Quantum mechanical calculations are carried out for a general three-photon process corresponding to the atomic transition sequence i → j → k → f, for nondegenerate states i, j, k, f and within the framework of the impact approximation. For comparison, analogous calculations are also performed using the substate (Weisskopf-Woolley) picture. It is Found that the quantum mechanical and the substate forms of generalized redistribution functions agree in the limit of no collisions, whereas in the presence of collisions they agree only if level interface phenomena of the collisional line broadening are negligible. The physical assumption underlying the substate picture are discussed in detail.

Absorption and emission line profile coefficients of multilevel atoms—I. Atomic profile coefficients

The line profile coefficients for absorption and emission appearing in the radiative transfer equation are formulated in terms of atomic line profile coefficients and velocity distribution functions. In order to derive the atomic profile coefficients of a multilevel atom, one defines generalized atomic redistribution functions that describe the correlations between photons involved in consecutive radiative transitions of the atom. Besides their dependence on the radiation field, the atomic line profile coefficients of a multilevel atom depend on the velocity distributions of the atoms in the various excitation states, in contrast to the case of a two-level atom where only the radiation intensity but not the velocity distributions affect the atomic emission profile. Closed expressions of the atomic profile coefficients in terms of generalized redistribution functions are obtained if stimulated emissions are neglected, and one is led to an iterative approximation scheme if stimulated emissions are taken into account. The possibility of a nonlocal character of the atomic profile coefficients is pointed out, and the effect of elastic, velocity-changing collisions with excited atoms is discussed. A major aim of this paper is to draw attention to the fact that ordinary redistribution functions that describe only the correlations between the absorbed and reemitted photons in the same spectral line are not sufficient to formulate the line profile coefficients of a multilevel atom.

Absorption and emission line profile coefficients of multilevel atoms—II. Velocity-averaged profile coefficients

Starting from the atomic profile coefficients of a multilevel atom derived in the previous first part of this paper, we consider the velocity-averaged line profile coefficients appearing in the radiative transfer equation for the important special case that the velocity distribution of atoms in the ground state is Maxwellian and that the streaming of excited atoms is negligible. Elastic velocity-changing collisions of excited atoms with other particles are taken into account in the framework of a strong-collision model. Neglecting stimulated emissions, we obtain explicit, albeit in some cases approximate, expressions for the line profile coefficients of a three-level atom in terms of the specific radiation intensity locally present. The emission and absorption profile coefficients are written in a form that exhibits the various physical effects responsible for deviations of these profiles from complete redistribution. The case of two-level atoms in the presence of elastic collisions with the excited atoms is also treated.

Hydride atomization in a cool hydrogen—oxygen flame burning in a quartz tube atomizer

A method of selenium hydride atomization using a cool highly fuel rich-hydrogen—oxygen diffusion flame burning in the inlet part of a T-shaped quartz tube placed in the optical axis of an atomic absorption spectrometer is investigated. It is shown that hydride atomization is caused by free radicals (H, OH) generated in the flame. Atomization (proceeding probably with 100% efficiency) is completed before the gases reach the optical axis. The life-time of free radicals is considerably reduced by admixture of even small amounts of oxygen to the hydrogen flush gas. A mathematical function describing the dependence of the selenium peak areas on the gas flow is derived and values for the selenium atomic absorption coefficient and rate constant for selenium decay by reactions on the tube walls are obtained by fitting experimental data to this function. The atomic absorption coefficient agrees well with a theoretically estimated value. The described method seems promising for absolute analysis by AAS.

Implications of a speed dependent modified Voigt function in atomic absorption spectrometry

A speed dependent modified Voigt function (MVF) is utilized to describe atomic spectral line profiles and compared to the well known Voigt function (VF). The air—acetylene flame has been used for these comparisons. In all cases the MVF differs from the VF in the calculation of atomic absorption coefficients (up to 15%), full-width half-maximum (FWHM) intensity values (up to 18%), and a-parameter values (up to 80%). The variation between the functions is mass dependent and decreases with atomic weight. The MVF is used to evaluate published lineshape data and reveals a significant difference in calculated collisional cross section values. In addition an experimental situation is evaluated using the MVF and VF to show that the perturber gas in a graphite furnace is important due to its influence on the absorption coefficient at line center.

Shock-tube atom kinetics of nitric oxide decomposition

The rate of formation of ground-state atomic oxygen, O(3P), has been measured behind shock waves in NO−Ar mixtures by atomic resonance absorption spectrophotometry. After experimental calibration of the atomic absorption coefficient, rate measurements were made over a temperature range of 2600°–6300°K, for 0.1%, 1.0%, and 10.0% mixtures of NO in Ar. The specific reaction-rate constants were first calculated, assuming that the dissociation proceeds by a simple bimolecular NO−Ar collision process. On this basis, Arrhenius plots of the data for the mixtures containing the higher concentrations of NO exhibit, at lower temperatures, rate constants which are much greater than the constants corresponding to the lower concentrations. At the higher temperatures, the rate constants converge. A chemical mechanism was then developed and employed to reproduce, by means of a computer calculation, the results obtained experimentally. The experimental data have been explained in terms of six reactions, of which two have been verified as the initial, rate-limiting steps of the dissociation process. These are: NO+NO→N2O+O(3P) and NO+Ar→N+O(3P)+Ar, for which the specific rate-constant expressions k=2.35×1010 exp(−29,000/RT) and k=1.37×1014 exp(−148,400/RT), respectively, have been obtained. The first of these two reactions is dominant at low temperatures and high NO concentrations, while the second predominates at high temperatures and low concentrations.

Atom Formation Rates behind Shock Waves in Oxygen

Atomic absorption spectrophotometry has been used to monitor the rate of oxygen atom formation behind shock waves in oxygen–argon mixtures. These observations were made by observing the time history of absorption at the oxygen atom triplet near 1300 Å. The atomic absorption coefficients were determined experimentally in the shock tube. These observations lead to an oxygen–argon dissociation rate given by kO2 − Ar = 1.85(± 14%) × 1011T0.5exp(− 95 700 / RT) cm3mole−1·sec−1 over the temperature range 2850°–5550°K. In addition this technique is shown to provide a powerful method for examining the vibration–dissociation coupling region. Measurements of the induction time for a steady-state atom formation rate to be established were obtained in the temperature range 2850°–5000°K. These measurements are compared with previously published experimental and theoretical induction times for dissociation in oxygen–argon mixtures.

Radiative Opacity in Stellar Atmospheres. I. Metal Absorption Coefficients

view Abstract Citations (69) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiative Opacity in Stellar Atmospheres. I. Metal Absorption Coefficients Travis, Larry D. ; Matsushima, Satoshi Abstract The results of numerical computations of the atomic absorption coefficients due to the elements C, N, 0, Na, Mg, Al, Si, Cr, Fe, and Ni are presented. The quantum defect method is used for the determina- tion of the photo-ionization cross-sections except in the cases of Cr, Fe, and Ni For these metals, detailed hydrogenic approximations are used owing to the inapplicability of the quantum defect method when LS coupling is not valid These cross-sections are used to compute the atomic absorption coefficients for temperatures ranging from 0 = 1 38 to 0 = 036, with an interval of M = 006, and a range in wave- lengths of XX1033-59958. The random errors involved in the empirical determination of the quantum defect are found to be comparable to the accuracy of the computed cross-sections estimated from com- parisons with experimentally determined values. Finally, the relative importance of metal absorptions in radiative opacity is examined for the atmospheric conditions of the solar-type and other spectral classes. I. INTRODUCTIO Publication: The Astrophysical Journal Pub Date: November 1968 DOI: 10.1086/149790 Bibcode: 1968ApJ...154..689T full text sources ADS | Related Materials (2) Part 2: 1968ApJ...154..715M Part 3: 1969ApJ...156..183M

Radiation transport in atomic plasmas

A general study of the radiation transport in a nonuniform plasma has been made including line and continuum processes. Available atomic absorption coefficient data from nonhydrogenic calculations and experiments for high temperature air are discussed. General relations are derived for the intensity, flux and flux divergence. Under uniform conditions, detailed transport comparisons are made with other theoretical predictions and recent experimental data. Also, results are presented for the power radiated from air for temperatures between 10 4 and 2×10 4 °K, pressures between 10 -1 and 10 2 atm, and plasma thickness between 1 and 30 cm. In addition, the flux has been obtained for a representative nonuniform shock layer condition and compared with the results from a uniform calculation.

Spectral absorption coefficients of carbon, nitrogen and oxygen atoms

A compilation of the spectral absorption coefficients of neutral and singly ionized carbon, nitrogen and oxygen atoms is presented. The continuum contributions resulting from bound-free and free-free transitions are presented in terms of an effective cross section summed over all permissible initial and final states. These effective continuum cross sections are tabulated spectrally over a frequency range 0.25 eV ≤ hv ≤ 20 eV for temperatures in the range 3000°K to 24 000°K at 1000°K intervals. The discrete, i.e. line, contributions resulting from bound-bound transitions are presented in terms of tabulations of absorption ƒ-numbers for all significant transitions (multiplet, supermultiplet, and transition arrays). Electron impact (half) half-widths and line shifts for Lorentzian line shapes are tabulated for all significant transitions for temperatures in the range 2500°K to 25 000°K.

Dynamical Models of Planetary Nebulae.

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamical Models of Planetary Nebulae. Sofia, S. ; Hunter, J. H. Abstract Dynamical models of the post-ejection stage of planetary nebulae have been built, for various initial conditions, taking detailed account of the most important heating and cooling processes. In computing the photo-ionization heating by the ultraviolet quanta of the central star, information about the flux and evolution of the star is utilized. The optically thin condition is used throughout. In computing the nebular cooling, the ionic balance is calculated at every mesh point in space and time, using the hydrogenic approximation for the atomic absorption coefficients. Free-free and 40 forbidden line transitions were taken into account, making allowance for collisional de-excitation. The model nebulae are made up of 10 elements and ions up to x =200 eV are considered. The dynamical models were advanced in time using a numerical code written in Eulerian variables. The code is highly accurate, utilizing fifth-order backward time differences to advance the variables in a predictor-corrector algorithm. The number of space mesh points is also variable in order to conserve computer time. While details of the results vary from model to model, the following general features are common to all of them: (a) Starting with a shell of a constant temperature, say 10 0000K, the expansion quickly cools the nebula much below the equilibrium condition given by the static model. Once the shell has expanded to a certain point, however, the photo-ionization heating dominates over expansion cooling and the temperature raises slowly towards the above-mentioned equilibrium value, which in general is reached when the nebula is too diluted to be seen. (b) Irrespective of the initial velocity profile a shock wave always develops in the forward direction. For some initial conditions there is a shock wave moving back towards the star. While for physical and numerical considerations we avoided using such initial conditions for long computer runs, in all cases studied in this investigation no substantial amount of matter falls back on the star, at variance with previously published results. Upon comparing the dynamical models with observations, it was found that a spherically symmetric nebular model is able to account for the observed radial dependence H~ emission. Thus, it is unnecessary to postulate the existance of torus-like structures. Comparison of theoretical lines profiles with observed line profiles were made at different evolutionary stages, and the agreement was good. Publication: The Astronomical Journal Pub Date: 1967 DOI: 10.1086/110465 Bibcode: 1967AJ.....72Q.830S full text sources ADS |

Summary-Introduction

This paper is an introduction to the astronomical material underlying the Varenna Symposium on Aerodynamical Phenomena in Stellar Atmospheres. The term « aerodynamical phenomena » rather than simply « velocity fields » is used in the title of the symposium to imply that primary concern centers as much on the physical phenomena and consequences associated with the presence of velocity fields as it does simply on the velocity fields themselves. To fully appreciate this distinction between aerodynamical phenomena and velocity fields from the astronomer's viewpoint, one must consider it against the background of the classical theory (*) of stellar atmospheres, which assumes that all the properties of the atmosphere are strictly controlled by the radiation field. The thermodynamic state of the classical atmosphere is fixed by the three conditions of radiative equilibrium (no energy transport other than by radiation), hydrostatic equilibrium (no mechanical momentum transport), and local thermodynamic equilibrium at a temperature fixed by the local energy-density of the radiation field (complete coupling between radiation field and atomic degrees of freedom). Analyses of stellar spectra under the framework of this classical atmospheric model take account of the presence of velocity fields (other than thermal) only in their effect upon the atomic absorption coefficient, not in their energetic or momentum coupling to the thermodynamic state of the atmosphere. Thus, if we become interested in aerodynamic phenomena in stellar atmospheres, we must investigate the possible perturbation these velocity fields may have upon the thermodynamic state of the atmosphere. We develop a primary concern with differential motions, velocity gradients, and dissipation mechanisms — all quantities which may produce a local non-relative energy source — rather than directing our attention only at stellar rotation and uniform expansion of an atmosphere. Thus, what we call aerodynamic phenomena embraces not only velocity fields but also their influence upon the thermodynamic state of the atmosphere.

THE STUDY OF GASEOUS MOTIONS IN STELLAR ATMOSPHERES: A REVIEW

If we define an eddy element, or simply an eddy, as an instantaneous association of material that moves in space as a whole, any motion in a medium can be represented by a superposition of the motions of eddies of different sizes. Mathematically this amounts to a Fourier analysis of the velocity field of the motion. In the case of the motions in stellar atmospheres we can divide the eddies, for empirical expedience, into two groups: (1) the small eddies with linear dimensions smaller than the thickness of the reversing layer and (2) the large eddies with linear dimensions greater than or equal to the thickness of the reversing layer. Such a classification is not a rigorous one because the thickness of the reversing layer is not a well-defined quantity. However, as a working model, this classification should be good enough to help us to describe the observed phenomena. Motions of both the large and small eddies in stellar atmospheres broaden the spectral lines, though in different manners. The small eddies, like thermal motion, broaden the effective atomic absorption coefficient along the line of sight in the reversing layer, while the large eddies impose no effect on the effective absorption coefficient. Consequently the former increase the equivalent width of the line as well as broaden it, but the latter only broaden the line without increasing its equivalent width. Indeed, it was a study of the equivalent widths of strong lines that led to the discovery of the turbulent motion of small eddies in stellar atmospheres,1 For weak lines for which the line depth is, in the first approximation, directly proportional to the line absorption coefficient, we should not expect to see much difference between the large and small eddies. Thus, wherever there is considerable motion in the atmosphere of a star whether in the form of large or small eddies the weak lines are invariably washed away. We can immediately see that we cannot distinguish, by observation, the small eddies of different sizes. As a matter of fact,

Continuous absorption of light in sodium vapour.

THE continuous absorption of light in sodium vapour, corresponding to the photo-ionization of the atom, has been measured with an apparatus generally similar to that used by Ditchburn, Tunstead and Yates1 for potassium vapour. Under the conditions of the present experiment, the absorption due to molecules is small compared with that due to atoms at the series-limit (2411 A.). After correcting for the molecular absorption, the atomic absorption coefficient at the series-limit (σl) is found to be 1.16 × 10−19 cm.2. This value is subject to an error of ± 10 percent, about half of which is due to uncertainty in the vapour pressure2. The variation of absorption coefficient with wave-length is shown in the accompanying graph, σλ/σl being plotted as a function of λ down to 2150 A. The accuracy of photometry decreases rapidly as the wave-length decreases beyond this point, and we can only state that the absorption lies between the two dotted curves. It appears probable that there is a minimum in the absorption curve at, or near, 2150 A.; but this cannot be stated with certainty.

On the Continuous Absorption Coefficient of the Negative Hydrogen Ion

In this paper the continuous absorption coefficient of the negative hydrogen ion, determined in an earlier paper (Ap. J., 102, 223, 1945) in terms of the matrix elements of the momentum operator, is further improved to take into account the effect of the static field of the hydrogen atom on the motion of the ejected electron. It is shown that for wave functions for the ground state of H- of the forms generally considered, the formula for the absorption cross-section can be reduced to the form K=3.7062/k(k2+0.05512)|∫∞0W2(r)χ1(r)dr|×10-18 cm2,where k denotes the momentum of the ejected electron in atomic units, W2(r) a certain weight function which can be tabulated, and χ1(r) the radial part of a p-spherical wave in the Hartree field of a hydrogen atom which tends to unit amplitude at infinity. The absorption cross-sections of H- have been evaluated according to the foregoing formula for various wave lengths. It is found that the new values are larger than those obtained with a plane-wave representation of the outgoing electron by about 5 per cent in the visual and the near infrared regions. The new absorption-curve places the maximum at about λ8500 A; at this wave length the atomic absorption coefficient has the value 4.52×10-17 cm2.