Spectral lines emission

The periodicity of structural regularities in spectra as related to the periodic law of the chemical elements is thus revealed in the verification of the displacement and alternation laws. Practically all the structures here referred to are in beautiful accord with the quantum theory of spectral line emission as developed by Bohr, Sommerfeld, Land and others. During the past three years more than 5000 spectral lines have been classified by Catalan, Gieseler, Kiess, Laporte, Meggers, Russell and Walters. With the further development of methods of attack, it seems probable that practically all complex spectra will be fully interpreted in a period of time not greater than that already expended in the classification of simpler spectra—especially if additional workers can be brought into this fascinating field. 

The practice of analytical spectroscopy preceded the development of the theories concerning the origin of spectra by a number of years. Bunsen and Kirchhoff studied spectra produced by salts and salt solutions heated in flames and noted the characteristic spectral line emissions of a number of elements. They also observed that substances absorb energy most strongly at the same wavelengths at which emission occurs. Their results led Kirchhoff to state that the power of emission is equal to the power of absorption for all... 

The Zeeman effect is exhibited when the intensity of an atomic spectral line, emission or absorption, is reduced when the atoms responsible are subjected to a magnetic field, nearby lines arising instead (Figure 11.2). This makes a powerful tool for the correction of background attenuation caused by molecules or particles that do not normally show... 

The characteristic lines observed in the absorption (and emission) spectra of nearly isolated atoms and ions due to transitions between quantum levels are extremely sharp. As a result, their wavelengths (photon energies) can be determined with great accuracy. The lines are characteristic of a particular atom or ion and can be used for identification purposes. Molecular spectra, while usually less sharp than atomic spectra, are also relatively sharp. Positions of spectral lines can be determined with sufficient accuracy to verify the electronic structure of the molecules. 

The emission yield, Ra, defined as the radiation of the spectral line, k, of an element, i, emitted per unit sputtered mass must be determined independently for each spectral line. The quantities g, and Ry are derived from a variety of different standard bulk samples with different sputtering rates. In practice, both sputtering rates and excitation probability are influenced by the working conditions of the discharge. Systematic variation of the discharge voltage, L/g, and current, I, leads to the empirical intensity expression [4.185] ... 

A distinction must be made between continuous sources (hydrogen or deuterium lamps, incandescent tungsten lamps, high pressure xenon lamps) and spectral line sources (mercury lamps), which deliver spectrally purer light in the region of their emission lines. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

With flame emission spectroscopy, there is greater likelihood of spectral interferences when the line emission of the element to be determined and those due to interfering substances are of similar wavelength, than with atomic absorption spectroscopy. Obviously some of such interferences may be eliminated by improved resolution of the instrument, e.g. by use of a prism rather than a filter, but in certain cases it may be necessary to select other, non-interfering, lines for the determination. In some cases it may even be necessary to separate the element to be determined from interfering elements by a separation process such as ion exchange or solvent extraction (see Chapters 6, 7). 

The Humphreys series is set of spectral lines in the emission spectrum of atomic hydrogen that ends in the fifth excited state. 

In the spectrum of atomic hydrogen, a violet line is observed at 434 nni. Determine the beginning and ending energy levels of the electron during the emission of energy that leads to this spectral line. 

Astronomers use spectroscopy to identify the composition of the sun and other stars. A striking example is the discovery of the element helium. In 1868, astrono-mers viewing a solar eclipse observed emission lines that did not match any known element. The English astronomer Joseph Lockyer attributed these lines to a new element that he named helium, from hellos, the Greek word for the sun. For 25 years the only evidence for the existence of helium was these solar spectral lines. 

Spectral overlap of emission and absorption wavelengths Is a potential cause of Interference In atomic absorption spectrometry (57) Thus, (a) the emission line of Fe at 352.424 nm Is close to the resonance line of N1 at 352.454, (b) the emission line of Sb at 217.023 nm Is close to the resonance line of Pb at 216.999 nm, and (c) the emission line of As at 228.812 nm Is close to the resonance line of Cd at 228.802 (57). To date, these practically coincident spectral lines have not been reported to be of practical Importance as sources of analytical Interference In atomic absorption analyses of biological materials. 

The stochastic theory of lineshape has been developed by Anderson and Weiss [157], by Kubo [158], and by Kubo and Tomita [159] in order to treat the narrowing of spectral lines by exchange or motion, a generalized formulation having been subsequently presented by Blume [31]. We consider below an application of the theory of Blume to the specific problem of relaxation between LS and HS states in Mossbauer spectra of powder materials which is based on the formulation by Blume and Tjon [32, 33], Accordingly, the probability of emission of a photon of wave vector Ik and frequency m is given as [160] ... 

The discovery of two other series of emission lines of hydrogen came later. They are named for their discoverers the Lyman series in the ultraviolet range and Paschen series in the infrared region. Although formulas were devised to calculate the spectral lines, the physics behind the math was not understood until Niels Bohr proposed his quantized atom. Suddenly, the emission spectrum of hydrogen made sense. Each line represented the energy released when an excited electron went from a higher quantum state to a lower one. 

FIG. 66. Effects of modulating the RF excitation frequency (a) deposition rate and average light intensity as a function of the modulation frequency, with the deposition rate at cw conditions indicated by the dotted line, (b) measured spectrally integrated emission and calculated production of SiH3 radicals as a function of time, at a modulation frequency of 50 kHz and a 509f duty cycle. [From A. C. W. Biebericher. J, Bezemer. W. F. van der Weg, and W. J. Goedheer, Appl. Phys. Lett. 76, 2002 (2000), 2000, American Institute of Physics, w ith permission.]... 

The emission line is centered at the mean energy Eq of the transition (Fig. 2.2). One can immediately see that I E) = 1/2 I Eq) for E = Eq E/2, which renders r the full width of the spectral line at half maximum. F is called the natural width of the nuclear excited state. The emission line is normalized so that the integral is one f l(E)dE = 1. The probability distribution for the corresponding absorption process, the absorption line, has the same shape as the emission line for reasons of time-reversal invariance. 

Spark sources are especially important for metal analysis. To date, medium-voltage sparks (0.5-1 kV) often at high frequencies (1 kHz and more), are used under an argon atmosphere. Spark analyses can be performed in less than 30 s. For accurate analyses, extensive sets of calibration samples must be used, and mathematical procedures may be helpful so as to perform corrections for matrix interferences. In arc and spark emission spectrometry, the spectral lines used are situated in the UV (180-380nm), VIS (380-550nm) and VUV (<180 nm) regions. Atomic emission spectrometry with spark excitation is a standard method for production and product control in the metal industry. 

The spectrum of atomic hydrogen, as observed in absorption or emission, arises from transitions between tile various possible states. In emission, a spectral line results from a transition such as n2 i and the application of Eq. (84) leads to the expression... 

In addition to measuring total recombination coefficients, experimentalists seek to determine absolute or relative yields of specific recombination products by emission spectroscopy, laser induced fluorescence, and optical absorption. In most such measurements, the products suffer many collisions between their creation and detection and nothing can be deduced about their initial translational energies. Limited, but important, information on the kinetic energies of the nascent products can be obtained by examination of the widths of emitted spectral lines and by... 

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