Atomic Absorption and Emission Spectra

The jump of an electron from one orbital to another is a transition between two energy states, such that an upward transition requires an input of energy, in the form of a photon of light for example. This is the absorption of light by the atom. A downward transition is accompanied by the release of energy, for instance in the shape of a photon of light. This is the process of emission or luminescence. 

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Calcium has exceptionally simple atomic absorption and emission spectra, and the 422.7 nm line is used for its determination by AAS, AES, and, very rarely, AFS. The next most sensitive absorption wavelength gives some 200 times poorer sensitivity, and is of no practical interest. The element is moderately well atomized in a fuel-rich air-acetylene flame, although the determination in this... 

The number of microstates resulting from the corresponding electron configuration varies between one for electron configurations with closed valence shells and 34 320 for Gd. N s can serve as a measure of the complexity of the atomic absorption (and emission) spectra. An element with a large number of microstates also has a large number of spectroscopic terms and atomic lines, because of the many different term combinations possible. 

Electronic spectra arise from transitions of electrons between energy levels. Transitions from lower to higher energy levels produce absorption spectra, while those from higher to lower energy levels give rise to emission spectra. Atomic absorption and emission spectra were discussed in Sections 1.4 and 4.3. In this section, we describe the electronic spectra of molecular species. 

Are all orbital transitions allowed in atomic absorption and emission spectra ... 

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. 

A helium +1 cation, like a hydrogen atom, has just one electron. Absorption and emission spectra show that He" has energy levels that depend on u, just like the hydrogen atom. Nevertheless, Figure 8 2 shows that the emission spectra of He and H differ, which means that these two species must have different energy levels. We conclude that something besides U influences orbital energy. 

Absorption and emission spectra of atoms and ions yield information about energy differences between orbitals, but they do not give an orbital s absolute energy. The most direct measurements of orbital energies come from a technique called photoelectron spectroscopy. 

Atomic spectra are much simpler than the corresponding molecular spectra, because there are no vibrational and rotational states. Moreover, spectral transitions in absorption or emission are not possible between all the numerous energy levels of an atom, but only according to selection rules. As a result, emission spectra are rather simple, with up to a few hundred lines. For example, absorption and emission spectra for sodium consist of some 40 peaks for elements with several outer electrons, absorption spectra may be much more complex and consist of hundreds of peaks. 

In contrast to Ag, these emission profiles are insensitive to variations of the excitation wavelength within the threefold structure of the 2P 2S absorption band. Simultaneous with the photolysis of any of the three 2P - 2S components, one observes gradual bleaching of all lines with concurrent formation of Ci where n =2-5 (34,56). A further intriguing observation concerns the appearance of a weak structured emission near 420 nm for excitations centered on the secondary atomic site band of Cu in all three rare gas films (Figure 4), which has been found from independent studies of the absorption and emission spectra of matrix entrapped CU2 to arise from the A-state of CU2 (34). 

However, in the sodium atom, An = 0 is also allowed. Thus the 3s —> 3p transition is allowed, although the 3s —> 4s is forbidden, since in this case A/ = 0 and is forbidden. Taken together, the Bohr model of quantized electron orbitals, the selection rules, and the relationship between wavelength and energy derived from particle-wave duality are sufficient to explain the major features of the emission spectra of all elements. For the heavier elements in the periodic table, the absorption and emission spectra can be extremely complicated - manganese and iron, for example, have about 4600 lines in the visible and UV region of the spectrum. 

Inner electrons are usually excited by X-rays. Atoms give characteristic X-ray absorption and emission spectra, due to a variety of ionization and possible inter-shell transitions. Two relevant refined X-ray absorption techniques, that use synchrotron radiation, are the so-called Absorption Edge Fine Structure (AEFS) and Extended X-ray Absorption Fine Structure (EXAFS). These techniques are very useful in the investigation of local structures in solids. On the other hand, X-Ray Fluorescence (XRF) is an important analytical technique. 

Octaethylporphinato complexes of the type [M(OEP)(02CMe)2] (M = Zr or Hf) have been prepared by reaction of H2OEP and [M(acac)4] in molten phenol at 210-240 °C followed by crystallization from pyridine-acetic acid-water mixtures. The complexes have been characterized by IR and mass spectra703,704 and by electronic absorption and emission spectra.703 They have an eight-coordinate square antiprismatic structure (48) in which the four porphinato nitrogen atoms occupy the coordination sites on one square face and the two bidentate acetate... 

In principle such upward or downward transitions can take place between any two energy states. The absorption spectrum of an atom consists of very sharp lines, the frequencies of which correspond to the difference of energies between the two states, E2 — Ex = hv. Similarly the luminescence spectrum of an atom consists of sharp emission lines of the same frequency. Figure 3.3 gives a simple picture of the energy states of an atom and of the transitions which can be observed in the absorption and emission spectra. The... 

The properties of absorption and luminescence emissions of atoms are important in analytical techniques as well as in spectroscopy in general. The absorption and emission spectra of atoms are line spectra which provide the unmistakable fingerprint of each element, and this is used in the analytical technique known as atomic absorption spectroscopy for example. Although the energy levels of atoms are shown as simple lines in a qualitative picture such as that of Figure 3.3, the absorption and emission lines which correspond to transitions between these levels are not infinitely narrow (that is, absolutely monochromatic) because of several effects. 

Fig. 5.1 Energies of orbitals available to the outer electron in sodium. Orbitals with different / values are shown in separate columns, in the atomic ground state, the 3s orbital Is occupied. Lines with arrows indicate some of the transitions observed in the absorption and emission spectra of sodium.

Modern atomic theory received a shot in the arm when it was recognized that the individual atom has light absorption and emission spectra occurring at narrow lines of the spectrum at specific wavelengths, as opposed to the broad bands typical of the polyatomic molecules and compounds. Since the line spectrum of each element is characteristic of that element, atomic spectroscopy can be used for precise elementary analysis of many types of chemically simple and complex materials. These studies make use of the wave character of light, as well as light s particle character. 

In Chapter 1, section 7, it was explained that very precise overlap of atomic absorption and emission profiles is required to obtain sensitive absorbance measurements. Absorption spectra of atoms at flame temperatures are much simpler than the emission spectra emitted by hollow cathode lamps. The possible transitions corresponding to electronic excitation of an atom may be shown as vertical lines on an energy level diagram, in which the vertical displacement... 

Photoluminescence from ambient temperature solutions of metal bis(l,2-dithiolenes) is rarely observed, and there have been only a few reports on luminescence of any type from these compounds. In addition to the weak emission (< ) = 10-5) seen for [Pt(mnt)2]2- (A.max = 775 nm), a similarly weak emission is observed for [Pt(qdt)2]2- (qdt = quinoxaline-2,3-dithiolate, 4) but at significantly higher energy (7,max = 606 nm) (58). Both absorption and emission spectra for the latter complex are highly dependent on solution pH, with protonation of one of the qdt nitrogen atoms leading to the shifts shown in... 

By suitable substituents, used at the same time to increase the yield and facilitate processibility. Substitution usually shifts emission toward the red (see Table 4). However, the absorption and emission spectra of phenylPPV (PPPV), in which the phenyl is substituted to one of the H atoms on the phenyl ring in the conjugated chain, is blue-shifted by several hundred angstroms [292]. 

Ytterbium atoms have been reacted with thermally generated hydrogen or deuterium atoms, with the resultant formation of YbH and YbD (198). The IR pydh stretching-frequency was observed at 1214.9 cm In addition, Yb atom and YbH absorption and emission spectra were observed. The magnetic parameters of YbH were determined from the esr spectra of the molecules (with Yb nuclear spin 1 = 0 and I = Vi) to be gn = 1.9953, gx = 1.9402, AjCH) = 226 MHz, Ax(H) = 224 MHz, Ax[ Yb (I = Vi)] = 5.266 GHz, A, [ Yb (I = Vi)] = 5.724 GHz. The hyperfine parameters indicated that the spin density is less than 20% on the hydrogen, and that the bonding is largely Yb H. ... 

This is what accounts for the discrete values of frequency v in emission spectra of atoms. Absorption spectra are correspondingly associated with the annihilation of a photon of the same energy and concomitant excitation of the atom from En to Em Fig. 1.9 is a schematic representation of the processes of absorption and emission of photons by atoms. Absorption and emission processes occur at the same set frequencies, as is shown by the two types of line spectra in Fig. 1.7. 

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