Absorption by free atoms

In order to measure absorption by free atoms it is first necessary to choose the correct wavelength at which this absorption can be measured. That wavelength is a critical property of the atoms and is given by the equation 

This relationship follows directly from quantum theory. It can be stated simply the energy of the radiation which the atom can absorb must equal the energy difference between the energy state the atom exists in before absorption and the energy state the atom exists in after absorption. 

The electronic states of atoms are well defined by atomic theory. As we proceed up the periodic table by increasing atomic numbers, electrons pro- 

In the case of sodium atoms two slightly different energy levels exist in the excited state and two slightly different absorption wavelengths are observed, at 589 nm and 589.5 nm. These wavelengths coincide exactly with the yellow sodium doublet emission lines observed when sodium atoms are heated. 

From the Grotrian diagram of an element, the absorption wavelengths of the atoms of that element can be deduced. Theoretically, any transitions between two states permitted by quantum theory can be used in AAS. In practice, however, it is found that this is not the case because the number of atoms in the upper excited state is extremely small and the degree of absorption by these few excited atoms is scarcely measurable. Consequently, absorption lines relating to transitions between an excited state and a higher excited state are insensitive and not analytically useful. 

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To a first approximation, absorption by free atoms is similar to absorption by molecules and there is a linear relationship between absorbance and the... 

Graphite Furnace Atomic Absorption Spectrometry GF A AS is based on absorption by free atoms, of the resonance lines characteristic of a given element, emitted by the radiation source [6, 72]. 

The ET AAS technique (see Fig. 5.2) is based on fast evaporation of samples to be analysed in a miniature tube furnace (6-8 mm in diameter and 20-30 mm in length) made of graphite [5]. A light beam from the source of a line spectrum (usually a hollow cathode lamp) passes through this tube and the value of the light absorption by free atoms of analyte is measured. A grating monochromator is used to separate the most sensitive resonance line from the atomic spectrum of the element emitted by the light source. 

Because atomic absorption involves the absorption of light by free atoms, some method is necessary for conversion of the determinant species in a sample into atoms. This is most commonly done by dissolving the sample, and spraying the resulting solution into a flame which is hot enough to convert the determinant to atoms. 

As the name implies, AAS is the measurement of absorption of radiation by free atoms. The total amount of absorption depends on the number of free atoms present and the degree to which the free atoms absorb the radiation. The key to understanding the application of atomic asborption spectrometry to analytical chemistry lies in understanding the factors which affect (1) the ability of the atoms to absorb and (2) the factors which affect the generation and loss of free atoms from a particular atom population. These two factors will be considered separately. 

Figure 10.2. Absorption of y-ray photons by free atoms E = excitation energy E = recoil energy D = line broadening by the Doppler effect.

The phenomenon of absorption of the central portion of the emission line by free atoms in the lamp is called self-reversal . Such absorption is not easily detectable, because it is at the very center of the emitted resonance line and very difficult to resolve. It is, of course, exactly the radiation that is most easily absorbed by the atoms of the sample. In practice, if the HCL is operated at too high a current, the self-reversal decreases the sensitivity of the analysis by removing absorbable light. It also shortens the life of the lamp significantly. 

Atomic absorption spectrometry (AAS). An analytical method for the determination of elements in small quantities. It is based on the absorption of radiation energy by free atoms. 

Fig. 3.4 The absorption edge structure of Cu illustrating the difference between the absorption in free atom case and the EXAFS case. Reproduced from Rehr and Albers [38] 2000 by the American Physictd Society...

Any polymer contains some inner free space free volume distributed in a dynamic manner between its molecular chains (see Section 23.2). When it is exposed to a fluid (liquid or gas) the physical possibility exists for fluid absorption by the polymer, if the fluid molecules or atoms are small enough to fit into local regions of this distributed space during kinetic movements. As this happens, subsequent kinetic chain motion must allow for the newly absorbed fluid molecules and, hence, the polymer s overall volume will adjust accordingly this action will coincide with the formation of more free space around these fluid molecules—so the polymer will swell a little. This process will be continued until an equilibrium is reached ( equilibrium swelling ), by which time the extent of swelling can be considerable. The amount of fluid taken up and the rate at which this happens are both important, and are discussed in this and following sections. 

Figure 4.10. Absorption of X-rays as a function of photon energy E = hv by a free atom and by atoms in a lattice. The fine structure, due to the interference of waves...

However, mathematics is essential to explain how structural data are derived from EXAFS. The EXAFS function, x(k), is extracted from the X-ray absorption spectrum in Fig. 4.10 by removing the approximately parabolic background and the step, i.e. the spectrum of the free atom. As in any scattering experiment, it is customary to express the signal as a function of the wavenumber, k, rather than of energy. The relation between k and the kinetic energy of the photoelectron is ... 

Hence, nuclear resonance absorption of y-photons (the Mbssbauer effect) is not possible between free atoms (at rest) because of the energy loss by recoil. The deficiency in y-energy is two times the recoil energy, 2Er, which in the case of Fe is about 10 times larger than the natural line width F of the nuclear levels involved (Fig. 2.4). 

Figure 5.1 Resonant absorption of y-radiation by a nucleus can only take place in the solid state because of recoil effects. The excited nucleus of a free atom emits a y-photon with an energy EirER, whereas the nucleus in the ground slate of a free atom can only absorb a photon if it has an energy equal to Eo+ER. As the linewidth of nuclear transitions is extremely narrow, the emission spectrum does not overlap with the absorption spectrum. In a solid, a considerable fraction of events occurs recoil free (ER=0), and here the emission spectrum overlaps completely with the absorption spectrum (provided source and absorber have the same chemical environment).

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