Atoms X-ray spectra

Relativity is expected to play an important role in several types of radiative processes in atoms. Its influence on the atomic levels fine structure has been most thoroughly investigated as its signature is easily evidenced in atomic x-ray spectra, [1], [2], [3], It manifests itself also in some delicate aspects of the chemical reactivity of the elements, [4], These effects arise from both the standard Dirac-like properties of electrons and from more sophisticated QED corrections. One of the major objective of the present paper is to show that the overall picture has dramatically changed recently, as a consequence of the considerable advances made in the design of ultra intense laser sources operated at intensities well beyond the so-called atomic unit of intensity la = 3.5 xlO16 W/cm2, [5]. 

As the Auger transitions do not change the shape of the angular momentum distribution, the particle quickly reaches the (/= — ) orbits (Hartmann 1989) and from those - owing to the Al = 1 rule - can make circular, (n, 1 = n — 1) —> (n — 1, Z = n — 2) transitions only (O Fig. 28.3). Typical exotic-atoms X-ray spectra are presented in O Fig. 28.4 note that the spectra are dominated by circular transitions from highly excited states. 

In an early 20 century study of atomic x-ray spectra, British physicist Henry Moseley discovered a relationship that replaced atomic mass as the criterion for ordering the elements. By what criterion are the elements now ordered in the periodic table Give an example of a sequence of element order that was confirmed by Moseley s findings. 

Whereas zirconium was discovered in 1789 and titanium in 1790, it was not until 1923 that hafnium was positively identified. The Bohr atomic theory was the basis for postulating that element 72 should be tetravalent rather than a trivalent member of the rare-earth series. Moseley s technique of identification was used by means of the x-ray spectra of several 2ircon concentrates and lines at the positions and with the relative intensities postulated by Bohr were found (1). Hafnium was named after Hafma, the Latin name for Copenhagen where the discovery was made. 

X-Rays. If an x-ray is emitted, it has an energy, AE, equal to the difference in the binding energies of the two atomic shells, E — Ej. If the original hole is in the K shell, the x-ray is called a K x-ray if the hole is in the L shell it is an L x-ray. Because the hole can be filled by an electron from any of the several outer shells, x-ray spectra contain a large number of discrete lines. 

Both the wavelength dispersive and energy dispersive spectrometers are well suited for quaUtative analysis of materials. Each element gives on the average only six emission lines. Because the characteristic x-ray spectra are so simple, the process of allocating atomic numbers to the emission lines is relatively simple and the chance of making a gross error is small. 

I9l 3 H. G. J. Moseley observed regularities in the characteristic X ray spectra of the elements he thereby discovered atomic numbers Z and provided justification for the ordina] sequence of the dements. 

One problem with Mendeleev s table was that some elements seemed to be out of place. For example, when argon was isolated, it did not seem to have the correct mass for its location. Its relative atomic mass of 40 is the same as that of calcium, but argon is an inert gas and calcium a reactive metal. Such anomalies led scientists to question the use of relative atomic mass as the basis for organizing the elements. When Henry Moseley examined x-ray spectra of the elements in the early twentieth century, he realized that he could infer the atomic number itself. It was soon discovered that elements fall into the uniformly repeating pattern of the periodic table if they are organized according to atomic number, rather than atomic mass. 

Now if either the elements were not characterized by these integers, or any mistake had been made in the order chosen or in the number of places left for unknown elements, these regularities would at once disappear . We can therefore conclude from the evidence of the X-ray spectra alone, without using any theory of atomic structure, that these integers are really characteristic of the elements. Further, as it is improbable that two different stable elements should have the same integer, three, and only three, more elements are likely to exist between Al and Au. As the X-ray spectra of these elements can be confidently predicted, they should not be difficult to find. The examination of keltium would be of exceptional interest, as no place has been assigned to this element. (For keltium see footnote1)... 

It is noted without further comment that Moseley was able to characterize his observed x-ray spectra in such a way that the individual elements are identified purely by their respective atomic numbers without any use of an atomic model. Note again, however, that the denominators in the Qk and Ql expressions are exactly the frequencies of the hydrogenic Lymann and Balmer alpha lines (Moseley s nomenclature) and that therefore Qk and Ql exactly equal the nuclear charge of a one-electron hydrogenic atom which would be deduced from the frequency v of its observed Lymann and Balmer alpha lines2. 

The basis for the claim of discovery of an element has varied over the centuries. The method of discovery of the chemical elements in the late eightenth and the early nineteenth centuries used the properties of the new sustances, their separability, the colors of their compounds, the shapes of their crystals and their reactivity to determine the existence of new elements. In those early days, atomic weight values were not available, and there was no spectral analysis that would later be supplied by arc, spark, absorption, phosphorescent or x-ray spectra. Also in those days, there were many claims, e.g., the discovery of certain rare earth elements of the lanthanide series, which involved the discovery of a mineral ore, from which an element was later extracted. The honor of discovery has often been accorded not to the person who first isolated the element but to the person who discovered the original mineral itself, even when the ore was impure and that ore actually contained many elements. The reason for this is that in the case of these rare earth elements, the earth now refers to oxides of a metal not to the metal itself This fact was not realized at the time of their discovery, until the English chemist Humphry Davy showed that earths were compounds of oxygen and metals in 1808. 

The problems associated with quantitative studies of structure based upon this viewpoint of X ray absorption-edge spectra may be similar to those encountered using electron beams of comparable energy, 3 to 100 ev., to carry out electron diffraction studies of crystal structure. Qualitatively, this analogy can be carried further, as both the X ray spectra and the electron beam diffraction in this energy range are influenced by only the first few atom layers. 

Additional experimental evidence for the preservation of electronic states upon formation of molecules is the insensitivity of X-ray spectra to the chemical enviromnent of an atom, implying that the ionization potentials of inner electrons are relatively unaffected by bonding. Indeed, Moseley s classic experiments on the relationship between X-ray frequencies and atomic number were carried out on atoms in a variety of states of chemical combination. 

Courtesy of Lyman C. Newell Henry Gwyn Jeffreys Moseley, 1887-1915. English physicist whc studied the X-ray spectra of more than fifty elements and discovered the relation existing between the atomic number of an element and the frequency of the X-rays which it emits when bombarded by cathode rays. At the age of twenty-seven years he was killed while in active service at the Dardanelles. 

The overall picture of the atom envisioned by Bohr was a dense nncleus of fixed charge surrounded by rings of electrons. The comphcated optical spectra and the simple x-ray spectra suggested that the ring closest to the nucleus was different than the outer rings. More theory and more observations were necessary to refine this picture, but the shell theory of electronic structure has persisted. 

The X-ray spectra of niobium have been investigated.4 Aston was unable to obtain a definite mass spectrum of the metal.5 The arrangement of the electron groups in the atom has been considered by Lesshdm and Samuel.4 Niobium is not radioactive.7... 

The X-ray spectra of tantalum have been investigated.1 The emission of electrons from tantalum when heated to high temperatures has received considerable investigation.2 Electron emission from the cold metal has been studied by Bother,3 and the arrangement of electron groups in the atom by Lessheim and Samuel.4 Tantalum is not radioactive.5... 

MOSLEY. HENRY 11887-1915). A British chemist who studied under Ernest Rutherford and brilliantly developed the application of X-ray spectra to the study of atomic structure his discoveries resulted in a more accurate positioning of elements in the periodic tahle hy closer determination of atomic numbers. Tragically for the development of seienee. Moscly was killed in action al Gallipoli in 1915. 

Since the X-ray spectral lines come from the inner electrons of the atoms, die lines are not related to the chemical properties of the elements or to the compounds in which they may reside. Because the characteristics of die X-ray spectra are associated with energies released through transitions of electrons within the inner shells of the atom, the spectra are simple. Most practical X-ray fluorescence analysis involves the detection of radiation release through electron transitions from outer shells to the K shell (K spectra), outer shells to die L shell (L spectra) and, in very few cases, from outer shells to the M shell (M spectra). 

Lines, corresponding to different transitions from initial states with vacancy in the shells with the same n, compose a series of spectra, e.g. K-, L-, M-series etc. Main diagram lines correspond to electric dipole ( 1) transitions between shells with different n. The lines of 2-transitions also belong to diagram lines. Selection rules of 1-radiation as well as the one-particle character of the energy levels of atoms with closed shells and one inner vacancy cause, as a rule, a doublet nature of the spectra, similar to optical spectra of alkaline elements. X-ray spectra are even simpler than optical spectra because their series consist of small numbers of lines, smaller than the number of shells in an atom. The main lines of the X-ray radiation spectrum, corresponding to transitions in inner shells, preserve their character also for the case of an atom with open outer shells, because the outer shells hardly influence the properties of inner shells. 

Most of these types of spectroscopy are of use in chemistry. The levels probed by low-energy photons are sensitive to the detailed structure of molecules, and can be used to help identify compounds that have been newly synthesized. Visible and UV spectroscopy is important in the study of chemical bonding. X-ray spectra are characteristic of particular atoms, and are important in some methods of chemical analysis. The application of the quantum theory to the appropriate types of energy levels is essential in all these applications. A few relevant examples will be given in later chapters. 

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