2.5.3.8. Wavelength Shifts

There are small but detectable changes in the energy of X-ray lines from pure element to various compounds of the element due to bonding and oxidation effects. Although X-ray lines are produced by inner-shell ionization and self-neutralization, the configuration of the inner shells is slightly influenced by the outer valence electrons (Figure 2.5.3.8a). Peak shifts are most apparent in the outermost transitions (M-lines) and in low-Z elements (B through F for K a, Al through Cl for K b) where the X-ray-producing inner shells are less shielded from the valence electrons (Figure 2.5.3.8b).

Figure 2.5.3.8a. Dependence of the profile of the Al-Kb peak on the involvement of the Al valence electrons in chemical bonding. The Kb X-rays are produced by transitions from the 3s and 3p levels to the 1s level. In Al metal, the 3s and 3p electrons are not involved in chemical bonding and the transitions produce a relatively narrow band of energies. In Al2O3, the valence electrons combine with the oxygen 2s and 2p electrons to fill molecular orbitals, spanning a wider energy band. The Kb emission becomes broader, asymmetric, and resolves into separate Kb and satellite Kb' peaks (after Williams 1987).

Figure 2.5.3.8b. Chemical effects on the emission spectra of magnesium (Mg), magnesium oxide (MgO), and magnesium fluoride (MgF2). Count rates are logarithmically plotted, and vertically shifted for easier comparison. After Heinrich (1981).

The oxidation state of an atom is reflected in the number of electrons in the valence shell and differences in the valence electrons produce changes in the overall electron cloud structure and minor wavelength shifts. For example, consider Fe²⁺ with 24 electrons and Fe³⁺ with 23 electrons; both have a nuclear charge produced by 26 protons. The fewer electrons of the Fe³⁺ atom individually "feel" more of the nuclear charge and the configuration of the shells is slightly different than for Fe²⁺. The difference results in a small change in the energy difference between Fe-La and Fe-Lb. Attempts have been unsuccessful to exploit the wavelength shift to determine the oxidation state of Fe in minerals. In part this is because the difference in the ratio of electrons to nuclear charge for the two oxidation states is small.

For lighter elements the wavelength shift is substantially larger, especially comparing metals with oxides. In minerals, the shift of S-Kb is appreciable when sulfides (S^2-) are compared with sulfates (S⁶⁺). Standards should be the same oxidation state as the unknowns to avoid problems when analyzing sulfur.

The coordination of an atom in a mineral may also effect the electron cloud configuration and thus the resulting X-ray lines. In tetrahedral coordination, a cation has 4 nearest neighbors, whereas in octahedral coordination there are 6. As with the oxidation effect, equal numbers of nuclear protons pull on electrons, while, in this case, different numbers of oxygen atoms pull out. With more oxygen atoms pulling out (6-fold vs. 4-fold coordination), the electrons are pulled away from the nucleus resulting in larger transitions (higher energy). The coordination effects are also more easily observed in lighter elements than heavy elements. For example, Al-Ka lines shift to varying degrees relative the line produced from Al metal. In feldspars (Al^IV) this shift is about 0.05 degrees toward higher energy; in kaolinite (Al^VI) it is about 0.11 degrees.

Copyright 1997-2003, James H. Wittke

Last update: 01/18/2006 01:47 PM.