X-ray Fluorescence

X-ray fluorescence is a process which occurs when an atom is ionised by a high energy source such as an electron beam or X-ray source. High-energy particles (e.g. accelerated electrons or X-ray photons) are able to energise the electrons within atoms sufficiently to move them out of the inner orbits to higher energy states (into the outer shells) or even out of the atom altogether, leaving the atom in an excited (or ionised) state. To stabilise the atom, electrons in the outer shells of the atom cascade down into the inner shells to fill the vacant electron shells and in doing so release a discrete amount of energy equal to the difference in energy between the donor shell and the vacant shell. In the inner shells (the K, L and M shells) of atoms, the energy released (measured in eV or KeV) is sufficient to produce an X-ray photon. The energy of this emitted photon increases as the atomic number increases whilst the wavelength of the emitted photon decreases. Because the possible electron transfers within an atom are different for every element (and get progressively more complex and energetic with increasing atomic weight), the radiation spectrum produced is specific to that element.

Radiation spectra can therefore be obtained by measuring the intensity of the radiation (i.e. the number of X-ray photons emitted) with respect to energy or wavelength. These radiation spectra comprise of X-ray "lines" (or a "family" of several lines) of specific frequency and energy which correspond to the type of electron transfers occurring within the atom; these X-ray lines are known as characteristic X-rays. The intensity of each line is proportional to the probability of that electron transition occurring within the atom i.e. it is more probable that electron vacancies in the K shell will be filled by electrons from the L shell rather than the M shell. Hence, the intensity of radiation produced by electrons moving from the L shell to the K shell is greater than that produced by electrons moving from the M to K shells. However, because the number of electron transfers occurring during ionisation of a single sample is extremely large, the uncertainty as to which electron transfer is likely to occur becomes insignificant (and the relative intensity of individual X-ray lines equals the probability of the corresponding electron transfer occurring). Therefore, the intensity of individual X-ray lines (or families of lines) produced by a single element within a sample is proportional to the abundance of that element within the sample. 

A shorthand notation is used for describing the different types of X-ray lines (siegbahn notation). The X-ray line is denoted by the letter of the vacant electron shell (K, L, M) followed by Greek letters (a, b, etc) indicating the shell from which the electron was donated. For instance, the X-ray line produced by the transfer of an electron from the L shell to fill a vacancy in the K shell will produce Ka radiation whilst an electron moving from the M shell into the K shell will produce a Kb X-ray line and so on. Light elements with few electron shells therefore have simple radiation spectra whilst heavy elements with several shells have quite complex radiation spectra with many X-ray lines (also known as an X-ray line "family").

For example:

Sulphur has 16 electrons (2 in the K shell, 8 in the L shell, and 6 in the M shell). Ionisation of a sulphur atom can therefore result in Ka, Kb, and La radiation only.

	Ka radiation is emitted when an L shell electron fills the vacant space in the K shell.
	Kb radiation is emitted when the an M shell electron fills the vacant space in the K shell.
	La radiation is emitted when an M shell electron fills the vacant space in the L shell.

Summary

X-ray fluorescence gives rise to radiation (of specific energy and wavelength) characteristic of individual elements, the intensity of which is proportional to the abundance of that element within a sample. Therefore, measuring either the wavelength or energy of the radiation emitted from an ionised sample allows accurate determination of the absolute abundance of particular elements within a sample.

This is a very simplified account of X-ray fluorescence; the L, M and higher shells are subdivided into 2 or more electron orbits and give rise to numerous La1, La2 etc X-ray lines (X-ray line families) again with energies characteristic of the atom.  These X-ray line families are of particular use in X-ray fluorescence spectrometry.